Zhang Y, DS Aidhy, T Varga, S Moll, PD Edmondson, F Namavar, K Jin, CN Ostrouchov, and WJ Weber. 2014. "The Effect Of Electronic Energy Loss On Irradiation-induced Grain Growth In Nanocrystalline Oxides." Physical Chemistry Chemical Physics. PCCP 16(17):8051-8059. doi:10.1039/c4cp00392f Abstract Grain growth of nanocrystalline materials is generally thermally activated, but can also be driven by irradiation at much lower temperature. In nanocrystalline ceria and zirconia, energetic ions deposit their energy to both atomic nuclei and electrons. Our experimental results have shown that irradiationinduced grain growth is dependent on the total energy deposited, where electronic energy loss and elastic collisions between atomic nuclei both contribute to the production of disorder and grain growth. Our atomistic simulations reveal that a high density of disorder near grain boundaries leads to locally rapid grain movement. The additive effect from both electronic excitation and atomic collision cascades on grain growth demonstrated in this work opens up new possibilities for controlling grain sizes to improve functionality of nanocrystalline materials.

Zhang Y, T Varga, M Ishimaru, PD Edmondson, H Xue, P Liu, S Moll, F Namavar, CM Hardiman, S Shannon, and WJ Weber. 2014. "Competing Effects Of Electronic And Nuclear Energy Loss On Microstructural Evolution In Ionic-covalent Materials." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 327:33-43. doi:10.1016/j.nimb.2013.10.095 Abstract Ever increasing energy needs have raised the demands for advanced fuels and cladding materials that withstand the extreme radiation environments with improved accident tolerance over a long period of time. Ceria (CeO2) is a well known ionic conductor that is isostructural with urania and plutonia-based nuclear fuels. In the context of nuclear fuels, immobilization and transmutation of actinides, CeO2 is a model system for radiation effect studies. Covalent silicon carbide (SiC) is a candidate for use as structural material in fusion, cladding material for fission reactors, and an inert matrix for the transmutation of plutonium and other radioactive actinides. Understanding microstructural change of these ionic-covalent materials to irradiation is important for advanced nuclear energy systems. While displacements from nuclear energy loss may be the primary contribution to damage accumulation in a crystalline matrix and a driving force for the grain boundary evolution in nanostructured materials, local non-equilibrium disorder and excitation through electronic While displacements from nuclear energy loss may be the primary contribution to damage accumulation in a crystalline matrix and a driving force for the grain boundary evolution in nanostructured materials, local non-equilibrium disorder and excitation through electronic energy loss may, however, produce additional damage or anneal pre-existing defect. At intermediate transit energies where electronic and nuclear energy losses are both significant, synergistic, additive or competitive processes may evolve that affect the dynamic response of materials to irradiation. The response of crystalline and nanostructured CeO2 and SiC to ion irradiation are studied under different nuclear and electronic stopping powers to describe some general material response in this transit energy regime. Although fast radiation-induced grain growth in CeO2 is evident with no phase transformation, different fluence and dose dependence on the growth rate is observed under Si and Au irradiations. While grain shrinkage and amorphization are observed in the nano-engineered 3C SiC with a high-density of stacking faults embedded in nanosize columnar grains, significantly enhanced radiation resistance is attributed to stacking faults that promote efficient point defect annihilation. Moreover, competing effects of electronic and nuclear energy loss on the damage accumulation and annihilation are observed in crystalline 4H-SiC. Systematic experiments and simulation effort are needed to understand the competitive or synergistic effects. energy loss may, however, produce additional damage or anneal pre-existing defect. At intermediate transit energies where electronic and nuclear energy losses are both significant, synergistic, additive or competitive processes may evolve that affect the dynamic response of materials to irradiation. The response of crystalline and nanostructured CeO2 and SiC to ion irradiation are studied under different nuclear and electronic stopping powers to describe some general material response in this transit energy regime. Although fast radiation-induced grain growth in CeO2 is evident with no phase transformation, different fluence and dose dependence on the growth rate is observed under Si and Au irradiations. While grain shrinkage and amorphization are observed in the nano-engineered 3C SiC with a high-density of stacking faults embedded in nanosize columnar grains, significantly enhanced radiation resistance is attributed to stacking faults that promote efficient point defect annihilation. Moreover, competing effects of electronic and nuclear energy loss on the damage accumulation and annihilation are observed in crystalline 4H-SiC. Systematic experiments and simulation effort are needed to understand the competitive or synergistic effects.

Zarkadoula E, R Devanathan, WJ Weber, M Seaton, I Todorov, K Nordlund, MT Dove, and K Trachenko. 2014. "High-energy radiation damage in zirconia: modeling results ." Journal of Applied Physics 115(8):083507. doi:10.1063/1.4866989 Abstract Zirconia has been viewed as a material of exceptional resistance to amorphization by radiation damage, and was consequently proposed as a candidate to immobilize nuclear waste and serve as a nuclear fuel matrix. Here, we perform molecular dynamics simulations of radiation damage in zirconia in the range of 0.1-0.5 MeV energies with the account of electronic energy losses. We find that the lack of amorphizability co-exists with a large number of point defects and their clusters. These, importantly, are largely disjoint from each other and therefore represent a dilute damage that does not result in the loss of long-range structural coherence and amorphization. We document the nature of these defects in detail, including their sizes, distribution and morphology, and discuss practical implications of using zirconia in intense radiation environments.

Jin K, Y Zhang, Z Zhu, DA Grove, H Xue, J Xue, and WJ Weber. 2014. "Electronic Stopping Powers For Heavy Ions In SiC And SiO2." Journal of Applied Physics 115(4):Article No. 044903. doi:10.1063/1.4861642 Abstract Accurate information on electronic stopping power is fundamental for broad advances in materials science, electronic industry, space exploration, and sustainable energy technologies. In the case of slow heavy ions in light targets, current codes and models provide significantly inconsistent predictions, among which the Stopping and Range of Ions in Matter (SRIM) code is the most commonly used one. Experimental evidence, however, has demonstrated considerable errors in the predicted ion and damage profiles based on SRIM stopping powers. In this work, electronic stopping powers for Cl, Br, I, and Au ions are experimentally determined in two important functional materials, SiC and SiO2, based on a single ion technique, and new electronic stopping power values are derived over the energy regime from 0 to 15 MeV, where large deviations from the SRIM predictions are observed. As an experimental validation, Rutherford backscattering spectrometry (RBS) and secondary ion mass spectrometry (SIMS) are utilized to measure the depth profiles of implanted Au ions in SiC for energies from 700 keV to 15MeV. The measured ion distributions by both RBS and SIMS are considerably deeper than the SRIM predictions, but agree well with predictions based on our derived stopping powers.

Xiao HY, Y Zhang, and WJ Weber. 2013. "Stability and Migration of Charged Oxygen Interstitials in ThO2 and CeO2." Acta Materialia 61(20):7639-7645. doi:10.1016/j.actamat.2013.09.001 Abstract Density functional theory calculations have been carried out to study the stability and migration of charged oxygen interstitials in ThO2 and CeO2. The calculations demonstrate that the oxygen interstitial is likely to lose electrons under p-type conditions and gain electrons under n-type conditions. Neutral and singly positive O–O〈1 1 0〉 split interstitials, and doubly negative octahedral () oxygen interstitial are found to be the lowest-energy configurations within a certain Fermi energy range. In both oxides, the is the most mobile, and the migration energies of the split oxygen interstitials in ThO2 are lower than in CeO2, indicating higher oxygen interstitial mobility in ThO2 than in CeO2.

Xiao HY, WJ Weber, and Y Zhang. 2013. "First-principles study of the stability and migration of Kr, I and Xe in ZrO2." Journal of Nuclear Materials 446(1-3):172-177. doi:10.1016/j.jnucmat.2013.11.044 Abstract The stability and migration of Kr, I and Xe in bulk ZrO2 and on the ZrO2(111) surface have been studied by standard density functional theory (DFT) and the DFT-D2 method that corrects for the van der Waals interaction. Both methods show that Kr and Xe prefer to incorporate in the bulk phase rather than adsorb on the surface, and Xe is very mobile in the bulk state. For Kr and Xe adsorption on the surface, van der Waals interaction dominates, causing the weak interaction between the adsorbate and substrate. Iodine is found to have comparable stability in both phases and forms <I-O> bonds with strong covalency. It exhibits higher mobility on the surface than in the bulk ZrO2, and diffusion from bulk-like state to surface state is an exothermic process. The fission product behavior in ZrO2 is shown to be a complicated synergetic effect of fission product atomic size, electron negativity, occupation site and phase structure of the host.

Qiao L, HY Xiao, SM Heald, ME Bowden, T Varga, GJ Exarhos, MD Biegalski, IN Ivanov, WJ Weber, T Droubay, and SA Chambers. 2013. "The impact of crystal symmetry on the electronic structure and functional properties of complex lanthanum chromium oxides." Journal of Materials Chemistry C 1(30):4527-4535. doi:10.1039/C3TC30883A Abstract Complex oxides exhibit a wide range of crystal structures, chemical compositions and physical properties. The underlying drivers determining the complicated structure-composition-property phase diagrams are the relative positions and orbital overlaps between the metal cations and the oxygen anions. Here we report a combined experimental and theoretical investigation of the structure and bonding in a series of lanthanum chromium oxides prepared by reactive molecular beam epitaxy. Of particular interest is the charge state and local coordination of the Cr. We have stabilized LaCrO3, LaCrO4 and La2CrO6 films by controlling the three elemental fluxes during deposition, and have carried out x-ray diffraction, x-ray photoemission, and x-ray absorption spectroscopy, as well as first-principles calculations, to determine structure, charge state, chemical bonding, and electronic structure. Significant changes in bonding character and orbital interaction are revealed with decreasing ligand symmetry from octahedral to tetrahedral Cr coordination. Both LaCrO4 and LaCrO6 with tetrahedrally coordinated Cr show strong pre-edge features in the Cr K-edge near-edge structure whereas LaCrO3 with octahedrally coordinated Cr exhibits very weak pre-edge features. The origin of these pre-edge features is discussed based on various selection rules and ligand symmetry. We also demonstrate an increase in cation-anion orbital hybridization and a decrease in long-range ligand coupling induced by this symmetry reduction. These in turn result in dramatic modifications of the macroscopic optical and magnetic properties.

Qiao L, HY Xiao, HM Meyer, JN Sun, CM Rouleau, AA Puretzky, DB Geohegan, IN Ivanov, M Yoon, WJ Weber, and MD Biegalski. 2013. "Nature of the Band Gap and Origin of the Electro-/Photo-Activity of Co3O4." Journal of Materials Chemistry C 1(31):4628-4633. doi:10.1039/c3tc30861h Abstract Co3O4 exhibits intriguing physical, chemical and catalytic properties and has demonstrated great potential for next-generation renewable energy applications. These interesting properties and promising applications are underpinned by its electronic structure and optical properties, which are unfortunately poorly understood and the subject of considerable debate over many years. Here, we unveil a consistent electronic structural description of Co3O4 by synergetic infrared optical and in situ photoemission spectroscopy as well as standard density functional theory calculations. In contrast to previous assumptions, we demonstrate a much smaller fundamental band gap, which is directly related to its efficient electro-/photoactivity. The present results may help to advance the fundamental understanding and provide guidance for the use of oxidematerials in photocatalysis and solar applications.

Moll SJ, Y Zhang, Z Zhu, PD Edmondson, F Namavar, and WJ Weber. 2013. "Comparison Between Simulated And Experimental Au-ion Profiles Implanted in nanocrystalline ceria." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 307:93-97. doi:10.1016/j.nimb.2012.12.119 Abstract Radiation response of nanocrystalline ceria films deposited on a silicon substrate was investigated under a 3-MeV Au-ion irradiation at 300 K. A uniform grain growth cross the ceria films is observed and effective densification of the ceria thin films occurs during irradiation. The Au ion profiling was measured by secondary ion mass spectrometry (SIMS) and compared to the Au ion distribution predicted by the Stopping and Range of Ions in Solids (SRIM) code. It is observed that the Au-ion penetration depth is underestimated in comparison with the SIMS measurements. An overestimation of the electronic stopping power for heavy incident ions in the SRIM program may account for the discrepancies between the calculations and the SIMS experimental results. This work presents an approach to compensate the overestimation of the electronic stopping powers in the SRIM program by adjusting the nanocrystalline ceria target density to better predict the ion implantation profile.

Jin K, Y Zhang, H Xue, Z Zhu, and WJ Weber. 2013. "Ion Distribution And Electronic Stopping Power For Au ions In Silicon Carbide." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 307:65-70. doi:10.1016/j.nimb.2013.02.051 Abstract Accurate knowledge of ion distribution and electronic stopping power for heavy ions in light targets is highly desired due to the large errors in prediction by the widely used Stopping and Range of Ions in Matter (SRIM) code. In this study, Rutherford backscattering spectrometry (RBS)and secondary ion mass spectrometry (SIMS) are used as complementary techniques to determine the distribution of Au ions in SiC with energie sfrom 700 keV to 15 MeV. In addition, asingle ion technique with an improved data analysis procedure is applied to measure the electronic stopping power for Au ions in SiC with energies up to ~70 keV/nucleon. Large overestimation of the electronic stopping power is found by SRIM prediction in the low energy regime up to ~50 keV/nucleon. The stopping power data and the ion ranges are crosschecked with each other and a good agreement is achieved.

Edmondson PD, NP Young, CM Parish, SJ Moll, F Namavar, WJ Weber, and Y Zhang. 2013. "Ion-Beam-Induced Chemical Mixing at a Nanocrystalline CeO2–Si Interface." Journal of the American Ceramic Society 96(5):1666-1672. doi:10.1111/jace.12214 Abstract Thin films of nanocrystalline ceria deposited onto a silicon substrate have been irradiated with 3 MeV Au+ ions to a total dose of 34 displacements per atom to examine the film/substrate interfacial response upon displacement damage. Under irradiation, a band of contrast is observed to form that grows under further irradiation. Scanning and high-resolution transmission electron microscopy imaging and analysis suggest that this band of contrast is a cerium silicate phase with an approximate Ce:Si:O composition ratio of 1:1:3 in an amorphous nature. The slightly nonstoichiometric composition arises due to the loss of mobile oxygen within the cerium silicate phase under the current irradiation condition. This nonequilibrium phase is formed as a direct result of ion-beam-induced chemical mixing caused by ballistic collisions between the incoming ion and the lattice atoms. This may hold promise in ion beam engineering of cerium silicates for microelectronic applications e.g., the fabrication of blue LEDs.

Xue H, Y Zhang, Z Zhu, W Zhang, IT Bae, and WJ Weber. 2012. "Damage Profiles and Ion Distribution in Pt-irradiated SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 286:114-118. doi:10.1016/j.nimb.2012.02.014 Abstract Single crystalline 6H-SiC samples were irradiated at 150 K with 2 MeV Pt ions. The local volume swelling was determined by electron energy loss spectroscopy (EELS), and a nearly sigmoidal dependence on irradiation dose is observed. The disorder profiles and ion distribution were determined by Rutherford backscattering spectrometry (RBS), transmission electron microscopy, and secondary ion mass spectrometry. Since the volume swelling reaches 12% over the damage region at high ion fluence, the effect of lattice expansion is considered and corrected for in the analysis of RBS spectra to obtain depth profiles. Projectile and damage profiles are estimated by SRIM (Stopping and Range of Ions in Matter).When compared with the measured profiles, the SRIM code predictions of ion distribution and the damage profiles are underestimated due to significant overestimation of the electronic stopping power for the slow heavy Pt ions. By utilizing the reciprocity method, which is based on the invariance of the inelastic energy loss in ion-solid collisions against interchange of projectile and target atom, a much lower electronic stopping power is deduced. A simple approach, based on reducing the density of SiC target in SRIM simulation, is proposed to compensate the overestimated SRIM electronic stopping power values, which results in improved agreement between predicted and measured damage profiles and ion ranges.

Xiao HY, Y Zhang, and WJ Weber. 2012. "Thermodynamic properties of CexTh1-xO2 solid solution from first-principles calculations." Acta Materialia 61(2):467-476. doi:10.1016/j.actamat.2012.09.050 Abstract A systematic study based on first-principles calculations along with a quasi-harmonic approximation has been conducted to calculate the thermodynamic properties of the CexTh1xO2 solid solution. The predicted density, thermal expansion coefficients, heat capacity and thermal conductivity for the CexTh1xO2 solid solution all agree well with the available experimental data. The thermal expansion coefficient for ThO2 increases with CeO2 substitution, and complete substitution shows the highest expansion coefficient. On the other hand, the mixed CexTh1xO2 (0 < x < 1) solid solution generally exhibits lower heat capacity and thermal conductivity than the ThO2 and CeO2 end members. Our calculations indicate a strong effect of Ce concentration on the thermodynamic properties of the CexTh1xO2 solid solution.

Xiao HY, Y Zhang, LL Snead, V Shutthanandan, H Xue, and WJ Weber. 2012. "Near-surface and bulk behavior of Ag in SiC." Journal of Nuclear Materials 420(1-3):123-130. doi:10.1016/j.jnucmat.2011.09.028 Abstract The diffusive release of fission products, such as Ag, from TRISO particles at high temperatures has raised concerns regarding safe and economic operation of advanced nuclear reactors. Understanding the mechanisms of Ag diffusion is thus of crucial importance for effective retention of fission products. Two mechanisms, i.e., grain boundary diffusion and vapor or surface diffusion through macroscopic structures such as nano-pores or nano-cracks, remain in debate. In the present work, an integrated computational and experimental study of the nearsurface and bulk behavior of Ag in silicon carbide (SiC) has been carried out. The ab initio calculations show that Ag prefers to adsorb on the SiC surface rather than in the bulk, and the mobility of Ag on the surface is high. The energy barrier for Ag desorption from the surface is calculated to be 0.85~1.68 eV, and Ag migration into bulk SiC through equilibrium diffusion process is not favorable. Experimentally, Ag ions are implanted into SiC to produce Ag profiles buried in the bulk and peaked at the surface. High-temperature annealing leads to Ag release from the surface region instead of diffusion into the interior of SiC. It is suggested that surface diffusion through mechanical structural imperfection, such as vapor transport through cracks in 2 SiC coatings, may be a dominating mechanism accounting for Ag release from the SiC in the nuclear reactor.

Xiao HY, Y Zhang, and WJ Weber. 2012. "Impact of Point Defects on Electronic Structure in Y₂Ti₂O₇." RSC Advances 2(18):7235-7240. doi:10.1039/C2RA21099A Abstract With many technologies and applications downscaling to nanometer dimensions, the influence of single point defects on electronic structure has shown an increasingly profound impact on optical and electrical properties, and advancing fundamental understanding is critical to defect engineering and control of materials properties. In the present study, first-principles calculations based on density functional theory (DFT) are carried out to study the effects of Ti point defects on the electronic structure of Y₂Ti₂O₇. In the literature, it has been demonstrated that conventional DFT tends to produce delocalized holes and electrons in defective oxide materials due to insufficient cancellation of the self-interaction energy and underestimation of the band gap, which results in an incorrect description of the electronic structure of the system. In an effort to better understand the accuracy of DFT in describing the behavior of Y₂Ti₂O₇ with point defects, the calculated results obtained from DFT and DFT+U methods are compared, including the geometrical distortion, the localization of the defect states and the position of the defect levels in the band gap. Using DFT, distorted geometries around the Ti vacancy and interstitial are found, along with localized oxygen holes and Ti electrons, both of which compare well with the DFT+U results, suggesting that the conventional DFT can be used to describe the localization of the Ti defects in Y₂Ti₂O₇. One major difference in the DFT and DFT+U calculations is the energy position of the defect levels, for which DFT+U results in the states positioned deep in the band gap. Since the DFT+U method suffers from the dependence of the results on the empirical parameter U and no experimental results on the energy position of the defect states are available to tune this U value, care must be taken in applying DFT+U to electronic structure calculations of Y₂Ti₂O₇ with point defects. Based on the DFT method, the most preferred charge state is determined by the formation energies for charged point defects. Moving the Fermi level across the band gap has slight effects on the charge state, and the Ti vacancy and interstitial are found to be in 24 and +4 charge states, respectively.

Xiao HY, Y Zhang, and WJ Weber. 2012. "Ab Initio Molecular Dynamics Simulations of Low-Energy Recoil Events in ThO2, CeO2, and ZrO2." Physical Review. B, Condensed Matter and Materials Physics 86(5):054109. doi:10.1103/PhysRevB.86.054109 Abstract Ab initio molecular dynamics simulations of low-energy recoil events in ThO2, CeO2, and ZrO2 have been carried out to determine the threshold displacement energies, resulting defect configurations, dynamics of defect generation, and role of charge transfer during the process. The results reveal that, in most cases, these fluorite structure oxides exhibit a similar response to low-energy recoils. A variety of different defect configurations are created, consisting mainly of vacancies and interstitials. Charge transfer occurs during the dynamic displacement process. Local charge redistribution leads to cation and O vacancies being negatively and positively charged, respectively. Likewise, due to charge redistribution, the cation and O interstitials are less positively and negatively charged, respectively, than the ions on lattice sites in perfect MO2.

Weber WJ, Y Zhang, and LM Wang. 2012. "Review of Dynamic Recovery Effects on Ion Irradiation Damage in Ionic-Covalent Materials." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 277:1-5. doi:10.1016/j.nimb.2011.12.043 Abstract Single crystalline samples of highly ionic Ca2La8(SiO4)6O2 and covalent 6H–SiC have been irradiated with different ions/energies to study the effects of dose, temperature, damage-energy density, and in-cascade ionization rate on the dynamics of irradiation-induced amorphization. Above temperatures of 100–150 K, the dose for complete amorphization, D, increases with temperature in a single stage and exhibits a strong dependence on the ratio of in-cascade recovery to displacement cross sections, σr/σd. A fit of a dynamic model for amorphization to these data indicates that irradiation-induced dynamicrecovery occurs with an activation energy of 0.15 ± 0.02 and 0.12 ± 0.01 eV for Ca2La8(SiO4)6O2 and 6H–SiC, respectively. Analysis of these data reveals that ionization processes are the dominant contributor to in-cascade recovery in Ca2La8(SiO4)6O2; while in 6H–SiC, ionization processes are less dominant.

Wang XJ, HY Xiao, X Zu, Y Zhang, and WJ Weber. 2012. "Ab initio molecular dynamics simulations of ion–solid interactions in Gd2Zr2O7 and Gd2Ti2O7." Journal of Materials Chemistry C 1(8):1665-1673. doi:10.1039/C2TC00192F Abstract The development of the ab initio molecular dynamics (AIMD) method has made it a powerful tool in describing ion–solid interactions in materials, with the determination of threshold displacement energies with ab initio accuracy, and prediction of a new mechanism for defect generation and new defective states that are different from classical molecular dynamics (MD) simulations. In the present work, this method is employed to study the low energy recoil events in Gd2Zr2O7 and Gd2Ti2O7. The weighted average threshold displacement energies in Gd2Zr2O7 are determined to be 38.8 eV for Gd, 41.4 eV for Zr, 18.6 eV for O48f, and 15.6 eV for O8b, which are smaller than the respective values of 41.8, >53.8, 22.6 and 16.2 eV in Gd2Ti2O7. It reveals that all the ions in Gd2Zr2O7 are more easily displaced than those in Gd2Ti2O7, and anion order–disorder is more likely to be involved in the displacement events than cation disordering. The average charge transfer from the primary knock-on atom to its neighbors is estimated to be [similar]0.15, [similar]0.11 to 0.27 and [similar]0.1 to 0.13 |e| for Gd, Zr (or Ti), and O, respectively. Neglecting the charge transfer in the interatomic potentials may result in the larger threshold displacement energies in classical MD.

Wang XJ, HY Xiao, X Zu, and WJ Weber. 2012. "A DFT + U study of cerium solubility in La₂Zr₂O₇." Journal of Nuclear Materials 424(1-3):69-74. doi:10.1016/j.jnucmat.2012.02.008 Abstract Density functional theory plus Hubbard U correction is employed to study the solubility of cerium in La₂Zr₂O₇. The results show that La₂Zr₂O₇ and Ce₂Zr₂O₇ form a solid solution over the whole range of cerium content. The solubility of Ce in La₂Zr₂O₇ can be partially attributed to the similar ionic radii of La³+ and Ce³+. Electronic structures of the La₂₋yCeyZr₂O₇ solid solution have been analyzed. The Ce 4ƒ states are found to be partially occupied, and Ce in the La₂₋yCeyZr₂O₇ solid solution exhibits a reduced charge state.

Lan C, J Xue, Y Zhang, JR Morris, Z Zhu, Y Gao, Y Wang, S Yan, and WJ Weber. 2012. "Molecular dynamics simulations of ion range profiles for heavy ions in light targets." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 286:45-50. doi:10.1016/j.nimb.2012.01.020 Abstract The determination of stopping powers for slow heavy ions in targets containing light elements is important to accurately describe ion-solid interactions, evaluate ion irradiation effects and predict ion ranges for device fabrication and nuclear applications. Recently, discrepancies of up to 40% between the experimental results and SRIM (Stopping and Range of Ions in Matter) predictions of ion ranges for heavy ions with medium and low energies (<25 keV/nucleon) in light elemental targets have been reported. The longer experimental ion ranges indicate that the stopping powers used in the SRIM code are overestimated. Here, a molecular dynamics simulation scheme is developed to calculate the ion ranges of heavy ions in light elemental targets. Electronic stopping powers generated from both a reciprocity approach and the SRIM code are used to investigate the influence of electronic stopping on ion range profiles. The ion range profiles for Au and Pb ions in SiC and Er ions in Si, with energies between 20 and 5250 keV, are simulated. The simulation results show that the depth profiles of implanted ions are deeper and in better agreement with the experiments when using the electronic stopping power values derived from the reciprocity approach. These results indicate that the origin of the discrepancy in ion ranges between experimental results and SRIM predictions in the low energy region may be an overestimation of the electronic stopping powers used in SRIM.

Edmondson PD, Y Zhang, SJ Moll, F Namavar, and WJ Weber. 2012. "Irradiation Effects on Microstructure Change in Nanocrystalline Ceria – Phase, lattice Stress, Grain Size and Boundaries." Acta Materialia 60(15):5408–5416. doi:10.1016/j.actamat.2012.07.010 Abstract With a wide variety of applications in numerous industries, ranging from biomedical to nuclear, ceramics such as ceria are key engineering materials. It is possible to significantly alter the materials functionality and therefore its applications by reducing the grain size to the nanometer size regime, at which point the unique varieties of grain boundaries and associated interfaces begin to dominate the material properties. Nanocrystalline films of cubic ceria deposited onto Si substrates have been irradiated with 3 MeV Au+ ions at temperatures of 300 and 400 K to evaluate their response to irradiation. It was observed that the films remained phase stable. Following a slight stress relief stage at low damage levels, the overall lattice is extremely stable up to high irradiation dose of ~34 displacements per atom. The grains were also observed to undergo a temperature-dependent grain growth process upon ion irradiation. This is attributed to a defect-driven mechanism in which the diffusion of defects from the collision cascade is critical. Formation of dislocations that terminate and stabilize at symmetric grain boundaries may be the limiting factor in the grain growth and overall energy reduction of the system. Utilizing ion modification, possible improvement of the adhesion of thin films and reduction of the probability of detrimental effects of stress-induced problems are discussed.

Edmondson PD, Y Zhang, SJ Moll, T Varga, F Namavar, and WJ Weber. 2012. "Anomalous grain growth in the surface region of a nanocrystalline CeO2 film under low-temperature heavy ion irradiation." Physical Review. B, Condensed Matter and Materials Physics 85(21):Article No. 214113. doi:10.1103/PhysRevB.85.214113 Abstract Grain growth and phase stability of nanocrystalline ceria are investigated under ion irradiation at different temperatures. Irradiations at temperatures of 300 and 400 K result in uniform grain growth throughout the film. Anomalous grain growth is observed in thin films of nanocrystalline ceria under 3 MeV Au+ irradiation at 160 K. At this low temperature, significant grain growth is observed within 100 nm from the surface, no obvious growth is detected in the rest of the films. While the grain growth is attributed to a defect-stimulated mechanism at room temperature and above, a defect diffusion-limited mechanism is significant at low temperature with the primary defect responsible being the oxygen vacancy. The nanocrystalline grains remain in the cubic phase regardless of defect kinetics.

Du J, R Devanathan, LR Corrales, and WJ Weber. 2012. "First-principles calculations of the electronic structure, phase transition and properties of ZrSiO4 polymorphs." Computational and Theoretical Chemistry 987(1):62–70. doi:10.1016/j.comptc.2011.03.033 Abstract First-principles periodic density functional theory (DFT) calculations have been performed to understand the electronic structure, chemical bonding, phase transition, and physical properties of the mineral zircon (in the chemical composition of ZrSiO4) and its high pressure phase reidite. Temperature effect on phase transition and thermal–mechanical properties such as heat capacity and bulk modulus have been studied by combining the equation of states obtained from DFT calculations with the quasi-harmonic Debye model to take into account the entropy contribution to free energy. Local density approximation (LDA) and generalized gradient approximation (GGA) DFT functionals have been systematically compared in predicting the structure and property of this material. It is found that the LDA functional provides a better description of the equilibrium structure and bulk modulus, while GGA predicts a transition pressure closer to experimental values. Both functionals correctly predict the relative stability of the two phases, with GGA giving slightly larger energy differences. The calculated band structures show that both zircon and reidite have indirect bandgaps and the reidite phase has a narrower bandgap than the zircon phase. The electronic density of states and atomic charges analyses show that bonding in the high-pressure reidite phase has a stronger covalent character.

Chang Y, Y Zhang, Z Zhu, PD Edmondson, and WJ Weber. 2012. "MeV Au Ion Irradiation in Silicon and Nanocrystalline Zirconia Film Deposited on Silicon Substrate." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 286:173-179. doi:10.1016/j.nimb.2012.01.017 Abstract Nanocrystalline zirconia (ZrO2) film with thickness of 305 nm deposited on a silicon substrate was irradiated with 2 MeV Au ions to different fluences at different temperatures. The implanted ion profiles were measured by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and simulated using the stopping and range of ions inmatter (SRIM) code, respectively. The experimental results show that a large fraction of the incident Au ions penetrates through the ZrO2 film and are deposited into the Si substrate. At the interface of ZrO2 and Si, a sudden decrease of Au concentration is observed due to the much larger scattering cross section of Au in ZrO2 than in Si. The depth profile of the Au ions is measured in both the ZrO2 films and the Si substrates, and the results show that the Au distribution profiles do not exhibit a dependence on irradiation temperature. The local Au concentration increases proportionally with the irradiation fluence, suggesting that no thermal or irradiation-induced redistribution of the implanted Au ions. However, the Au concentration in the ZrO2 films, as determined by SIMS, is considerably lower than that predicted by the SRIM results, and the penetration depth from the SIMS measurements is much deeper than that from the SRIM predictions. These observations can be explained by an overestimation of the electronic stopping power, used in the SRIM program, for heavy incident ions in light targets. Over-estimation of the heavy-ion electronic stopping power may lead to errors in local dose calculation and underestimation of the projected range of slow heavy ions in targets that contain light elements. A quick estimate based on a reduced target density may be used to compensate the overestimation of the electronic stopping power in the SRIM program to provide better ion profile prediction.

Zhang Y, PD Edmondson, T Varga, SJ Moll, F Namavar, C Lan, and WJ Weber. 2011. "Structural Modification of Nanocrystalline Ceria by Ion Beams." Physical Chemistry Chemical Physics. PCCP 13(25):11946-11950. doi:10.1039/c1cp21335k Abstract Using energetic ions, we have demonstrated effective modification of grain size in nanocrystalline ceria in the critical region for controlling exceptional size-dependent electronicionic conductivity. The grain size increases and follows an exponential law as a function of ion fluence that increases with temperature, while the cubic phase is stable under the irradiation. The unique self-healing response of radiation damage at grain boundaries is utilized to control the grain size at the nanoscale.

Xiao HY, and WJ Weber. 2011. "Pressure Induced Structural Transformation in Gd2Ti2O7 and Gd2Zr2O7." Journal of Physics. Condensed Matter 23(3):Article No. 035501. doi:10.1088/0953-8984/23/3/035501 Abstract Ab initio total energy calculations have been performed to study the phase stability of Gd2Ti2O7 and Gd2Zr2O7 pyrochlores over the pressure range from 0 to 60 GPa. Both compounds are unstable under pressure, and phase transformations to the defect-cotunnite structure are predicted. The phase transformation pressure of 43.6 GPa for Gd2Ti2O7 is considerably larger than the value of 13 GPa for Gd2Zr2O7, in good agreement with experiments. The decreased structural stability of Gd2Zr2O7 under pressure, relative to Gd2Ti2O7, is a consequence of the lower compressibility of the langZr–Orang bond and the higher compressibility of the langGd–Orang bond. In addition, the Gd 4f electrons are found to have only a small effect in determining the pressure induced phase transformation.

Xiao HY, and WJ Weber. 2011. "Oxygen Vacancy Formation and Migration in CexTh₁_xO₂ Solid Solution." Journal of Physical Chemistry B 115(20):6524-6533. doi:10.1021/jp202016s Abstract A local-density approximation with the Hubbard U correction (LDA+U) method has been employed to investigate oxygen vacancy formation and migration in CexTh₁_xO₂. The addition of CeO₂ into ThO₂ significantly decreases the oxygen vacancy formation and migration energies. ThO₂ containing 50% CeO₂ exhibits the lowest calculated formation energy, 3.7 eV, and the lowest calculated migration energy, 0.2 eV, occurs for a CeO₂ content of 75%, suggesting that introducing CeO2 into ThO2 promotes the formation of mobile oxygen vacancies. If the ceria content is less than about 35%, the reduced CexTh₁_xO₂ becomes antiferromagnetic (AFM), whereas the ferromagnetic (FM) state dominates for x values above about 35%, which may allow the tailoring of magnetic properties by varying the CeO₂ content.

Weber WJ, Y Zhang, HY Xiao, and LM Wang. 2011. "Dynamic Recovery in Silicate-Apatite Structures Under Irradiation and Implications for Long-Term Immobilization of Actinides." RSC Advances 2:595-604. doi:10.1039/C1RA00870F Abstract The irradiation responses of Ca₂La₈(SiO₄)₆O₂ and Sr₂Nd₈ (SiO₄)₆O₂ with the apatite structure are investigated to predict their long-term behaviour as host phases for immobilization of actinide elements from the nuclear fuel cycle. Different ions and energies are used to study the effects of dose, temperature, atomic displacement rate and ionization rate on irradiation-induced amorphization and recrystallization. The dose for amorphization increases with temperature in two stages, below and above 150 K. In the high temperature stage relevant to actinide immobilization, the increase of amorphization dose with temperature exhibits a strong dependence on the ratio of ionization rate to displacement rate for the different ions. Data analysis using a dynamic model for amorphization reveals that ionization-induced processes, with activation energy of 0.15 ± 0.02 eV, dominate dynamic recovery for ions from Ne through Xe. For heavier Au ions or for alpha-recoil nuclei emitted in alpha decay of actinides, ionization becomes less dominant and dynamic recovery is controlled primarily by thermally-driven processes. In post-irradiation annealing studies of amorphous samples, epitaxial thermal recrystallization is observed at 1123 K, and irradiation-enhanced nucleation of nanocrystallites is observed under irradiation with heavier ions. The recrystallization temperature under irradiation decreases with increasing ion mass to a value of ~ 823 K, which also defines the thermally-driven critical temperature for amorphization under irradiation with heavy ions. Some partial recovery due to alpha particle irradiation at 300 K is observed that suggests a self-healing mechanism in apatite phases containing actinides. Based on the results and dynamic model, the temperature and time dependences of amorphization in silicate-apatite host phases for actinide immobilization are predicted.

Toulemonde M, WJ Weber, G Li, V Shutthanandan, P Kluth, T Yang, Y Wang, and Y Zhang. 2011. "Synergy of Nuclear and Electronic Energy Losses in Ion-irradiation Processes: the Case of Vitreous Silicon Dioxide." Physical Review. B, Condensed Matter 83(5):054106- 054106-9. doi:10.1103/PhysRevB.83.054106 Abstract Structural modification of vitreous SiO2 by Au ion irradiation is investigated over an energy regime (~ 0.3-15 MeV) where the decrease of the nuclear energy loss with increasing energy is compensated by the increase of the electronic energy loss, leading to a nearly constant total energy loss of ~ 4 keV/nm. The radii of damaged zones resulting from the ion impact, deduced from changes in infrared bands as a function of ion fluence, decrease from 4.9 nm at 0.3 MeV to 2.5 and 2.6 nm at 9.8 MeV and 14.8 MeV, respectively. Based on previous data where vitreous SiO2 was irradiated with much higher energy Au ions, the damage zone radius increases from 2.4 nm at 22.7 MeV to 5.4 nm at 168 MeV, and a U-shaped dependence on energy within experimental uncertainty is observed in the energy region from 0.3 MeV to 168 MeV. The current results demonstrate that large damage radii at low and high ion energy can be explained by the elastic or inelastic thermal spike model, respectively. In the transition regime where both nuclear and electronic energy loss are significant, an unified thermal spike model consisting a coherent synergy of the elastic collision spike model with the inelastic thermal spike model is suggested to interpret and describe the radius evolution from the nuclear to the electronic energy regime.

Griscom DL, and WJ Weber. 2011. "Electron Spin Resonance Study of Fe3+ and Mn2+ Ions in 17-Year-Old Nuclear-Waste-Glass Simulants Containing PuO2 with Different Degrees of 238Pu Substitution." Journal of Non-crystalline Solids 357(5):1437-1451. doi:10.1016/j.jnoncrysol.2010.11.017 Abstract Three samples of a model nuclear waste glass, DRG-P1, P2, and P3, were prepared at PNNL in 1982 with identical chemical compositions but were respectively batched with 0.0, 0.1, and 0.9 wt% of 238PuO2 (half life 87.8 years) partially replacing the 1.0 wt% 239PuO2 present in DRG-P1. In 1999, samples of these glasses were sent to the Naval Research Laboratory for electron spin resonance (ESR) measurements. No radiation-induced point defects were observed. Profound alpha decay-induced changes in the ESR spectra of the batched iron-group ions were found. The spectra recorded for DRG-P1 were shown by absolute spin counts to have ESR intensities equivalent to ~85% of the sum of the batched 8.28 mole% Fe3+ and 2.79 mole% Mn2+, assuming all of those ions to behave as paramagnetic S=5/2 states at room temperature. Separate experiments and calculations ruled out the possibility of precipitated magnetite-like precipitates comprising even so much as 0.01% of the total iron. A relatively weak ESR spectral feature observed at g=4.3 is the known signature of dilute Fe3+ in glasses. However, the strongest ESR signal was found to be characterized by a first-derivative zero crossing at g=2.06 and a peak-to-peak derivative linewidth of ~150 mT, which is virtually invariant in shape with both measurement temperature and alpha-decay dose. It was discovered that these broad line shapes could be accurately simulated as weighted sums of Lorentzian shape functions of differing widths but having the same g value. The absence of any measurable anisotropy in the broad line, coupled with the temperature invariance of its width, imply the existence of extremely strong exchange interactions within clusters of Fe3+, Fe2+, Mn2+, and Ni2+ ions characterized by extremely short-range magnetic order. The result is a speromagnetic system rather than exhibiting a distinct Néel temperature. The most evident ESR effect of 17 years of 238Pu decay is the (irreversible) lowering of the intensity of the "broad line" in rough proportion to the amount of 238Pu in the sample, with concomitant increases in the amplitude of the g=4.3 feature. It was additionally observed that cooling these glasses to successively lower temperatures gives rise to reversible lowering of the broad-line intensity and increasing of the strength of the g=4.3 feature when compared with theoretical expectation for non-interacting paramagnets. The truly remarkable observation that the broad lines could be simulated as weighted sums of pure Lorentzian functions of differing widths fortuitously opened the way for high precision measurements of the ESR intensities of experimental spectra that are far broader than the magnetic field range of the available laboratory electromagnets. The areas under the simulated absorption curves fitted to the experimental spectra in the manner described provided an empirical measure of the degrees to which the present model nuclear waste glasses had been affected by alpha-decay self irradiation. Specifically, the broad-line ESR integrated-intensity data as a function of 238Pu alphadecay dose (proportional to the 238Pu doping level in these fixed-time experiments) proved to be accurately fitted by a simple saturating exponential function asymptotic to zero for infinite-time self irradiation. This result promises a precise means of extrapolating thousands of years into the future the process of "super-vitrification" that results from the creation and rapid quenching of thermal spikes due to alpha decay in glasses immobilizing 239Pu and other actinide elements. In addition, because the ESR spectra of several very different candidate high-level nuclear waste (HLW) glass compositions containing even higher amounts of Fe2O3 are also shown here to be decomposable into sums of pure Lorentzians, the analytical method we describe should be applicable to these and many other HLW glasses containing both iron-group oxides and radionuclides.

Gao F, YL Xie, SN Kerisit, LW Campbell, and WJ Weber. 2011. "Yield, variance and spatial distribution of electron–hole pairs in CsI." Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment 652(1):564-567. doi:10.1016/j.nima.2010.08.063 Abstract A Monte Carlo (MC) method previously developed has been applied to simulate the interaction of photons, with energies ranging from 50 eV to ~ 1 MeV, with CsI and the subsequent electron cascades. The MC model has been employed to compute nano-scale spatial distributions of electron-hole pairs and important intrinsic properties, including W, the mean energy per electron-hole pair, and the Fano factor, F. W exhibits discontinuities at the shell edges that follow the photoionization cross sections and decreases with increasing photon energy (from ~19 to 15 eV), with an asymptotic value of 15.2 eV at high energy. This decrease may contribute the initial rise in relative light yield with incident energy observed experimentally for CsI, thus suggesting that nonlinearity may be associated with intrinsic properties of the material at low energies. F is calculated to increase with increasing energy and has an asymptotic value of 0.28. A significant number of electron-hole pairs are produced through the different ionization channels of core shells and corresponding relaxation processes, which may explain why F is larger for CsI than for Si or Ge. Finally, the calculated spatial distributions show that the electron-hole pairs are primarily distributed along fast electron tracks. These spatial distributions constitute important input for large-scale simulations of electron-hole pair transport.

Gao F, HY Xiao, and WJ Weber. 2011. "Ab initio molecular dynamics simulations of low energy recoil events in ceramics ." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 269(14):1693-1697. doi:10.1016/j.nimb.2011.01.131 Abstract The recent progress in the use of large-scale ab initio molecular dynamics (AIMD) to investigate low energy recoil events and determine threshold displacement energies, Ed, in ceramics is reviewed. In general, Ed shows a significant dependence on recoil direction and atom. In 3C-SiC, the minimum Ed for both C and Si atoms is found along the <100> direction, with a value of 20 and 49 eV, respectively. The results demonstrate that significant charge transfer occurs during the dynamics process, and defects can enhance charge transfer to surrounding atoms, which provides important insights into the formation of charged defects. It is found that the C vacancy is a positively charged defect, whereas the Si vacancy is in its neutral state. The minimum Ed in GaN is determined to be 17 and 39 eV for N and Ga atoms, respectively, both along the direction. The average Ed for N atoms (32.4 eV) is smaller than that for Ga atoms (73.2 eV). It is of interest to note that the N defects created along different crystallographic directions have a similar configuration (a N-N dumbbell configuration), but various configurations for Ga defects are formed. In Y2Ti2O7 prochlore, the minimum Ed for Y atoms is determined to be 27 eV for a recoil along the <100> direction, 31.5 eV for Ti atoms along the <100> direction, 14.5 eV for O48f atoms along the <110> direction and 13 eV for O8b atoms along the <111> direction. The average Ed values determined are 32.7, 34.2, 14.2 and 16.1 eV for yttrium, titanium, O48f and O8b atoms, respectively. Cation interstitials at vacant 8a sites, which are generally occupied by oxygen anions, and at the bridge sites between two neighboring cations along the <010> direction are observed after low energy recoil events.

Edmondson PD, WJ Weber, F Namavar, and Y Zhang. 2011. "Lattice Distortions and Oxygen Vacancies Produced in Au+-Irradiated Nanocrystalline Cubic Zirconia." Scripta Materialia 65(8):675-678. doi:10.1016/j.scriptamat.2011.07.010 Abstract The oxygen ion conductivity, attributed to an oxygen vacancy mechanism, of yttria-stabilized zirconia membranes used in solid oxide fuel cells is restricted due to trapping limitations. In this work, a high concentration of oxygen vacancies has been deliberately introduced into nanocrystalline stabilizer-free zirconia through ion-irradiation. Oxygen vacancies with different charge states can be produced by varying irradiation temperatures. Due to the reduced trapping sites and high oxygen vacancy concentration, this work suggests that the efficiency of solid oxide fuel cells can be improved.

Edmondson PD, Y Zhang, F Namavar, CM Wang, Z Zhu, and WJ Weber. 2011. "Defect- and Strain-enhanced Cavity Formation and Au Precipitation at nano-crystalline ZrO2/SiO2/Si Interfaces ." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 269(2):126-132. doi:10.1016/j.nimb.2010.10.014 Abstract Defect- and strain-enhanced cavity formation and Au precipitation at the interfaces of a nanocrystalline ZrO2/SiO2/Si multilayer structure resulting from 2 MeV Au+ irradiation at temperatures of 160 and 400 K have been studied. Under irradiation, loss of oxygen is observed, and the nanocrystalline grains in the ZrO2 layer increase in size. In addition, small cavities are observed at the ZrO2/SiO2 interface with the morphology of the cavities being dependent on the damage state of the underlying Si lattice. Elongated cavities are formed when crystallinity is still retained in the heavily-damaged Si substrate; however, the morphology of the cavities becomes spherical when the substrate is amorphized. With further irradiation, the cavities appear to become stabilized and begin to act as gettering sites for the Au. As the cavities become fully saturated with Au, the ZrO2/SiO2 interface then acts as a gettering site for the Au. Analysis of the results suggests that oxygen diffusion along the grain boundaries contributes to the growth of cavities and that oxygen within the cavities may affect the gettering of Au. Mechanisms of defect- and strain-enhanced cavity formation and Au precipitation at the interfaces will be discussed with focus on oxygen diffusion and vacancy accumulation, the role of the lattice strain on the morphology of the cavities, and the effect of the binding free energy of the cavities on the Au precipitation.

Costantini JM, F Beuneu, SE Morrison-Smith, R Devanathan, and WJ Weber. 2011. "Paramagnetic defects in electron-irradiated yttria-stabilized zirconia: Effect of yttria content." Journal of Applied Physics 110(12):Article No. 123506. doi:10.1063/1.3666062 Abstract We have studied the effect of the yttria content on the paramagnetic centres in electron-irradiated yttria-stabilized zirconia (ZrO2: Y3+) or YSZ. Single crystals with 9.5 mol% or 18 mol% Y2O3 were irradiated with electrons of 1.0, 1.5, 2.0 and 2.5 MeV. The paramagnetic centre production was studied by X-band EPR spectroscopy. The same paramagnetic centres were identified for both chemical compositions, namely two electron centres, i.e. i) F+-type centres (involving singly ionized oxygen vacancies), and ii) so-called T centres (Zr3+ in a trigonal symmetry site), and hole-centres. A strong effect is observed on the production of hole-centres which are strongly enhanced when doubling the yttria content. However, no striking effect is found on the electron centres (except the enhancement of an extra line associated to the F+-type centres). It is concluded that hole-centres are produced by inelastic interactions, whereas F+-type centres are produced by elastic collisions with no effect of the yttria content on the defect production rate. In the latter case, the threshold displacement energy (Ed) of oxygen is estimated from the electron-energy dependence of the F+-type centre production rate, with no significant effect of the yttria content on Ed. An Ed value larger than 120 eV is found. Accordingly, classical molecular dynamics (MD) simulations with a Buckingham-type potential show that Ed values for Y and O are likely to be in excess of 200 eV. It is concluded that F+-type centres might be actually oxygen divacancies (F2+-type centres). Due to the difficulty in displacing O or Y atoms, the radiation-induced defects may alternatively be a result of Zr atom displacements for Ed = 80 ± 1 eV with subsequent defect re-arrangement.

Zhang Y, and WJ Weber. 2010. "Stopping of Ions in Nanomaterials." In Ion Beams in Nanoscience and Technology, ed. R. Hellborg, H. J. Whitlow, and Y. Zhang, pp. 87-93. Springer, Berlin, Germany. Abstract The stopping of ions in solids is due to the energy loss as a result of the resistance to ion passage of the electronic and ionic nuclei in the material. When an ion penetrates a solid, it experiences a number of collisions. Energetic charged particles interact with both electrons and atoms in materials. Kinetic energy transfers to atoms can result in displacement of atoms from their original sites; thereby forming atomic-scale defects in the structure. Energy transfers to the target electrons (either bound or free) produces electron-hole pairs that can result in charging of pre-existing defects, localized electronic excitations, rupture of covalent and ionic bonds, enhanced defect and atomic diffusion, increased free energy, changes in phase transformation dynamics, as well as formation of atomic-scale defects. Such atomic collisions and ionization processes can modify the physical and chemical behavior of nanomaterials. This box will discuss irradiation-induced defect, address nanostructure engineering and radiation effects in nanomaterials, as well as the scientific challenges of ion-solid interactions.

Zhang J, M Lang, RC Ewing, R Devanathan, WJ Weber, and M Toulemonde. 2010. "Nanoscale Phase Transitions under Extreme Conditions within an Ion Track." Journal of Materials Research 25(7):1344-1351. Abstract The dynamics of track development due to the passage of relativistic heavy ions through solids is a long-standing issue relevant to nuclear materials, age-dating of minerals, space exploration, and nanoscale fabrication of novel devices. We have integrated experimental and simulation approaches to investigate nanoscale phase transitions under the extreme conditions created within single tracks of relativistic ions in Gd2O3(TiO2)x and Gd2Zr2-xTixO7. Track size and internal structure depend on energy-density deposition, irradiation temperature, and material composition. Molecular dynamics methods based on the thermal spike model have simulated, for the first time, the internal structure of individual tracks, consistent with experimental observations. Individual ion tracks have nanoscale core-shell structures that provide a unique record of the phase transition pathways under extreme conditions.

Zhang Y, W Jiang, CM Wang, F Namavar, PD Edmondson, Z Zhu, F Gao, J Lian, and WJ Weber. 2010. "Grain growth and phase stability of nanocrystalline cubic zirconia under ion irradiation." Physical Review. B, Condensed Matter 82(18):184105:1-7. doi:10.1103/PhysRevB.82.184105 Abstract Grain growth, oxygen stoichiometry and phase stability of nanostructurally-stabilized zirconia (NSZ) in pure cubic phase are investigated under 2 MeV Au ion bombardment at 160 and 400 K to doses up to 35 displacements per atom (dpa). The NSZ films are produced by ion-beam-assisted deposition technique at room temperature with an average grain size of 7.7 nm. The grain size increases with dose, and follows a power law (n=6) to a saturation value of ~30 nm that decreases with temperature. Slower grain growth is observed under 400 K irradiations, as compared to 160 K irradiations, indicating that thermal grain growth is not activated and defect-stimulated grain growth is the dominating mechanism. While cubic phase is perfectly retained and no new phases are identified after the high-dose irradiations, reduction of oxygen in the irradiated NSZ films is detected. The ratio of O to Zr decreases from ~2.0 for the as-deposited films to ~1.65 after irradiation to ~35 dpa. Significant increase of oxygen vacancies in nanocrystalline zirconia suggests substantially enhanced oxygen diffusion under ion irradiation, a materials behavior far from equilibrium. The oxygen deficiency may be essential in stabilizing cubic phase to larger grain sizes.

Zhang Y, J Jagielski, IT Bae, X Xiang, L Thome, G Balakrishnan, DM Paul, and WJ Weber. 2010. "Damage evolution in Au-implanted Ho2Ti2O7 titanate pyrochlore." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 268(19):3009-3013. doi:10.1016/j.nimb.2010.05.029 Abstract Damage evolution at room temperature in Ho2Ti2O7 single crystals is studied under 1 MeV Au2+ ion irradiation by Rutherford backscattering spectroscopy along the <001> direction. For a better determination of ion-induced disorder profile, an iterative procedure and a Monte Carlo code (McChasy) were used to analyze ion channeling spectra. A disorder accumulation model, with contributions from the amorphous fraction and the crystalline disorder, is fit to the Ho damage accumulation data. The damage evolution behavior indicates that the relative disorder on the Ho sublattice follows a nonlinear dependence on dose and that defect-stimulated amorphization is the primary amorphization mechanism. Similar irradiation behavior previously was observed in Sm2Ti2O7. A slower damage accumulation rate for Ho2Ti2O7, as compared with damage evolution in Sm2Ti2O7, is mainly attributed to a lower effective cross section for defect-simulated amorphization.

Zhang Y, M Ishimaru, J Jagielski, W Zhang, Z Zhu, LV Saraf, W Jiang, L Thome, and WJ Weber. 2010. "Damage and Microstructure Evolution in GaN under Au Ion Irradiation." Journal of Physics D. Applied Physics 43(8):085303. doi:10.1088/0022-3727/43/8/085303 Abstract Damage and microstructure evolution in gallium nitride (GaN) under Au+ ion irradiation has been investigated using complementary electron microscopy, secondary ion mass spectrometry and ion beam analysis techniques. Epitaxially-grown GaN layers (2-um-thick) have been irradiated by 2.0 MeV Au ions to 1.0 × 1015 and 1.4 × 1015 cm-2 at 155 K and 7.3 × 1015 cm-2 at 200 K. The irradiation-induced damage has been analyzed by Rutherford backscattering spectroscopy in a channeling direction (RBS/C). For a better determination of ion-induced disorder profile, an iterative procedure and a Monte Carlo code (McChasy) are combined to analyze the ion channeling spectra. With increasing irradiation dose, separated amorphous layers develop from the sample surface and near the damage peak region. Formation of large nitrogen bubbles with sizes up to 70 nm is observed in the buried amorphous layer, while the surface layer contains small bubbles with diameter of a few nanometers due to significant nitrogen loss from the surface. Volume expansion from 3% to 25% in the irradiated region is suggested by cross sectional transmission electron microscope and RBS/C measurement. The anomalous shape of the Au distributions under three irradiations indicates out-diffusion of Au toward sample surface. The results from the complementary techniques suggest that nitrogen is retained in the damaged GaN where the crystallinity is preserved. Once the amorphous state is reached in the surface region, GaN starts to decompose and nitrogen escapes from the surface. Furthermore, experimental results show considerable errors in both disorder profile and ion range predicted by the Stopping and Range of Ions in Matter code, indicating a significant overestimation of electronic stopping powers of Au ions in GaN.

Xiao HY, FX Zhang, F Gao, M Lang, RC Ewing, and WJ Weber. 2010. "Zirconate pyrochlores under high pressure." Physical Chemistry Chemical Physics. PCCP 12(39):12472-12477. doi:10.1039/c0cp00278j Abstract Ab initio total-energy calculations and x-ray diffraction measurements have been combined to study the phase stability of zirconate pyrochlores (A2Zr2O7; A=La, Nd and Sm) under pressures up to 50 GPa. Phase transformations to the defect-cotunnite structure are theoretically predicted at pressures of 22, 20 and 18 GPa, in excellent agreement with the experimentally determined values of 21, 22 and 18 GPa for La2Zr2O7, Nd2Zr2O7 and Sm2Zr2O7, respectively. Analysis of the elastic properties indicates that elastic anisotropy may be one of the driving forces for the pressure-induced cubic-to-noncubic phase transformation.

Xiao HY, F Gao, and WJ Weber. 2010. "Threshold displacement energies and defect formation energies in Y2Ti2O7." Journal of Physics. Condensed Matter 22(41):Article No.: 415801. doi:10.1088/0953-8984/22/41/415801 Abstract Ab initio molecular dynamics simulations have been carried out to determine both the threshold displacement energies Ed and corresponding defect configurations, and ab initio methods have been used to accurately determine the formation energies in Y2Ti2O7. The minimum Ed is found to be 27 eV for a Y recoil along the <100> direction, 31.5 eV for Ti atoms along the <100> direction, 14.5 eV for O48f atoms along the <110> direction and 13 eV for O8b atoms along the <111> direction. The average Ed value along three directions determined is 35.1, 35.4, 17.0 and 16.2 eV for yttrium, titanium, O48f and O8b atoms, respectively. Cation interstitials at vacant 8a sites, which are generally occupied by oxygen anions, and at the bridge sites between two neighboring cations along the <010> direction are observed after low energy recoil events. A systematic study of the defect formation energies suggests that cation interstitials, which are located at 8a sites and bridge sites along the <010> direction, and in split configurations along the <010>, <110> or <111> direction, are all stable in these configurations. It is suggested that the relative stability of cation interstitials may provide a pathway of driving ion-irradiation induced amorphization in Y2Ti2O7.

Xiao HY, XD Jiang, G Duan, F Gao, XT Zu, and WJ Weber. 2010. "First-principles calculations of pressure-induced phase transformation in AlN and GaN." Computational Materials Science 48(4):768-772. doi:10.1016/j.commatsci.2010.03.028 Abstract Ab initio total energy calculations have been carried out to study pressure-induced wurtzite and zinc-blende to rocksalt phase transformation in AlN and GaN. The effects of d electrons on the phase transition pressure and pressure coefficients of band gap have been studied. It is shown that the presence of Ga 3d electrons do have a certain effect on the transition pressure, while other factors such as the variation of charge distribution with pressure should also be considered to explain the higher phase transition pressure for GaN than AlN. Our calculations also show that Ga 3d electrons affect the pressure coefficients of band gap slightly.

Xiao HY, F Gao, XT Zu, and WJ Weber. 2010. "Ab initio molecular dynamics simulation of structural transformation in zinc blende GaN under high pressure." Journal of Alloys and Compounds 490(1-2):537-540. doi:10.1016/j.jallcom.2009.10.076 Abstract High-pressure induced zinc blende to rocksalt phase transition in GaN has been investigated by ab initio molecular dynamics method to characterize the transformation mechanism at the atomic level. It was shown that at 100 GPa GaN passes through tetragonal and monoclinic states before rocksalt structure is formed. The transformation mechanism is consistent with that for other zinc blende semiconductors obtained from the same method. Detailed structural analysis showed that there is no bond breaking involved in the phase transition.

Weber WJ, and F Gao. 2010. "Irradiation-induced defect clustering and amorphization in silicon carbide." Journal of Materials Research 25(12):2349-2353. Abstract Previous computer simulations of multiple 10 keV Si cascades in 3C-SiC demonstrated that many damage-state properties exhibit relatively smooth, but noticeably different, dose dependencies. Recent analysis of these archived damage-state properties reveals more complex relationships between system energy, swelling, energy per defect, relative disorder, elastic modulus and elastic constant, C11. These relationships provide evidence for the onset of defect clustering and amorphization processes, both of which appear to be driven by local energy and elastic instabilities from the accumulation of defects. The results provide guidance on experimental approaches to reveal the onset of these processes.

Wang Z, J Li, F Gao, and WJ Weber. 2010. "Tensile and compressive mechanical behavior of twinned silicon carbide nanowires." Acta materialia 58(6):1963-1971. doi:10.1016/j.actamat.2009.11.039 Abstract Molecular dynamics simulations with the Tersoff potential were used to study the response of twinned SiC nanowires under tensile and compressive strains. The critical strain of the twinned nanowires can be enhanced by twin-stacking faults, and their critical strains are larger than those of perfect nanowires with the same diameters. Under axial tensile strain, the bonds of the nanowires are just stretched before failure. The failure behavior is found to depend on the twin segment thickness and the diameter of the nanowires. An atomic chain is observed for the thin nanowires with small twin segment thickness under tension strain. Under axial compressive strain, the collapse of the twinned SiC nanowires exhibits two differently failure modes, depending on the length and diameter of the nanowires, i.e. shell buckling for short length nanowires and columnar buckling for longer length nanowires.

Wang Z, C Zhang, J Li, F Gao, and WJ Weber. 2010. "First principles study of electronic properties of gallium nitride nanowires grown along different crystal directions." Computational Materials Science 50(2):Pages 344-348 . doi:10.1016/j.commatsci.2010.08.024 Abstract The electronic properties of hydrogen-saturated GaN nanowires with different orientations and sizes are investigated using first-principles calculations, and three types of nanowires oriented along the [001], [110] and [1-10] crystal directions are considered. The electronic properties show little dependence on orientation. The hydrogen-saturated GaN nanowires show semiconducting behavior with a direct band gap larger than that of bulk wurtzite GaN. Quantum confinement leads to a decrease in the band gap of the nanowires with increasing nanowire size. The [001]-oriented nanowires with hexagonal cross sections are energetically more favorable than the [100]- and [1-10]-oriented nanowires with triangular cross-sections.

Wang Z, J Li, F Gao, and WJ Weber. 2010. "Defects in gallium nitride nanowires: first principles calculations." Journal of Applied Physics 108(4):Article No. 044305. doi:10.1063/1.3476280 Abstract Atomic configurations and formation energies of native defects in an unsaturated GaN nanowire grown along the [001] direction and with (100) lateral facets are studied using large-scale ab initio calculation. Cation and anion vacancies, antisites and interstitials in the neutral charge state are all considered. The nitrogen related defects are more stable than the gallium related defects under nitrogen-rich conditions. The configurations of these defects in the core region of the nanowire are same as those in the bulk GaN. The relaxation of vacancies is generally small, but the relaxation around antisite defects is large. The nitrogen interstitial relaxes into a split interstitial configuration. The configurations of the defects in the outermost free surface region are different than those in the core. A Ga atom on the outmost surface is replaced by a Ga interstital, and is ejected on to the surface to become an adsorbed atom. A gallium atom at the outermost surface can also be ejected out to become an adsorbed atom. Nitrogen interstitials form a split-interstitial configuration with one of the nearest-neighbor nitrogens. For a Ga vacancy at the edge of the side plane of the nanowire, nitrogen atom at a gallium site and nitrogen interstitial often induced the formation of N2 molecules with low formation energy, which agrees well with experiment findings [Nano Letters 9, 1844 (2009)].

Wang Z, J Li, F Gao, and WJ Weber. 2010. "Codoping of magnesium with oxygen in gallium nitride nanowires." Applied Physics Letters 96(10):Article No. 103112. doi:10.1063/1.3318462 Abstract Co-doping of p-type GaN nanowires with Mg and oxygen was investigated using first-principles calculations. The Mg becomes a deep acceptor in GaN nanowires with high ionization energy due to the quantum confinement. The ionization energy of Mg doped GaN nanowires containing passivated Mg-O complex decreases with increasing the diameter, and reduces to 300 meV as the diameter of the GaN nanowire is larger than 2.01 nm, which indicates that Mg-O co-doping is suitable for achieving p-type GaN nanowires with larger diameters. The co-doping method to reduce the ionization energy can be effectively used in other semiconductor nanostructures.

Wang Z, J Li, F Gao, and WJ Weber. 2010. "Charge Separation of Wurtzite/Zinc-blende Heterojunction GaN Nanowires ." Chemphyschem 11(15):3329-3332. doi:10.1002/cphc.201000244 Abstract The electronic properties of wurtzite/zinc-blende (WZ/ZB) heterostructure GaN are investigated using first-principles methods. A small component of ZB stacking formed along the growth direction in the WZ GaN nanowires does not show a significant effect on the electronic property, whereas a charge separation of electrons and holes occurs along the directions perpendicular to the growth direction in the ZB stacking. The later case provides an efficient way to separate the charge through controlling crystal structure. These results should have significant implications for most state of the art excitonic solar cells and the tuning region in tunable laser diodes.

Moreira PA, R Devanathan, and WJ Weber. 2010. "Atomistic Simulation of Track Formation by Energetic Recoils in Zircon." Journal of Physics. Condensed Matter 22:Art. No. 395008. Abstract We have performed classical molecular dynamics simulations of fission track formation in zircon. We simulated the passage of a swift heavy ion through crystalline zircon using cylindrical thermal spikes with energy deposition (dE/dx) of 2.5 to 12.8 keV/nm and radius of 3 nm. At a low dE/dx of 2.55 keV/nm, the structural damage recovered almost completely and a damage track was not produced. At higher values of dE/dx, tracks were observed and the radius of the track increased with increasing dE/dx. Our structural analysis shows amorphization in the core of the track and phase separation into Si-rich regions near the center of the track and Zr-rich regions near the periphery. These simulations establish a threshold dE/dx for fission-track formation in zircon that is relevant to thermo-chronology and nuclear waste immobilization.

Li Z, S Wang, Z Wang, XT Zu, F Gao, and WJ Weber. 2010. "Mechanical behavior of twinned SiC nanowires under combined tension-torsion and compression-torsion strain." Journal of Applied Physics 108(1):Art. No. 013504. doi:10.1063/1.3456002 Abstract The mechanical behavior of twinned silicon carbide (SiC) nanowires under combined tension-torsion and compression-torsion is investigated using molecular dynamics simulations with an empirical potential. The simulation results show that both the tensile failure stress and buckling stress decrease under combined tension-torsional and combined compression-torsional strain, and they decrease with increasing torsional rate under combined loading. The torsion rate has no effect on the elastic properties of the twinned SiC nanowires. The collapse of the twinned nanowires takes place in a twin stacking fault of the nanowires.

Jiang W, H Wang, I Kim, Y Zhang, and WJ Weber. 2010. "Amorphization of nanocrystalline 3C-SiC irradiated with Si+ ions." Journal of Materials Research 25(12):2341-2348. doi:10.1557/JMR.2010.0311 Abstract Irradiation induced amorphization in nanocrystalline and single crystal 3C-SiC has been studied using 1 MeV Si+ ions under the identical irradiation conditions at room temperature and 400 K. The disordering behavior has been characterized using in-situ ion channeling and ex-situ x-ray diffraction methods. The results show that, compared to single crystal 3C-SiC, full amorphization of small 3C-SiC grains (~3.8 nm in size) occurs at a slightly lower dose at room temperature. For grains with sizes of 3.0 - 3.8 nm, the amorphization dose is lower at room temperature than 400 K. A significantly lower dose for amorphization of smaller grains (2.0 nm in size) is observed at 400 K. The behavior has been interpreted based on the competition between the interface and interior amorphization.

Gao F, D Chen, W Hu, and WJ Weber. 2010. "Energy Dissipation and Defect Generation for Nanocrystalline Silicon Carbide." Physical Review. B, Condensed Matter and Materials Physics 81(18):Article No.184101. doi:10.1103/PhysRevB.81.184101 Abstract Large-scale molecular dynamics simulations have been employed to study defect generation and primary damage state in nano crystalline (NC) SiC of average grain diameters from 5 to 21 nm. Primary knock-on atom (PKA) kinetic energies of 10 keV are simulated, and cascade structures in NC SiC with a grain size smaller than 12 nm are generally different from those generated in single crystalline SiC. It is found that the local stresses near the grain boundaries (GBs) strongly affect the behavior of the PKA and secondary recoil atoms (SRAs), and the GBs act as sinks for deposition of kinetic energy. A striking feature is that the PKA and SRAs preferentially deposit energy along the GBs for grains with average size less 12 nm, which results in atomic displacements primarily within the GBs; whereas for larger grain sizes, most defects are produced within the grains. There exists a crossover in defect production, which is manifested in switching from grain boundary damage to grain damage. The most common defects created in NC SiC are antisite defects, following by vacancies and interstitials, in contrast to those produced in a single crystalline SiC, where the dominate defects are Frenkel pairs. Defect production efficiency increases with increasing the grain size, with a typical value of 0.18 for small grains and rising to 0.5 for larger grains.

Devanathan R, and WJ Weber. 2010. "Simulation of collision cascades and thermal spikes in ceramics." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 268(19):2857-2862. doi:10.1016/j.nimb.2010.05.047 Abstract Classical molecular dynamics simulations have been employed to examine defect production by energetic recoils in UO2, Gd2Ti2O7, Gd2Zr2O7 and ZrSiO4. These atomistic simulations provide details of the nature and size distribution of defect clusters produced in collision cascades. The accommodation of recoil damage by lower energy cation exchange and greater occupation of anion structural vacancies is a contributing factor for the greater radiation tolerance of Gd2Zr2O7 relative to Gd2Ti2O7. In addition, electronic energy loss processes in UO2 has been modeled in the form of a thermal spike to study the details of track formation and track structure. For thermal spikes with energy deposition of 4 keV/nm in UO2, a track was not formed and mainly isolated Frenkel pairs are produced.

Devanathan R, WJ Weber, and JD Gale. 2010. "Radiation tolerance of ceramics—Insights from atomistic simulation of damage accumulation in pyrochlores." Energy & Environmental Science 3(10):1551-1559. doi:10.1039/C0EE00066C Abstract We have used molecular dynamics simulations to examine the effects of radiation damage accumulation in two pyrochlore-structured ceramics, namely Gd2Ti2O7 and Gd2Zr2O7. It is well known from experiment that the titanate is susceptible to radiation-induced amorphization, while the zirconate does not go amorphous under prolonged irradiation. Our simulations show that cation Frenkel pair accumulation eventually leads to amorphization of Gd2Ti2O7. Anion disorder occurs with cation disorder. The amorphization is accompanied by a density decrease of about 12.7% and a decrease of about 50% in the elastic modulus. In Gd2Zr2O7, amorphization does not occur, because the residual damage is not sufficiently energetic to drive the material amorphous. Subtle differences in damage accumulation and annealing between the two pyrochlores lead to drastically different radiation response as the damage accumulates.

Zhang Y, J Lian, Z Zhu, WD Bennett, LV Saraf, JL Rausch, CA Hendricks, RC Ewing, and WJ Weber. 2009. "Response of Strontium Titanate to Ion and Electron Irradiation." Journal of Nuclear Materials 389(2):303-310. doi:10.1016/j.jnucmat.2009.02.014 Abstract Response of strontium titanate (SrTiO3) to ion and electron irradiation is studied at room temperature. For an accurate energy to depth conversion and a better determination of ion-induced disorder profile from Rutherford backscattering spectrometry measurement, a detailed iterative procedure is described and applied to ion channeling spectra to determine the dechanneling yield and the disorder profiles for the Sr and Ti sublattices. The result shows a large underestimation in disorder depth, ~ 40% at the damage peak, which indicates a large overestimation of the electronic stopping power for 1.0 MeV Au ions in SrTiO3 predicted by the SRIM (Stopping and Range of Ions in Matter) code. Overestimation of heavy ion stopping power may lead to an overestimation of the critical dose for amorphization. The current study also demonstrates possible ionization effects in SrTiO3 under ion and electron irradiation. Pre-amorphized SrTiO3 exhibits strong ionization-induced epitaxial recovery at the amorphous/crystalline interface under electron irradiation.

Zhang Y, and WJ Weber. 2009. "Response of Materials to Single Ion Events." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 267(8-9):1705-1712. doi:10.1016/j.nimb.2009.01.104 Abstract Response of materials to single radiation events is fundamental to research and many technological applications that involve energetic particles. Ion-solid interactions lead to energy loss of ions, production of electron-hole pairs, and light emission from excitation-induced luminescence. Employing a unique time-of-flight system, material response to single ion irradiation has been utilized to measure electronic energy loss, and to evaluate materials performance for radiation detection. Measurements of electronic energy loss of single ions in a thin ZrO2 foil over a continuous energy range exhibit good agreement with SRIM predictions for He and Be ions. For O and F ions, slight over- and under-estimation of SRIM prediction is evident at energies around 250 (near the stopping maximum) and above 800 keV/nucleon, respectively. For a Si semiconductor detector, its response to single ion irradiation shows that pulse height defect is clear for elements heavier than Si, and nonlinear energy response is significant for all elements at energies below ~ 150 keV/nucleon. For a single crystal CsI:Tl scintillator, the response to H ion events is used to determine relative light yield and absolute energy resolution over a wide energy region, where energy resolution of ~ 5.3% is achieved at 2 MeV.

Zhang M, RC Ewing, LA Boatner, EKH Salje, WJ Weber, P Daniel, Y Zhang, and IE Farnan. 2009. "Pb+ irradiation of synthetic zircon (ZrSiO4): Infrared spectroscopic investigation - Reply." American Mineralogist 94(5-6):856-858. doi:10.2138/am.2009.542 Abstract We appreciate the opportunity to respond to the comment by Nasdala (2009) concerning our interpretation of infrared spectra used to investigate the change in the structure of Pb-irradiated zircon as a function of increasing fluence (Zhang et al. 2008a, 2008b). Nasdala is correct in cautioning experimentalists to carefully match the analytical technique to the expected irradiation damage profile in order to optimally probe the irradiation effects, and in fact, this point was emphasize by Ewing et al. (2003) in a review of radiation effects in zircon. However, Nasdala’s discussion fails to fully appreciate three important points: i) There is a difference between in situ irradiations of TEM samples that must be electron transparent, ~200 nm thick, as were completed by Weber et al. (1994), and more bulk-like irradiations that were completed in the Zhang et al. (2008a and 2008b) studies; ii) The particle-solid interactions change along the path of an implanted ion, that is the distribution and nature of the damage changes with depth as the ion loses energy, resulting in the greatest number of ballistic interactions near the end of the particle trajectory (see Figure 1 of Ewing et al. 2003); iii) In comparing natural zircon damaged by alpha-decay events with ion irradiated zircon, one must be aware that the recoil nucleus and the alpha particle cause different types of damage, and the use of the Pb-implantation experiment is meant to simulate only the alpha-recoil damage.

Zhang Y, X Xiang, JL Rausch, XT Zu, and WJ Weber. 2009. "Ion Technique for Identifying Gamma Detector Candidates." IEEE Transactions on Nuclear Science 56(3):920-925. doi:10.1109/TNS.2008.2011640 Abstract Recent demands for radiation detector materials with better energy resolution at room temperature have prompted research efforts on both accelerated material discovery and efficient analysis techniques. Ions can easily deposit their energy in thin films or small crystals and the radiation response can be used to identify material properties relevant to detector performance. In an effort to identify gamma detector candidates using small crystals or film samples, an ion technique is developed to measure relative light yield and energy resolution of candidate materials and to evaluate radiation detection performance. Employing a unique time-of-flight (TOF) telescope, light yield and energy resolution resulting from ion excitation are investigated over a continuous energy region. The efficiency of this ion technique is demonstrated using both organic (plastic scintillator) and inorganic (CaF2:Eu, YAP:Ce, CsI:Tl and BGO) scintillators.

Zhang ZL, HY Xiao, XT Zu, F Gao, and WJ Weber. 2009. "First-principles calculation of structural and energetic properties for A2Ti2O7 (A =Lu, Er, Y, Gd, Sm, Nd, La)." Journal of Materials Research 24(4):1335-1341. Abstract A first-principles method has been employed to investigate the structural and energetic properties for A2Ti2O7 (A=Lu, Er, Y, Gd, Sm, Nd, La), including the formation energies of the cation antisite-pair, the anion Frenkel pair that defines anion-disorder, and the coupled cation antisite-pair/anion-Frenkel. It is proposed that the <A-O48f> interaction may have more significant influence on the radiation resistance behavior of titanate pyrochlores, although the <Ti-O48f> interactions are relatively much stronger than the <A-O48f> interactions. It is found that the defect formation energies are not simple functions of the A-site cation radii. The formation energy of the cation antisite-pair increases continuously as the A-site cation varies from Lu to Gd, and then decreases continuously with the variation of the A-site cation from Gd to La, in excellent agreement with the radiation-resistance trend of the titanate pyrochlores. The band gaps in these pyrochlores were also measured, and the band gap widths changed continuously with cation radius.

Zhang Y, IT Bae, K Sun, CM Wang, M Ishimaru, Z Zhu, W Jiang, and WJ Weber. 2009. "Damage Profile and Ion Distribution of Slow Heavy Ions in Compounds." Journal of Applied Physics 105(10):104901:1-12. doi:10.1063/1.3118582 Abstract Slow heavy ions inevitably produce a significant concentration of defects and lattice disorder in solids during their slowing-down process via ion-solid interactions. For irradiation effects research and many industrial applications, atomic defect production, ion range and doping concentration are commonly estimated by the Stopping and Range of Ions in Matter (SRIM) code. In this study, ion-induced damage and projectile ranges of low energy Au ions in SiC are determined using complementary ion beam and microscopy techniques. Considerable errors in both disorder profile and ion range predicted by the SRIM code indicate an overestimation of the electronic stopping power, by a factor of 2 in most cases, in the energy region up to 25 keV/nucleon. Such large discrepancies are also observed for slow heavy ions, including Pt, Au and Pb ions, in other functional materials, such as GaN, AlN and SrTiO3. Due to the importance of these materials for advanced device and nuclear applications, better electronic stopping cross section predictions, based on a reciprocity principle developed by Sigmund, is suggested with fitting parameters for possible improvement.

Yu J, R Devanathan, and WJ Weber. 2009. "Unified Interatomic Potential for Zircon, Zirconia and Silica Systems." Journal of Materials Chemistry 19(23):3923-3930. doi:10.1039/b902767j Abstract We report the development of a unified force field with high transferability and reliability to model both structures and mechanical properties of ZrSiO4, ZrO2 and SiO2 based on the success of the BKS potential for SiO2.1 The thermal expansion, relative stability and phase transition properties are consistent with experimental data and DFT calculations. The zircon to reidite transition pressure is 6.7 GPa. Amorphization of zircon results in volume expansion of 11% and decrease in the bulk modulus of 60%. Si polymerization and Zr under-coordination were observed in amorphous ZrSiO4 and in the damage produced by a 10 keV Zr recoil in crystalline ZrSiO4.

Yu J, R Devanathan, and WJ Weber. 2009. "First-principles study of defects and phase transition in UO2." Journal of Physics. Condensed matter 21(43):Art. No. 435401. doi:10.1088/0953-8984/21/43/435401 Abstract The electronic properties, structure and phase transformation of UO2 have been studied from first principles by the all-electron projector-augmented-wave (PAW) method. The generalized gradient approximation (GGA)+U formalism has been used to account for the strong on-site Coulomb repulsion among the localized U 5f electrons. GGA+U gives an antiferromagnetic insulating ground state for the effective Hubbard parameter Ueff ≥2.0 eV and this ordering is consistent with experimental measurement. Our results also reveal that by choosing an appropriate Ueff =3.0 eV it is possible to consistently describe structural properties of UO2 and model phase transition processes. The distribution of the local electrostatic potential indicated that the phase transition pressure for UO2 under operant conditions is about 20 GPa. In addition, the formation energies of intrinsic defects, which play a critical role in UO2 fuel under operant conditions, are found to be depended on whether the environment is under U-rich condition or O-rich condition.

Xiao HY, F Gao, XT Zu, and WJ Weber. 2009. "Threshold displacement energy in GaN; Ab initio molecular dynamics study." Journal of Applied Physics 105(12):123527, 1-5. doi:10.1063/1.3153277 Abstract Large-scale ab initio molecular dynamics method has been used to determine the threshold displacement energies, Ed, along five specific directions and to determine the defect configurations created during low energy events. The Ed shows a significant dependence on direction. The minimum Ed is determined to be 39 eV along the <-1010> direction for a gallium atom and 17.0 eV along the <-1010> direction for a nitrogen atom, which are in reasonable agreement with the experimental measurements. The average Ed values determined are 73.2 and 32.4 eV for gallium and nitrogen atoms, respectively. The N defects created at low energy events along different crystallographic directions have a similar configuration (a N-N dumbbell configuration), but various configurations for Ga defects are formed in GaN.

Xiao HJ, F Gao, XT Zu, and WJ Weber. 2009. "Ab initio molecular dynamics simulation of pressure-induced zinc blende to rocksalt phase transition in SiC." Journal of Physics. Condensed matter 21(24):Article Number: 123527. Abstract The high-pressure induced phase transformation from the zinc blende to rocksalt structure in SiC has been studied by ab initio molecular dynamics. The simulations show that SiC passes through tetragonal and then monoclinic intermediate states before finally forming the rock salt structure at 160 GPa. The mechanism for this phase transformation agrees well with recent ab initio MD simulations, in which the applied pressure was as high as ~600 GPa, but in the present study the transformation occurs at much lower pressure. It is found that the phase transition has to overcome an energy barrier of 0.44 eV/pair.

Xiao HY, F Gao, and WJ Weber. 2009. "Ab initio investigation of phase stability of Y2Ti2O7 and Y2Zr2O7 under high pressure." Physical Review. B, Condensed Matter and Materials Physics 80(21):Art. No. 212102. doi:10.1103/PhysRevB.80.212102 Abstract The phase stabilities of Y2Ti2O7 and Y2Zr2O7 under high pressure were investigated by ab initio methods. Pyrochlore-structured Y2Ti2O7 and defect-fluorite Y2Zr2O7 exhibit different responses to high pressure. Both the defect-fluorite and defect-cotunnite structures are energetically more stable at high pressure in Y2Ti2O7, but comparison with experimental results suggest that only the transformation to the defect-fluorite structure is kinetically favored. For Y2Zr2O7, the defect-fluorite phase should undergo a structural transformation to the defect-cotunnite state under high pressure.

Weber WJ, A Navrotsky, SV Stefanovsky, ER Vance, and EY Vernaz. 2009. "Materials Science of High-Level Nuclear Waste Immobilization." MRS Bulletin 34(1):46-53. Abstract With the increasing demand for the development of more nuclear power comes the responsibility to address the technical challenges of immobilizing high-level nuclear wastes in stable solid forms for interim storage or disposition in geologic repositories. The immobilization of high-level nuclear wastes has been an active area of research and development for over 50 years. Borosilicate glasses and complex ceramic composites have been developed to meet many technical challenges and current needs, although regulatory issues, which vary widely from country to country, have yet to be resolved. Cooperative international programs to develop advanced proliferation-resistant nuclear technologies to close the nuclear fuel cycle and increase the efficiency of nuclear energy production might create new separation waste streams that could demand new concepts and materials for nuclear waste immobilization. This article reviews the current state-of-the-art understanding regarding the materials science of glasses and ceramics for the immobilization of high-level nuclear waste and excess nuclear materials and discusses approaches to address new waste streams.

Wang Z, S Wang, J Li, F Gao, and WJ Weber. 2009. "Structure and electronic properties of saturated and unsaturated gallium nitride nanotubes." Journal of Physical Chemistry C 113(44):19281-19285. doi:10.1021/jp907657z Abstract The atomic and electronic structures of saturated and unsaturated GaN nanotubes along the [001] direction with (100) lateral facets are studied using first-principles calculations. Atomic relaxation of nanotubes shows that appreciable distortion occurs in the unsaturated nanotubes. All the nanotubes considered, including saturated and unsaturated ones, exhibit semiconducting, with a direct band gap. Surface states arisen from the threefold-coordinated N and Ga atoms at the lateral facets exist inside the bulk-like band gap. When the nanotubes saturated with hydrogen, these dangling bond bands are removed from the band gap, but the band gap decreases with increasing the wall thickness of the nanotubes.

Wang Z, F Gao, JINGBO Li, XT Zu, and WJ Weber. 2009. "Stone-Wales Defects Created by Low Energy Recoils in Single-walled Silicon Carbide Nanotubes." Journal of Applied Physics 106(8):Art. No. 084305. doi:10.1063/1.3238307 Abstract The defect creation at low energy events was studied using density functional theory molecular dynamics simulations in silicon carbide nanotubes, and the displacement threshold energies determined exhibit a dependence on sizes, which decrease with decreasing diameter of the nanotubes. The Stone-Wales (SW) defect, which is a common defect configurations induced through irradiation in nanotubes, has also been investigated, and the formation energies of the SW defects increase with increasing diameter of the nanotubes. The mean threshold energies were found to be 23 and 18 eV for Si and C in armchair (5,5) nanotubes.

Wang Z, F Gao, J Li , XT Zu, and WJ Weber. 2009. "Controlling Electronic Structures by Irradiation in Single-walled SiC Nanotubes: A First-Principles Molecular Dynamics Study." Nanotechnology 20(7):Art. No. 075708. doi:10.1088/0957-4484/20/7/075708 Abstract Using first principles molecular dynamics simulations, the displacement threshold energy and defect configurations are determined in SiC nanotubes. The simulation results reveal that a rich variety of defect structures (vacancies, Stone-Wales defects, and antisite defects) are formed with threshold energies of from 11 to 64 eV. The threshold energy shows an anisotropic behavior and exhibits a dramatic decrease with decreasing tube diameter. The electronic structure can be altered by the defects formed by irradiation, which suggests that the electron irradiation may be a way to use defect engineering to tailor electronic properties of SiC nanotubes.

Moreira PA, R Devanathan, J Yu, and WJ Weber. 2009. "Molecular dynamics simulation of threshold displacement energies in zircon." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 267(20):3431-3436. doi:10.1016/j.nimb.2009.07.023 Abstract Molecular-dynamics simulations were used to examine the displacement threshold energy (Ed) surface for Zr, Si and O in zircon using two different interatomic potentials. For each sublattice, the simulation was repeated from different initial conditions to estimate the uncertainty in the calculated value of Ed. The displacement threshold energies vary considerably with crystallographic direction and sublattice. The average displacement energy calculated with a recently developed transferable potential is about 120 and 60 eV for cations and anions, respectively. The oxygen displacement energy shows good agreement with experimental estimates in ceramics.

Lian J, J Zhang, F Namavar, Y Zhang, F Lu, H Haider, K Garvin, WJ Weber, and RC Ewing. 2009. "Ion beam-induced amorphous-to-tetragonal phase transformation and grain growth of nanocrystalline zirconia ." Nanotechnology 20(24):245303, 1-7. doi:10.1088/0957-4484/20/24/245303 Abstract Nanocrystalline zirconia has recently attracted extensive research interest due to its unique mechanical, thermal and electrical properties as compared to bulk zirconia counterparts, and it is of particular importance to control the phase stability of different polymorphs (amorphous, cubic, tetragonal and monoclinic phases) at different size regimes. In this paper, we performed ion beam bombardments on bilayers (amorphous and cubic) of pure nano-zirconia using 1 MeV Kr2+ irradiation. Transmission electron microscopy (TEM) analysis reveals that amorphous zirconia transforms to a tetragonal structure under irradiation at room temperature, suggesting that the tetragonal phase is more energetically favorable under these conditions. The final grain size of the tetragonal zirconia can be controlled by irradiation conditions. The irradiation-induced nanograins of tetragonal ZrO2 are stable at ambient conditions and maintain their physical integrity over a long period of time after irradiation. These results demonstrated that ion-beam modification methods provide the means to control the phase stability and structure of zirconia polymorphs.

Lang M, J Lian, J Zhang, F Zhang, WJ Weber, C Trautmann, and RC Ewing. 2009. "Single-Ion Tracks in Gd2Zr2-xTixO7 Pyrochlores Irradiated with Swift Heavy Ions." Physical Review. B, Condensed Matter and Materials Physics 79(22):article no. 224105. doi:10.1103/PhysRevB.79.224105 Abstract Swift xenon ions (1.43 GeV) were used to systematically investigate the radiation response of pyrochlores in the Gd2Zr2-xTixO7 binary in the electronic energy loss regime. Ion-induced structural modifications were characterized by synchrotron x-ray diffraction, Raman spectroscopy and transmission electron microscopy. In general, individual ion tracks consist of amorphous domains (in the core of the track) and are surrounded by a crystalline, but disordered structure (track shell), and finally, by a defect-rich region (track halo). The size and structure of these concentric regions, which form dynamically in single ion tracks, depend on the chemical composition. The overall extent of the radiation damage strongly increases with Ti-content, and this confirms the enhanced radiation stability of Zr-rich pyrochlore, even within single swift heavy ion tracks.

Jiang W, H Wang, I Kim, IT Bae, G Li, P Nachimuthu, Z Zhu, Y Zhang, and WJ Weber. 2009. "Response of Nanocrystalline 3C Silicon Carbide to Heavy-Ion Irradiation." Physical Review. B, Condensed Matter 80(16):Art. No.161301(R). doi:10.1103/PhysRevB.80.161301 Abstract Nanostructured materials are generally believed to be more radiation resistant. This study reports on Au ion induced amorphization in nanocrystalline 3C-SiC, characterized using x-ray diffraction, transmission electron microscopy and Raman spectroscopy. Full amorphization at room temperature occurs at a comparable dose to that for bulk SiC single crystals. The behavior is attributed to a high ion flux and sluggish migration of point defects produced during irradiation. The results may have a significant implication of using nanophased SiC in extremely high radiation environments.

Jiang W, WJ Weber, J Lian, and NM Kalkhoran. 2009. "Disorder accumulation and recovery in gold-ion irradiated 3C-SiC." Journal of Applied Physics 105(1):Art. No. 013529. doi:10.1063/1.3055797 Abstract A single-crystal 3C-SiC film on the Si/SiO2/Si (SIMOX) substrate was irradiated in different areas at 156 K with Au2+ ions to low fluences. The disorder profiles as a function of dose on both the Si and C sublattices have been determined in situ using a combination of 0.94 MeV D+ Rutherford backscattering spectrometry and nuclear reaction analysis in channeling geometry along the <100>, <110> and <111> axes. The results indicate that for the same damage state, the level of disorder on the Si sublattice in 3C-SiC follows a decreasing order along the <111>, <100> and <110> axes, while that on the C sublattice shows comparable values. Similar levels of Si and C disorder are observed along the <111> axis over the applied dose range. However, the level of C disorder is higher than that of Si disorder along either <100> or <110>. The amount of disorder recovery during thermal annealing processes depends on the sublattice (Si or C) and crystallographic orientation. Room-temperature recovery occurs for both sublattices in 3C-SiC irradiated to a dose of 0.047 dpa or lower. Significant recovery is observed along all directions during thermal annealing at 600 K. The results will be discussed and compared to those for 6H- and 4H-SiC under similar irradiation conditions.

Jiang W, and WJ Weber. 2009. "Anisotropy of disorder accumulation and recovery in 6H-SiC irradiated with Au2+ ions at 140 K." Journal of Nuclear Materials 389(2):332-335. doi:10.1016/j.jnucmat.2009.02.023 Abstract Single crystal <0001>-oriented 6H-SiC was irradiated with Au2+ ions to fluences of 0.032, 0.058 and 0.105 ions/nm2 at 140 K and was subsequently annealed at various temperatures up to 500 K. The relative disorder on both the Si and C sublattices has been determined simultaneously using in-situ D+ ion channeling along the <0001> and <2-201> axes. A higher level of disorder on both the Si and C sublattices is observed along the <2-201>. There is a preferential C disordering and more C interstitials are aligned with <0001>. Room-temperature recovery along <2-201> occurs, which is associated with the <0001>-aligned interstitials that annihilate due to close-pair recombination. Disorder recovery between 400 and 500 K is primarily attributed to annihilation of interstitials that are misaligned with <0001>. Effects of stacking order in SiC on disorder accumulation are insignificant; however, noticeable differences of low-temperature recovery in Au2+-irradiated 6H-SiC and 4H-SiC are observed.

Ishimaru M, Y Zhang, and WJ Weber. 2009. "Ion-beam-induced chemical disorder in GaN." Journal of Applied Physics 106(5):053513, 1-4. doi:10.1063/1.3212555 Abstract Atomistic structures of high-energy ion irradiated GaN have been examined using transmission electron microscopy (TEM). Single crystalline GaN substrates were irradiated at cryogenic temperature with 2 MeV Au2+ ions to a fluence of 7.35x1015 Au/cm2. Cross-sectional TEM observations revealed that damaged layers consisting of amorphous and nanocrystalline phases are formed at the surface and buried depth of the as-irradiated GaN substrate. Atomic radial distribution functions of the amorphous/poly-nanocrystalline regions showed that not only heteronuclear Ga-N bonds but also homonuclear Ga-Ga bonds exist within the first coordination shell. It was found that the ratio of heteronuclear-to-homonuclear bonds, i.e., the degree of chemical disorder is different between the surface and buried damaged layers. The alternation of chemical disorder was attributed to the difference in the defect formation processes between these layers.

Gao F, WJ Weber, HY Xiao, and XT Zu. 2009. "Formation and properties of defects and small vacancy clusters in SiC: Ab initio calculations ." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 267(18):2995-2998. doi:10.1016/j.nimb.2009.06.018 Abstract Large-scale ab initio simulation methods have been employed to investigate the configurations and properties of defects in SiC. Atomic structures, formation energies and binding energies of small vacancy clusters have also been studied as a function of cluster size, and their relative stabilities are determined. The calculated formation energies of point defects are in good agreement with previously theoretical calculations. The results show that the most stable configuration of a di-vacancy cluster consists of two C vacancies located at second nearest neighbor sites, while a di-vacancy with two Si vacancies is not stable and may dissociate at room temperature. In general, the formation energies of small vacancy clusters increase with size, but the formation energies for clusters with a Si vacancy and n C vacancies (VSi-nVC) are much smaller than those with a C vacancy and n Si vacancies (VC-nVSi). These results demonstrate that the VSi-nVC clusters are more stable than the VC-nVSi clusters in SiC, and provide possible nucleation sites for larger vacancy clusters or voids to grow. For these small vacancy clusters, the binding energy decreases with increasing cluster size, and ranges from 2.5 to 4.6 eV. These results indicate that the small vacancy clusters in SiC are stable at temperatures up to 1900 K, which is consistent with experimental observations.

Gao F, HY Xiao, XT Zu, M Posselt, and WJ Weber. 2009. "Defect-Enhanced Charge Transfer by Ion-Solid Interactions in SiC using Large-Scale Ab Initio Molecular Dynamics Simulations." Physical Review Letters 103(2):Article number: 027405. doi:10.1103/PhysRevLett.103.027405 Abstract Large-scale ab initio molecular dynamics simulations of ion-solid interactions in SiC reveal that significant charge-transfer occurs between atoms and defects can enhance charge transfer to surrounding atoms. The results demonstrate that charge transfer to and from recoiling atoms can alter the energy barriers and dynamics for stable defect formation. The present simulations illustrate in detail the dynamic processes for charged defect formation. The averaged values of displacement threshold energies along four main crystallographic directions are smaller than those determined by empirical potentials due to charge transfer effects on recoil atoms.

Devanathan R, J Yu, and WJ Weber. 2009. "Energetic recoils in UO2 simulated using five different potentials." Journal of Chemical Physics 130(17):Art. No. 174502. doi:10.1063/1.3125967 Abstract This report presents the results of classical molecular dynamics simulations of the diffuse pre-melting transition, melting, and defect production by 1 keV U recoils in UO2 using five different rigid-ion potentials. The experimentally-observed pre-melting transition occurred for all five cases. For all the potentials studied, dynamic defect annealing is highly effective and is accompanied by replacement events on the anion sublattice. The primary damage state after ~15 ps consists of isolated Frenkel pairs and interstitial and vacancy clusters of various sizes. The average displacement energy varies from ~28 to ~83 eV and the number of Frenkel pairs is different by a factor of three depending on the choice of potential. The size and spatial distribution of vacancy and interstitial clusters is drastically different for the potentials studied. The results provide statistics of defect production. They point to a pressing need to determine defect formation, migration and binding energies in UO2 from first principles and to develop reliable potentials based on this data for simulating microstructural evolution in nuclear fuel under operating conditions.

Bae IT, W Jiang, CM Wang, WJ Weber, and Y Zhang. 2009. "Thermal evolution of microstructure in ion-irradiated GaN." Journal of Applied Physics 105(8):083514, 1-7. doi:10.1063/1.3106606 Abstract The thermal evolution of the microstructure created by irradiation of a GaN single crystal with 2 MeV Au2+ ions at 150 K is characterized following annealing at 973 K using transmission electron microscopy. In the as-irradiated sample characterized at 300 K, Ga nanocrystals with the diamond structure, which is an unstable configuration for Ga, are directly observed together with nitrogen bubbles in the irradiation-induced amorphous layer. Upon thermal annealing, the thickness of the amorphous layer decreases by ~13.1 %, and nano-beam electron diffraction analysis indicates no evidence for residual Ga nanocrystals, but instead reveals a mixture of hexagonal and cubic GaN phases in the annealed sample. Nitrogen molecules, captured in the as-irradiated bubbles, appear to debond and react with the Ga nanocrystals during the thermal annealing to form crystalline GaN. In addition, electron energy loss spectroscopy measurements reveal an atomic volume change of 18.9 % for the as-irradiated amorphous layer relative to the virgin single crystal GaN. This relative swelling of the damaged layer reduces to 7.7 % after thermal annealing. Partial recrystallization and structural relaxation of the GaN amorphous state are believed responsible for the volume change.

Bae IT, WJ Weber, and Y Zhang. 2009. "Direct measurement of local volume change in ion-irradiated and annealed SiC." Journal of Applied Physics 106(12):123525, 1-5. doi:10.1063/1.3272808 Abstract Depth profiles of local volume expansions are precisely measured in 6H-SiC after irradiation at 150 K with 2 MeV Pt ions and following annealing at 770 K using transmission electron microscopy equipped with electron energy loss spectroscopy. It is found that the depth profile of local volume expansion from the as-implanted sample matches well with the depth profile of irradiation-induced local disorder measured by Rutherford backscattering spectrometry. Further, the local volume expansion increases linearly with local dose up to ~10%. By systematically comparing the depth profiles of local volume expansion and local relative disorder, it is revealed that the atomic volume of amorphous SiC continues to increase until it saturates at ~15% due to the increased chemical short-range disorder. This is believed to be one of the reasons for significant scatter in values of volume expansion previously reported for the irradiation-induced amorphous state of SiC.

Wang Z, F Gao, X Zu, and WJ Weber. 2008. "Physical Properties of GaN Nanotubes as Revealed by Computer Simulation." Chapter 5 in One-Dimensional Nanostructures, Lecture Notes in Nanoscale Science and Technology, vol. 3, ed. Zhiming M. Wang, pp. 97-126. Springer, New York, NY. Abstract Single-crystalline wurtzite GaN nanotubes have been synthesized recently with proposed applications in nanoscale electronics, optoelectronics and the biochemical-sensing field. Molecular dynamics methods with a Stillinger-Weber potential are used to investigate the melting behavior, thermal conductivity and mechanical properties of these wurtzite-type single crystalline GaN nanotubes. Four major topical areas are summarized in this chapter. (1) The melting temperature of the GaN nanotubes increases with the thickness of the nanotubes to a saturation value, which is close to the melting temperature of bulk GaN. The simulations result reveal that the nanotubes begin to melt at the surface, and then the melting rapidly extends to the interior of the nanotubes as the temperature increases. (2) The thermal conductivity of nanotubes is smaller than that of the bulk GaN single crystal. The thermal conductivity is also found to decrease with temperature and increase with increasing wall thickness of the nanotubes. The change of phonon spectrum and surface inelastic scattering may account for the reduction of thermal conductivity in the nanotubes, while thermal softening and high frequency phonon interactions at high temperatures may provide an explanation for its decrease with increasing temperature. (3) At low temperatures, the simulation results show that the nanotubes exhibit brittle properties; whereas at high temperatures, they behave as ductile materials. The brittle to ductile transition temperature generally increases with increasing wall thickness of the nanotubes and increasing strain rate. (4) The simulation temperature, tube length and strain rate affect the buckling behavior of GaN nanotubes. The critical stress decreases with the increase of simulation temperature and tube length. The dependence of buckling on tube length is consistent with the analysis of equivalent continuum structures using Euler buckling theory.

Zhang J, Q Wei, J Lian, W Jiang, WJ Weber, and RC Ewing. 2008. "Self-Assembly of Well-Aligned 3C-SiC Ripples by Focused Ion Beam." Applied Physics Letters 92(19):193107, 1-3. doi:10.1063/1.2927473 Abstract Well-aligned ripple structures on single crystal 3C-SiC surface were created and imaged in real time by focused ion beam (FIB) bombardment. Ex situ atomic force microscopy was applied to investigate topography. The ripple structure was formed by ion sputtering beyond a critical incidence angle (~50°), and its characteristic wavelength varied from 158 nm to 296 nm with the incidence angle and ion beam flux. Furthermore, the geometry, ordering and homogeneity of the self-assembled surface ripples can be well controlled by varying the ion beam incidence angle and beam current, as required for the formation of nanostructures used in SiC optical and electronic applications.

Zhang Y, X Xiang, and WJ Weber. 2008. "Scintillation Response of CaF2 to H and He over a Continuous Energy Range." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 266(12-13):2750-2753. doi:10.1016/j.nimb.2008.03.110 Abstract Recent demands for new radiation detector materials with improved -ray detection performance at room temperature have prompted research efforts on both accelerated material discovery and efficient techniques that can be used to identify material properties relevant to detector performance. New material discovery has been limited due to the difficulties of large crystal growth to completely absorb energies; whereas high-quality thin films or small crystals of candidate materials can be readily produced by various modern growth techniques. In this work, an ion-scintillator technique is demonstrated that can be applied to study scintillation properties of thin films and small crystals. The scintillation response of a benchmark scintillator, europium-doped calcium fluoride (CaF2:Eu), to energetic proton and helium ions is studied using the ion-scintillator approach based on a time of flight (TOF) telescope. Excellent energy resolution and fast response of the TOF telescope allow quantitative measurement of light yield, nonlinearity and energy resolution over an energy range from a few tens to a few thousands of keV.

Zhang M, LA Boatner, E Salje, RC Ewing, P Daniel, WJ Weber, Y Zhang, and IE Farnan. 2008. "Micro-Raman and Micro-Infrared Spectroscopic Studies of Pb- and Au-Irradiated ZrSiO4: Optical Properties, Structural Damage and Amorphization ." Physical Review. B, Condensed Matter and Materials Physics 77(14):144110. doi:10.1103/PhysRevB.77.144110 Abstract The optical properties of damaged, periodic and aperiodic domains created by Pb+ (280 keV) and Au4+ (10 MeV) implantation of zircon were studied using micro-infrared (IR) and micro-Raman spectroscopy. The Pb+- and Au4+-irradiations caused a dramatic decrease in the IR reflectivity similar to that observed for metamict natural zircon. The irradiation with 10 MeV Au4+ ions (to fluences of 1x10^15 Au4+ ions/cm2) also results in the formation of an amorphized phase similar to that observed in metamict zircon. These results show that high-energy, heavy-ion irradiations provide a good simulation of the ballistic effects of the recoil nucleus of an alpha-decay event, and in both cases, the result is the creation of aperiodic domains. Additional IR and Raman features were recorded in samples irradiated with 280 keV Pb+ ions (to fluences of 1x10^14 and 1x10^15 Pb+ ions/cm2), indicating the formation of an irradiation-induced new phase(s). The frequencies of the features are consistent with lead silicates, ZrO2 and SiO2. The results show that spectral features of the Au4+- and Pb+-irradiated zircon are different from those of quenched ZrSiO4 melts, and the finding further confirms that the amorphous state produced by high-energy ion irradiations is structurally different from the glassy state that results from quenching a high temperature melt. In contrast to significant changes in the frequency and width of the Raman 3 band observed in metamict zircon, the Pb+- and Au4+-irradiations do not cause similar variations, indicating that the remaining zircon crystalline domains in irradiated samples have a crystalline structure with fewer defects than those of metamict zircon.

Zhang Y, M Elfman, BD Milbrath, and WJ Weber. 2008. "Evaluate Scintillation Response Over a Continuous Energy Region." IEEE Transactions on Nuclear Science 55(3):1097-1101. doi:10.1109/TNS.2008.922821 Abstract A recently developed fast analysis technique utilizing a time of flight (TOF) telescope is demonstrated to obtain relevant quantitative data on material scintillation response to energetic He particles. With superior energy resolution and fast response of the TOF telescope, energy of individual particle before impinging on a scintillating crystal can be determined with a high counting rate, which allows quantitative study of material performance over a continuous energy range in a relatively short time. Scintillation performances in terms of light output, nonlinearity and energy resolution in bismuth germinate (BGO) and europium-doped calcium fluoride (CaF2:Eu) Crystals are demonstrated, and the corresponding energy resolution is compared with gamma-ray tests on the same crystals.

Zhang Y, IT Bae, and WJ Weber. 2008. "Atomic Collision and Ionization Effects in Oxides." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 266(12-13):2828-2833. doi:10.1016/j.nimb.2008.03.197 Abstract Irradiation with ions and electrons provides accelerated study of radiation damage in nuclear materials, such as those proposed for immobilization of actinides and long-lived fission products. The effects of ion irradiation in SrTiO3, Sm2Ti2O7 and Sr2Nd8(SiO4)6O2, as representative materials, are studied using 1 MeV Au+ ions. The irradiation-induced disorder, due to atomic collisions processes, increases nonlinearly with irradiation dose and is well described by a disorder accumulation model that includes contributions from amorphous domains, point defects and defect clusters. Ionization from 200 keV electrons induces recrystallization at the amorphous/crystalline (a/c) interface that exhibits several distinct stages associated with residual defect annihilation near the interface, epitaxial regrowth at the interface, and a surface-stabilized amorphous state. Understanding ionization effects and the coupled effects of electronic and atomic dynamics on material behavior is a challenging area for scientific research.

Xiao HY, F Gao, LM Wang, XT Zu, Y Zhang, and WJ Weber. 2008. "Structural Phase Transitions in High-Pressure Wurtzite to Rocksalt Phase in GaN and SiC." Applied Physics Letters 92(24):Art. No. 241909. doi:10.1063/1.2938724 Abstract Ab initio molecular dynamics simulations are employed to study the atomistic mechanisms and pathways of high-pressure phase transformation in GaN and SiC. Our simulations bring a fundamental level of understanding of the wurtzite to rocksalt phase transformation that undergoes inhomogeneous displacements via a tetragonal atomic configuration, and suggest that the transition path may be independent of the presence of d electrons on the cation in GaN. The discrepancies between experimental and theoretical studies of transition paths are discussed.

Xiao HY, XT Zu, F Gao, and WJ Weber. 2008. "First-principles study of energetic and electronic properties of A2Ti2O7 (A=Sm, Gd, Er) pyrochlore." Journal of Applied Physics 104(7):Art. No. 073503. doi:10.1063/1.2986156 Abstract First-principles calculations have been carried out to study the electronic properties of A2Ti2O7 (A=Sm, Gd, Er) pyrochlores. It was found that f electrons have negligible effect on the structural and energetic properties, but have significant effect on the electronic properties. Density of state analysis shows that A-site 4f electrons do take part in the chemical bonding. Also, we found that <Gd-O48f> bond is less covalent than <Sm-O48f> and <Er-O48f> bonds, while <Ti-O48f> bond in Gd2Ti2O7 is more covalent. It was proposed that for A2Ti2O7 (A = Sm, Gd, Er) pyrochlores, <Ti-O48f> bonds may play more significant role in determining their radiation resistance to amorphization.

Xiao HY, XT Zu, F Gao, and WJ Weber. 2008. "Ab initio calculations of structural and energetic properties of defects in gallium nitride ." Journal of Applied Physics 103(12):123529. doi:10.1063/1.2947604 Abstract Ab initio total energy calculations have been performed to investigate the properties of intrinsic defects in GaN. It is found that the nitrogen defects are more stable than the Ga defects under nitrogen-rich conditions, and the results are generally consistent with those obtained by recent first-principles calculations. For the four types of nitrogen interstitials investigated, relaxation of all configurations leads to a N–N<11-20> split configuration. The most stable configuration for Ga interstitials is the Ga octahedral interstitial, but the energy difference between the octahedral and tetrahedral configurations is small (<0.35 eV) and depends on the basis set employed. While the _N–N_ bond distance in the N–N split interstitial is very close to that of a free N2 molecule, the Mulliken charge analysis indicates that the N atoms are partially charged, which is in contrast with previous theoretical suggestions. Based on the calculated results, the relative stabilities of various defects in GaN are determined.

Weber WJ, L Wang, Y Zhang, W Jiang, and IT Bae. 2008. "Effects of dynamic recovery on amorphization kinetics in 6H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 266(12-13):2793-2796. doi:10.1016/j.nimb.2008.03.119 Abstract The irradiation-induced amorphization in single crystal 6H-SiC has been previously studied as a function of irradiation temperature for electrons and ions ranging from Ne to Au. Analysis of these data in terms of a dynamic model for amorphization reveals that the amorphization dose increases and critical amorphization temperature decreases as the ratio of in-cascade ionization to displacement rates increases. Model fits to the data yield values for the ratio of radiation-induced cross section to damage cross section and an activation energy of 0.12 ± 0.01 eV for irradiation-induced recovery. The critical temperature exhibits a linear dependence on the ln(cross-section ratio), consistent with the model. The results also indicate that damage rate effects associated with thermal recovery may play a role near the critical temperature.

Wang Z, XT Zu, HY Xiao, F Gao, and WJ Weber. 2008. "Tuning the band structures of single walled silicon carbide nanotubes with uniaxial strain: a first principles study." Applied Physics Letters 92(18):183116. doi:10.1063/1.2924307 Abstract Electronic band structures of single-walled silicon carbide nanotubes are studied under uniaxial strain using first principles calculations. The band structure can be tuned by mechanical strain in a wide energy range. The band gap decreases with uniaxial tensile strain, but initially increases with uniaxial compressive strain and then decreases with further increases in compressive strain. These results may provide a way to tune the electronic structures of silicon carbide nanotubes, which may have promising applications in building nanodevices.

Wang Z, XT Zu, L Yang, F Gao, and WJ Weber. 2008. "Orientation and temperature dependence of the tensile behavior of GaN nanowires: an atomistic study." Journal of Materials Science. Materials in Electronics 19(8-9):863-867. doi:10.1007/s10854-007-9526-8 Abstract Gallium nitride (GaN) is a high-temperature semiconductor material of considerable interest. It emits brilliant light and has been considered as a key material for the next generation of high frequency and high power transistors that are capable of operating at high temperatures. Due to its anisotropic and polar nature, GaN exhibits direction-dependent properties. Growth directions along [001], [1-10] and [110] directions have all been synthesized experimentally. In this work, molecular dynamics simulations are carried out to characterize the mechanical properties of GaN nanowires with different orientations at different temperatures. The simulation results reveal that the nanowires with different growth orientations exhibit distinct deformation behavior under tensile loading. The nanowires exhibit ductility at high deformation temperatures and brittleness at lower temperature. The brittle to ductile transition (BDT) was observed in the nanowires grown along the [001] direction. The nanowires grown along the [110] direction slip in the {010} planes, whereas the nanowires grown along the [1-10] direction fracture in a cleavage manner under tensile loading.

Wang Z, XT Zu, F Gao, and WJ Weber. 2008. "Nanomechanical Behavior of Single Crystalline SiC Nanotubes Revealed by Molecular Dynamics Simulations." Journal of Applied Physics 104(9):093506. doi:10.1063/1.3005979 Abstract Molecular dynamics simulations with Tersoff potentials were used to study the response of single crystalline SiC nanotubes under tensile, compressive, torsional, combined tension-torsional and combined compression-torsional strains. The simulation results reveal that the nanotubes deform through bond-stretching and breaking and exhibit brittle properties under uniaxial tensile strain, except for the thinnest nanotube at high temperatures, which fails in a ductile manner. Under uniaxial compressive strain, the SiC nanotubes buckle with two modes, i.e. shell buckling and column buckling, depending on the length of the nanotubes. Under torsional strain, the nanotubes buckle either collapse in the middle region into a dumbbell-like structure for thinner wall thicknesses or fail by bond breakage for the largest wall thickness. Both the tensile failure stress and buckling stress decrease under combined tension-torsional and combined compression-torsional strain, and they decrease with increasing torsional rate under combined loading.

Wang Z, X Zu, L Yang, F Gao, and WJ Weber. 2008. "Molecular dynamics simulation on the buckling behavior of GaN nanowires under uniaxial compression." Physica. E, Low-dimensional systems & nanostructures 40(3):561-566. doi:10.1016/j.physe.2007.08.040 Abstract Molecular dynamics simulation is one of the most promising methods for investigating the mechanical behavior of nanostructures, such as nanowires and nanotubes. Atomistic simulations are performed to investigate the buckling properties of [001], [1ī0] and [110] oriented GaN nanowires under uniaxial compression, these three types of nanowires correspond to experimentally synthesized nanowires. The effects of simulation temperature, and wire length on the buckling behavior are investigated. The simulation results show that critical stress decreases with the increase of wire length, which is in agreement with the Euler theory. Buckling occurs as a result of dynamic processes, buckling strain (and corresponding stress) decreases as temperature is increased.

Wang Z, XT Zu, F Gao, and WJ Weber. 2008. "Mechanical behavior of gallium nitride nanotubes under combined tension-torsion: An atomistic simulation." Journal of Applied Physics 103(1):013505, (3 pages). doi:10.1063/1.2828169 Abstract The tensile mechanical behavior of single crystalline gallium nitride (GaN) nanotubes under combined tension-torsion is investigated using molecular dynamics simulations with an empirical potential. The simulation results show that a small torsion rate (<0.010° ps-1) does not affect the tensile behavior of GaN nanotube, i.e. the nanotubes show brittle properties at low temperatures; whereas at high temperatures, they behave as ductile materials. However, the failure stress decreases with increasing rate of torsion above 0.010° ps-1, and the nanotube fails in a different manner. The torsion rate has no effect on the elastic properties of GaN nanotubes.

Wang Z, XT Zu, L Yang, F Gao, and WJ Weber. 2008. "Buckling of GaN nanotubes under uniaxial compression." Solid State Communications 146(7-8):293-297. doi:10.1016/j.ssc.2008.03.004 Abstract Molecular dynamics simulations with a Stillinger-Weber potential are used to study the buckling behavior of single crystalline GaN nanotubes under uniaxial compression. The simulation results show the buckling behavior of GaN nanotubes is strongly dependent on the simulation temperature, tube length and strain rate. The buckling stress decreases with the increase of simulation temperature and tube length. The dependence of buckling on the tube length is consistent with the analysis of equivalent continuum structures using Euler buckling theory.

Wang Z, XT Zu, F Gao, and WJ Weber. 2008. "Atomistic simulations of the mechanical properties of silicon carbide nanowires." Physical Review. B, Condensed Matter and Materials Physics 77(22):224113, 1-10. doi:10.1103/PhysRevB.77.224113 Abstract Molecular dynamics methods using the Tersoff bond-order potential are performed to study the nanomechanical behavior of [111]-oriented β-SiC nanowires under tension, compression, torsion, combined tension-torsion and combined compression-torsion. Under axial tensile strain, the bonds of the nanowires are just stretched before the failure of nanowires by bond breakage. The failure behavior is found to depend on size and temperatures. Under axial compressive strain, the collapse of the SiC nanowires by yielding or column buckling mode depends on the length and diameters of the nanowires, and the latter is consistent with the analysis of equivalent continuum structures using Euler buckling theory. The nanowires collapse through a phase transformation from crystal to amorphous structure in several atomic layers under torsion strain. Under combined loading the failure and buckling modes are not affected by the torsion with a small torsion rate, but the critical stress decreases with increasing the torsion rate. Torsion buckling occurs before the failure and buckling with a big torsion rate. Plastic deformation appears in the buckling zone with further increasing the combined loading.

Wang Z, XT Zu, F Gao, and WJ Weber. 2008. "Atomistic Level Studies on the Tensile Behavior of GaN Nanotubes under Uniaxial Tension." The European Physical Journal. B. 61(4):413-418. doi:10.1140/epjb/e2008-00091-3 Abstract Molecular dynamics method with the Stillinger-Weber (SW) potential has been employed to study the responses of GaN nanotubes (Ga NNTs) to a uniaxial tensile load along the axial direction. It has been revealed that GaNNTs exhibits a completely different tensile behavior at different temperatures, i.e., ductility at higher deformation temperatures and brittleness at lower temperatures, leading to a brittle to ductile transition (BDT). Both the BDT temperature and the critical stress increases with increasing thickness of GaNNTs and the critical stress at higher tmperature are lower than those at lower temperature. These results on the tensile behaviors of GaNNTs in an atomic level will provide a good reference to its promising applications.

Rondinella VV, TA Wiss, JP Hiernaut, S Lutique, P Raison, D Staicu, WJ Weber, and T Fanghanel. 2008. "Factors Affecting the Stability of Matrix Materials for Actinides Transmutation and Conditioning." Transactions of the American Nuclear Society 98(1):831-832. Abstract The minimization of the long-term radiotoxicity of high level nuclear waste is an important criterion adopted for the development of advanced fuel cycles for the new generations of nuclear reactors. Pu recycling as fuel, and transmutation of Minor Actinides (MA: Np, Am, and in some concepts also Cm) in reactors and/or MA burners are the steps considered to achieve this goal. U-free compounds are considered as matrices for Pu, MA burning. In some cases, these matrices are envisaged also for the conditioning and immobilization of radionuclides in final disposal concepts. The list of properties of a good inert matrix includes good chemical compatibility with the actinides, easy and economical processes of fabrication and, if required, reprocessing, and good thermo-mechanical performance in-pile, in terms of thermal transport, swelling and high temperature stability. In addition, the material must retain the good properties under the cumulative effect of radiation damage, and fission product accumulation. Since good radiation resistance materials usually exhibit poor thermal transport, in some concepts the actinides are stabilized in a host phase (e.g. zirconia) dispersed in a high thermal conductivity matrix (either ceramic or metallic).

Milbrath BD, AJ Peurrung, M Bliss, and WJ Weber. 2008. "Radiation Detector Materials: An Overview." Journal of Materials Research 23(10):2561-2581. doi:10.1557/JMR.2008.0319 Abstract This review describes the current state of radiation detection material science, with particular emphasis on national security needs and the goal of identifying the challenges and opportunities that this area represents for the materials science community. Radiation detector materials physics is reviewed, which sets the stage for performance metrics that determine the relative merit of existing and new materials. Semiconductors and scintillators represent the two primary classes of radiation detector materials that are of interest. The state-of-the-art and limitations for each of these materials classes are presented, along with possible avenues of research. Novel materials that could overcome the need for single crystals will also be discussed. Finally, new methods of material discovery and development are put forward – the goal being to provide more predictive guidance and faster screening of candidate materials – and thus ultimately the faster development of superior radiation detection materials.

Ishimaru M, A Hirata, M Naito, IT Bae, Y Zhang, and WJ Weber. 2008. "Direct observations of thermally induced structural changes in amorphous silicon carbide." Journal of Applied Physics 104(3):033503, 1-5. doi:10.1063/1.2960342 Abstract Thermally induced structural relaxation in amorphous silicon carbide (SiC) has been examined by means of in situ transmission electron microscopy (TEM). The amorphous SiC was prepared by high-energy ion-beam-irradiation into a single crystalline 4H-SiC substrate. Cross-sectional TEM observations and electron energy-loss spectroscopy measurements revealed that thermal annealing induces a remarkable volume reduction, so-called densification, of amorphous SiC. From radial distribution function analyses using electron diffraction, notable changes associated with structural relaxation were observed in chemical short-range order. On the basis of the alteration of chemical short-range order, we discuss the origin of thermally induced densification in amorphous SiC.

Gao F, LW Campbell, YL Xie, R Devanathan, AJ Peurrung, and WJ Weber. 2008. "Electron-Hole Pairs Created by Photons and Intrinsic Properties in Detector Materials." IEEE Transactions on Nuclear Science 55(3):1079-1085. doi:10.1109/TNS.2007.908917 Abstract A Monte Carlo (MC) code has been developed to simulate the interaction of gamma-rays with semiconductors and scintillators, and the subsequent energy partitioning of fast electrons. The results provide insights on the processes involved in the electron-hole pair yield and intrinsic variance through simulations of full electron energy cascades. The MC code has been applied to simulate the production of electron-hole pairs and to evaluate intrinsic resolution in a number of semiconductors. In addition, the MC code is also able to consider the spatial distribution of electron-hole pairs induced by photons and electrons in detector materials, and has been employed to obtain details of the spatial distribution of electron-hole pairs in Ge, as a benchmark case. The preliminary results show that the distribution of electron-hole pairs exhibit some important features; (a) the density of electron-hole pairs along the main electron track is very high and (b) most electron-hole pairs produced by interband transitions are distributed at the periphery of the cascade volume. The spatial distribution and density of thermalized electron-hole pairs along the primary and secondary tracks are important for large scale simulations of electron-hole pair transport.

Gao F, Y Zhang, M Posselt, and WJ Weber. 2008. "Computational Study of Anisotropic Epitaxial Recrystallization in 4H-SiC." Journal of Physics. Condensed matter 20(12):Art. No. 125203. doi:10.1088/0953-8984/20/12/125203 Abstract Two nano-sized amorphous layers were employed within a crystalline cell to study anisotropic expitaxial recrystallization using molecular dynamics (MD) methods in 4H-SiC. Both amorphous layers were created with the normal of the amorphous-crystalline (a-c) interfaces along the [0001] direction, but one with a microscopic extension long the [0001] direction, i.e. the dimension along the [-12-10] direction is much larger than that along the [-12-10] direction (Ix model), and another with a microscopic extension long the [-1010] direction (Iy model). The amorphous layer within the Ix model can be completely recrystallized at 2000 K within achievable simulation time, and the recrystallization is driven by a step-regrowth mechanism. On the other hand, the nucleation and growth of secondary ordered phases are observed at high temperatures in the Iy model. The temperature for recrystallization of the amorphous layer into high quality 4H-SiC is estimated to be below 1500 K. As compared with other models, it is found that the regrowth rates and recrystallization mechanisms strongly depend on the orientation of 4H-SiC, whereas the activation energy spectra for recrystallization processes are independent on a specific polytypic structure, with activation energies ranging from 0.8 to 1.7 eV.

Devanathan R, and WJ Weber. 2008. "Dynamic annealing of defects in irradiated zirconia-based ceramics." Journal of Materials Research 23(3):593-597. doi:10.1557/JMR.2008.0104 Abstract We have observed self-healing behavior in large scale molecular dynamics simulations of 30 keV Zr recoils in pure zirconia and 10 mole % yttria-stabilized zirconia. Our results reveal that dynamic annealing is highly effective during the first 5 ps of damage evolution, especially in the presence of oxygen structural vacancies introduced by aliovalent doping (Y3+ substitution for Zr4+). The presence of mobile oxygen vacancies results in near complete recovery of damage. Damage recovery on the cation sublattice is assisted by the anion sublattice recovery, which explains the remarkable radiation tolerance of stabilized zirconia. Ceramics engineered to heal themselves in this fashion hold great promise for use in high-radiation environments or for safely encapsulating high-level radioactive waste over geological time scales.

Bae IT, Y Zhang, WJ Weber, M Ishimaru, Y Hirotsu, and M Higuchi. 2008. "Temperature dependence of electron-beam induced effects in amorphous apatite ." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 266(12-13):3037-3042. doi:10.1016/j.nimb.2008.03.160 Abstract Irradiation effects on pre-amorphized Sr2Nd8(SiO4)6O2 have been investigated under 200 and 300 keV electron-beam irradiation at 130 and 480 K using in situ transmission electron microscopy. At 480 K, recrystallization occurred from the amorphous/crystalline interface under both 200 and 300 keV e-beam irradiation. At 130 K, the 200 keV e-beam irradiation induced recrystallization only; however, 300 keV e-beam irradiation induced both recrystallization and an electron hammering effect in the amorphous material that resulted in radial expansion perpendicular to the incident electron-beam direction and shrinkage parallel to the electron-beam direction. Ionization-induced processes and knock-on displacement damage are suggested to be the mechanisms for the recrystallization and the electron hammering effect, respectively.

Bae IT, Y Zhang, WJ Weber, M Ishimaru, Y Hirotsu, and M Higuchi. 2008. "Ionization-induced effects in amorphous apatite at elevated temperatures." Journal of Materials Research 23(4):962-967. doi:10.1557/JMR.2008.0114 Abstract Electron-beam induced effects in pre-amorphized Sr2Nd8(SiO4)6O2 is investigated in situ using transmission electron microscopy with 200 keV electrons at temperatures ranging from 380 to 780 K. While epitaxial recrystallization is observed from amorphous/crystalline toward surface within the electron irradiated area with its rate increasing as temperature increases from 380 to 580 K, structural contrast features (i.e., O deficient amorphous materials) as well as recrystallization are observed even outside of the irradiation area at temperatures from 680 to 780 K. Ionization-induced processes and local non-stoichiometry induced by oxygen migration and desorption are possible mechanisms for the electron-beam induced recrystallization and for the formation of the structural contrast features, respectively.

Weber WJ, LR Corrales, R Devanathan, F Gao, and HL Heinisch. 2007. "" Abstract The Eighth International Conference on Computer Simulation of Radiation Effects in Solids (COSIRES 2006) was hosted by Pacific Northwest National Laboratory in Richland, WA, USA from 18-23 June 2006. This conference series started in 1992 in Berlin, Germany. Since then, the conference has been held biennially in Santa Barbara, CA, USA (1994); Guildford, UK (1996); Okayama, Japan (1998); State College, PA, USA (2000); Dresden, Germany (2002); and Helsinki, Finland (2004). The COSIRES conferences are the foremost international forum on the theory, development and application of advanced computer simulation methodologies to achieve fundamental understanding and predictive modeling of the interaction of energetic particles and clusters with solids. This scope includes subsequent defect interactions and microstructure evolution as functions of environmental variables, such as energy and temperature, and over multiple spatial and temporal scales. Fundamental understanding and interpretation of radiation effects processes are often not accessible by experimental methods, since they occur on very small time and length scales. Thus, these computer simulation methodologies have proven to be very useful for the study of fundamental processes and, as can be observed in these proceedings, they are now becoming increasingly applied to the development of predictive models of materials processing and performance for advanced industrial and energy-production technologies. By all indications, this is an active and thriving area of research with potential for growth. We greatly appreciated the opportunity to host 96 participants from 15 different countries, one third of whom were graduate students or post docs who provided youthful enthusiasm and new perspectives to the presentations and discussions. The success of the meeting was to a large extent made possible by generous financial support from the Pacific Northwest National Laboratory, Cray, Inc., and IBM. We are very grateful for the support received, as well as the efforts provided for the meeting by the international advisory committee, program committee and most of all the local organizing committee. Without the assistance of the local organizing committee, the meeting would not have run as smoothly and pleasantly as it did. We especially want to thank Yanwen Zhang for managing the manuscript submission and review process, Sue Finch for financial management, and Shirley Alderson for hotel, event, and tour arrangements.

Zhang Y, J Jensen, G Possnert, DA Grove, IT Bae, and WJ Weber. 2007. "Stopping power measurements of He ions in Si and SiC by time-of-flight spectrometry." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 261(1-2):1180-1183. doi:10.1016/j.nimb.2007.04.276 Abstract Electronic energy loss is the fundamental mechanism accountable for the response of materials to ions. Helium particles are a product from alpha decay in nuclear waste materials, and helium ions are widely used as projectiles in ion beam analysis. Current work introduces a straightforward approach to determine electronic stopping powers of He ions in Si and SiC over a continuous range of energies. In transmission geometry, the energy loss of He ions in self-supporting stopping foils of Si and SiC is measured using a Time-of-Flight (TOF) set-up. The energy of individual heavy ions prior to impingement on the foil is determined from its TOF data; the exit energy after the stopping foil is essentially measured using the Si detector, for which every channel has been calibrated using TOF data without the stopping foil present. The measured stopping powers demonstrate excellent repeatability of this experimental approach and good reliability as compared with pervious data from the literature and theoretical predictions.

Zhang Y, BD Milbrath, WJ Weber, M Elfman, and HJ Whitlow. 2007. "Radiation detector resolution over a continuous energy range." Applied Physics Letters 91(09):094105, 1-3. doi:10.1063/1.2776978 Abstract Urgent need for new radiation detector materials with excellent energy resolution at room temperature has prompted research efforts on both accelerated materials discovery and efficient techniques that can investigate candidate materials to identify material properties relevant to detector performance. In this work, an ion-beam approach is demonstrated to obtain energy resolution in both semiconductor detectors and scintillators over a continuous energy range. For semiconductors, the energy resolution of a silicon detector was measured as a function of He+ ion energy, and the values from an extrapolation to high energies are in good agreement with the literature data from alpha measurements. For the scintillators, benchmark crystals of BGO, YAP(Ce) and CsI(Tl) subject to He+ irradiation were investigated, and the corresponding energy resolution is compared with gamma-ray tests on the same crystals. The agreement of energy resolution between the ion and gamma measurements indicates that the ion approach can be used to predict energy resolution of candidate materials in thin film form (a few tens m thick) or a small crystals (a few mm3), when large crystals necessary for gamma-ray testing are unavailable.

Zhang Y, F Gao, R Devanathan, and WJ Weber. 2007. "A Fast Screening Technique to Evaluate Detector Response." Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment 579(1):108-112. doi:10.1016/j.nima.2007.04.019 Abstract A fast screening technique to evaluate detector response was demonstrated using a silicon detector. Pulse height was measured for H, He, Be, C, O, Mg, Si, Ni, Zr and Au ions over a wide energy range using a time-of-flight (TOF) telescope. Using scattering or recoil process, a secondary beam with a continuous energy distribution but low intensity is generated to avoid direct beam exposure of the Si detector. Prior to impinging on the Si detector, the energy of individual ions is determined from the time of flight and its tabulated isotopic mass. The pulse height-energy calibration for ions with a given atomic number can be described by a linear relationship with small systematic deviations. For particles that have the same velocity (~500 keV/nucleon), a non-linear dependence on efficiency of electron–hole pair collection is observed as a function of electronic stopping power. The detector response is studied using He ions, and the measured energy resolution is given as functions of deposition energies over a wide energy range.

Wang Z, F Gao, JP Crocombette, X Zu, L Yang, and WJ Weber. 2007. "Thermal Conductivity of GaN Nanotubes Simulated by Nonequilibrium Molecular Dynamics." Physical Review. B, Condensed Matter 75(12):Art. No. 153303. doi:10.1103/PhysRevB.75.153303 Abstract Thermal conductivity of GaN nanotubes along the tube axis is investigated over the temperature range of 600K-2300K using homogeneous nonequilibrium molecular dynamics. In general, the thermal conductivity of nanotubes is smaller than that for the bulk GaN single crystal. The thermal conductivity is also found to decrease with temperature and increase with increasing wall thickness of the nanotubes. The change of phonon spectrum and surface inelastic scattering may account for the reduction of thermal conductivity in the nanotubes, while thermal softening and high frequency phonon interactions at high temperatures may provide an explanation for its decrease with increasing temperature.

Wang Z, X Zu, F Gao, and WJ Weber. 2007. "Size dependence of melting of GaN nanowires with triangular cross sections." Journal of Applied Physics 101(4):043511. doi:10.1063/1.2512140 Abstract Molecular dynamics simulations have been used to study the melting of GaN nanowires with triangular cross-sections. The curve of the potential energy, along with the atomic configuration is used to monitor the phase transition. The thermal stability of GaN nanowires is dependent on the size of the nanowires. The melting temperature of the GaN nanowires increases with the increasing of area cross-section of the nanowires to a saturation value. An interesting result is that of the nanowires start to melt from the edges, then the surface, and extends to the inner regions of nanowires as temperature increases.

Wang Z, X Zu, F Gao, and WJ Weber. 2007. "Atomistic Study of the Melting Behavior of Single Crystalline Wurtzite Gallium Nitride Nanowires." Journal of Materials Research 22(3):742-747. doi:10.1557/JMR.2007.0095 Abstract Molecular dynamics (MD) simulation was used to study the melting behavior of GaN nanowires with Stillinger-Webber (SW) potential. Our results reveal that the melting of nanowires starts from the surface, and rapidly extends to the inner regions of nanowires as temperature increases. The melting temperature of GaN nanowires is lower than that of the bulk GaN, which may associate with large surfaces of nanowires. The melting temperatures increase to saturation values ~3100K and ~2900K when the diameters of nanowires are larger than 3.14 and 4.14 nm for nanowires with [100]- and [110]-oriented lateral facets, respectively.

Wang Z, X Zu, L Yang, F Gao, and WJ Weber. 2007. "Atomistic Simulations of the Size, Orientation and Temperature Dependence of Tensile Behavior in GaN Nanowires." Physical Review. B, Condensed Matter and Materials Physics 76(4):Art. No. 045310. doi:10.1103/PhysRevB.76.045310 Abstract Molecular dynamics simulations with Stillinger-Weber potentials were used to study the response of wurtzite-type single crystalline GaN nanowires to a tensile strain along the axial direction. Nanowires with axial orientations along the [0001], [1 00] and [11 0] crystallographic directions, which correspond to experimentally synthesized nanowires, were studied. The results reveal that the nanowires with different axial orientations show distinctly different deformation behavior under loading. The brittle to ductile transition (BDT) was observed in the nanowires oriented along the [0001] direction and the BDT temperatures lie in the temperature range between 1500 and 1800 K. The nanowires oriented along the [11 0] direction exhibit slip in the {01 0} planes; whereas the nanowires oriented along the [1 00] direction fracture in a cleavage manner under tensile loading. It should be emphasized that multiple yield stresses were observed during different stages in the [11 0]-oriented nanowires. In general, Young's modulus of the GaN nanowires decreases with decreasing diameter of the nanowires.

Wang Z, X Zu, F Gao, WJ Weber, and JP Crocombette. 2007. "Atomistic Simulation of the Size and Orientation Dependences of Thermal Conductivity in GaN Nanowires." Applied Physics Letters 90(16):Art. No. 161923. doi:10.1063/1.2730747 Abstract The thermal conductivity of GaN nanowires has been determined computationally, by applying nonequilibrium atomistic simulation methods using the Stillinger-Weber [Phys. Rev. B 31, 5262 (1985)] potentials. The simulation results show that the thermal conductivity of the GaN nanowires is smaller than that of a bulk crystal and increases with increasing diameter. Surface scattering of phonons and the high surface to volume ratios of the nanowires are primarily responsible for the reduced thermal conductivity and its size dependence behavior. The thermal conductivity is also found to decrease with increasing temperature, which is due to phonon-phonon interactions at high temperatures. The thermal conductivity also exhibits a dependence on axial orientation of the nanowires.

Rong Z, F Gao, and WJ Weber. 2007. "Monte Carlo Simulations of Defect Recovery within a 10 keV Collision Cascade in 3C-SiC." Journal of Applied Physics 102(10):103508, 1-7. doi:10.1063/1.2812701 Abstract A kinetic lattice Monte Carlo (KLMC) model is developed to investigate the recovery and clustering of defects during annealing of a single 10 keV cascade in cubic silicon carbide. The 10 keV Si cascade is produced by molecular dynamics (MD), and a method of transferring the defects created by MD simulations to the KLMC model is developed. The KLMC model parameters are obtained from molecular dynamics simulations and ab initio calculations of defect migration, recombination and annihilation. The defects are annealed isothermally from 100 K to 1000 K in the KLMC model. Two distinct recovery stages for close Frenkel pairs are observed at about 200 and 550 K, and the growth of complex clusters is observed above 400 K. These simulation results are in good agreement with available experimental results.

Oda T, Y Oya, S Tanaka, and WJ Weber. 2007. "Validation of Potential Models for Li2O in Classical Molecular Dynamics Simulation." Journal of Nuclear Materials 367-370:263-268. doi:10.1016/j.jnucmat.2007.03.139 Abstract Four Buckingham-type pairwise potential models for Li2O were assessed by molecular static and dynamics simulations. In the static simulation, all models afforded acceptable agreement with experimental values and ab initio calculation results for the crystalline properties. Moreover, the superionic phase transition was realized in the dynamics simulation. However, the Li diffusivity and the lattice expansion were not adequately reproduced at the same time by any model. When using these models in future radiation simulation, these features should be taken into account, in order to reduce the model dependency of the results.

Jiang W, P Nachimuthu, WJ Weber, and L Ginzbursky. 2007. "Variation in lattice parameters of 6H-SiC irradiated to extremely low doses." Applied Physics Letters 91(9):091918, 1-3. doi:10.1063/1.2778630 Abstract Irradiation of 6H-SiC single crystals was performed using 4 MeV H+ ions at 340 and 210 K. The changes in lattice parameters in the basal plane and along the c-axes were measured as a function of dose using high-resolution x-ray diffraction. The c-axis lattice parameter increases monotonically with the increasing dose, while a-axis lattice parameter decreases at extremely low doses. An initial volumetric contraction of the unit cell is observed. The decrease in the a parameter may originate from the irradiation-induced vacancies and the possible formation of antisite defects that cause the lattice structure on the basal plane to shrink.

Jiang W, IT Bae, and WJ Weber. 2007. "Disordering and Dopant Behaviour in Au+ Ion-Irradiated AlN." Journal of Physics. Condensed matter 19(35):356207, 1-10. doi:10.1088/0953-8984/19/35/356207 Abstract Single-crystal AlN films on SiC were irradiated at 145 K with 1.0 MeV Au+ ions to a wide range of ion fluences. The accumulation of disorder on both the Al and N sublattices in AlN has been investigated in situ using conventional Rutherford backscattering spectrometry (RBS) and non-RBS along the <0001>-axial channelling direction. The results suggest that a disorder saturation stage is attained following an initial disorder increase at intermediate doses (< 10 dpa). A continuously amorphized layer was not formed in AlN for doses up to 208 dpa. Similar disordering behaviour is observed for the Al and N sublattices. The lattice disorder produced at 145 K is thermally stable at room temperature; further irradiation does not induce disorder recovery. The microstructures in the irradiated AlN exhibit both amorphous and crystalline domains at the stage of disorder saturation. The implanted Au does not show significant redistribution during the ion irradiation or room-temperature annealing.

Jiang W, Y Zhang, MH Engelhard, WJ Weber, GJ Exarhos, J Lian, and RC Ewing. 2007. "Behavior of Si and C atoms in ion amorphized SiC." Journal of Applied Physics 101(2):Art. No. 023524. doi:10.1063/1.2431941 Abstract Single crystal 6H-SiC wafers were fully amorphized at room temperature or 200 K using 1.0 or 2.0 MeV Au+ ion irradiation. The thickness of the amorphized layers has been determined using Rutherford backscattering spectrometry under ion channeling conditions. Microstructures of the irradiated SiC have been examined using cross-sectional transmission electron microscopy. The depth profiles of both the Si and C atoms have been studied using both x-ray photoelectron spectroscopy (XPS) and time-of-flight energy elastic recoil detection analysis. Neither Si nor C in the amorphized SiC exhibits a significant mass transport by diffusion during the irradiation and subsequent storage at room temperature. There is no observable phase segregation of either Si or C in the amorphized SiC. Ar+ ion sputtering leads to modifications of the composition, structure and chemical bonding at the 6H-SiC surface. The Si-Si bonds at the sputtered surface (amorphized) do not appear, as suggested by the XPS; however, Raman backscattering data reveals the existence of the Si-Si bonds in the bulk amorphized SiC, in addition to the C-C and Si-C bonds that the XPS also identified.

Gao F, LW Campbell, R Devanathan, YL Xie, LR Corrales, AJ Peurrung, and WJ Weber. 2007. "Monte Carlo Method For Simulating Gamma-Ray Interaction With Materials: A Case Study on Si." Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment 579(1):292-296. doi:10.1016/j.nima.2007.04.063 Abstract In the present work, a Monte Carlo (MC) method has been developed to simulate various quantum mechanical processes for energy loss of photons and fast electrons. The MC model is demonstrated by application to the interaction of photons with silicon over the energy range from 50 eV to 2 MeV and the following subsequent electron cascades. The electron cascade process is commonly represented by two macroscopic parameters, the mean energy required to create an electron-hole pair, W, and the Fano factor, F, describing the electron yield and its variance. At energies lower than 5 keV, W generally decreases with increasing photon energy (from 3.96 to 3.58 eV), and it exhibits a sawtooth variation, as observed previously. However, discontinuities at the shell edges follow the photoionization cross section, in contrast to previous results. The function, F(Ep), initially increases with increasing photon energy, Ep, to a maximum value of 0.187 around 155 eV, and then decreases at higher energies. Above the K shell edge, F has a value of 0.135. These results are consistent with experimental observations. The simulated distribution indicates that the interband transition and plasmon excitation are the most important mechanisms of electron-hole pair creation, while core shell ionization appears to be significant only at high energies.

Gao F, LW Campbell, R Devanathan, YL Xie, Y Zhang, AJ Peurrung, and WJ Weber. 2007. "Gamma-Ray Interaction in Ge: A Monte Carlo Simulation." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 255(1):286-290. doi:10.1016/j.nimb.2006.11.031 Abstract The interaction of X-ray and gamma-ray photons with materials is of fundamental interest to many fields. The interaction of these primary photons with atoms results in the creation of fast electrons. Electron-solid interaction can be determined experimentally by measuring physical quantities, such as scattering cross section, but the partitioning of energy loss into different quantum mechanical processes is important to determine the mean energy required to create an electron-hole pair, W, and the intrinsic variance (or Fano factor, F) for radiation detectors. In the present work, a Monte Carlo (MC) method previously developed has been employed to simulate the interaction of photons with Ge over the energy range from 50 eV to ~1 MeV and the subsequent electron cascades. Various quantum mechanical processes for energy loss of fast electrons, which control the broadening of Fano factor, are investigated in detail. At energies lower than 1 keV, W generally decreases with increasing photon energy from 2.95 to 2.75 in Ge, whereas it has a constant value of 2.64 eV for higher energies. Also, the function, F, decreases with increasing photon energy. Above the L shell edge, F has a value of 0.11 that is smaller than that in Si (0.14). However, F exhibits a sawtooth variation, and discontinuities at the shell edges follow the photoionization cross section. These results are in good agreement with experimental measurements. The simulated distribution indicates that the interband transition and plasmon excitation are the most important mechanisms of electron-hole pair creation in Ge, while core shell ionization appears to be significant only at high energies.

Gao F, Y Zhang, R Devanathan, M Posselt, and WJ Weber. 2007. "Atomistic Simulations of Epitaxial Recrystallization in 4H-SiC along the [0001] Direction." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 255(1):136-140. doi:10.1016/j.nimb.2006.11.016 Abstract Molecular dynamics (MD) methods have been employed to study the epitaxail recrystallization and amorphous-to-crystalline (a-c) transition in 4H-SiC, with simulation times of up to a few hundred ns and at temperatures of 1500 and 2000 K. Three nano-sized amorphous layers with the normal of a-c interfaces along the [-12-10 ], [-1010] and [0001] directions, respectively, were created within a crystalline cell to investigate the anisotropies of recrystallization processes. The recovery of bond defects at the interfaces is an important process driving the initial epitaxial recrystallization of the amorphous layers. The amorphous layers with the a-c interface normal along the [-12-10] direction can be completely recrystallized at the temperatures of 1500 and 2000 K, and along the [0001] direction at 2000 K. However, the recrystallized region is defected with dislocations and stacking faults. The temperatures required for complete recrystallization are in good agreement with those observed in experiments. On the other hand, the recrystallization processes for the a-c interface normal along [-1010] direction are hindered by the nucleation of polycrystalline phases. These secondary ordered phases have been identified as 4H- and 3C-SiC with different crystallographic orientations to the original 4H-SiC. The bond mismatches at the interfaces between different microcrystals result in the formation of a number of stacking faults. The temperature is an important parameter to control the nucleation of the secondary ordered phase, whereas the size of amorphous region has a significant effect on their growth. These results are in good agreement with the previous experimental observations.

Gao F, J Du, EJ Bylaska, M Posselt, and WJ Weber. 2007. "Ab Initio Atomic Simulations of Antisite Pair Recovery in Cubic Silicon Carbide." Applied Physics Letters 90(22):Art. No. 221915. doi:10.1063/1.2743751 Abstract The thermal stability of an antisite pair in 3C-SiC is studied using ab initio molecular dynamics within the framework of density functional theory. The lifetime of the antisite pair configuration is calculated for temperatures between 1800 and 2250 K, and the effective activation energy for antisite pair recombination is determined to be 2.52 eV. The recombination energy path and static energy barrier are also calculated using the nudged elastic band method, along with the dimer method to accurately locate the transition states. The consistency of the results suggests that the antisite pair cannot be correlated with the DI photoluminescence center, as proposed by previously theoretical interpretations. An extended exchange mechanism is found for the antisite pair recombination, and this may be a dominant mechanism for antisite pair recombination and diffusion of impurities in compound semiconductors.

Farnan IE, HM Cho, and WJ Weber. 2007. "QUANTIFICATION OF ACTINIDE ALPHA-RADIATION DAMAGE IN MINERALS AND CERAMICS." Nature 445:190-193. doi:10.1038/nature05425 Abstract There are large amounts of heavy alpha-emitters in nuclear waste and nuclear materials inventories stored in various sites around the world. These include plutonium and minor actinides such as americium and curium. In preparation for geological disposal there is a consensus that actinides that have been separated from spent nuclear fuel should be immobilised within mineral-based ceramics rather than glass. Over the long-term, the alpha-decay taking place in these ceramics will severely disrupt their crystalline structure and reduce their durability. A fundamental property in predicting cumulative radiation damage is the number of atoms permanently displaced per alpha–decay. Currently, this number is estimated as 1000-2000 atoms/alpha decay event. Here, we report nuclear magnetic resonance, spin-counting experiments that measure close to 5000 atoms/alpha decay event in radiation damaged natural zircons. New radiological NMR measurements on highly radioactive, 239Pu zircon show damage similar to that created by 238U and 232Th in mineral zircons at the same dose, indicating no significant effect of dose rate. Based on these measurements, the initially crystalline structure of a 10 wt% 239Pu zircon would be amorphous after only 1400 years in a geological repository. These measurements establish a basis for assessing the long-term structural durability of actinide-containing ceramics based on an atomistic understanding of the fundamental damage event.

Devanathan R, and WJ Weber. 2007. "Radiation Effects in a Model Ceramic for Nuclear Waste Disposal." JOM. The Journal of the Minerals, Metals and Materials Society 59(4):32-35. Abstract The safe immobilization of nuclear waste in geological repositories is one of the major scientific challenges facing humanity today. Crystalline ceramics hold the promise of locking up actinides from nuclear fuel and excess weapons plutonium in their structure thereby isolating them from the environment. In this paper, we discuss the atomistic details of radiation damage in a model ceramic, zircon.

Devanathan R, F Gao, and WJ Weber. 2007. "Atomistic modeling of amorphous silicon carbide using a bond-order potential." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 255(1):130-135. doi:10.1016/j.nimb.2006.11.045 Abstract Molecular dynamics simulations were performed with a Brenner-type bond-order potential to study the melting of silicon carbide (SiC), the structure of amorphous SiC produced by quenching from the melt, and the evolution of the amorphous state after isochronal annealing at elevated temperatures. The simulations reveal that SiC melts above 3700 K with an enthalpy of fusion of about 0.6 eV/atom. The density of the quenched liquid is about 2820 kg/m3, in excellent agreement with the experimental value for SiC amorphized by neutron irradiation. In addition to the loss of long-range order, the quenched liquid shows short-range disorder as measured by the C homonuclear bond ratio. Upon annealing, there is partial recovery of shortrange order.

Crocombette JP, G Dumazer, NQ Hoang, F Gao, and WJ Weber. 2007. "Molecular Dynamics Modeling of the Thermal Conductivity of Irradiated SiC as a Function of Cascade Overlap." Journal of Applied Physics 101(2):023527. doi:10.1063/1.2431397 Abstract SiC thermal conductivity is known to decrease under irradiation. To understand this effect, we study the variation of the thermal conductivity of cubic SiC with defect accumulation induced by displacement cascades. We use an empirical potential of the Tersoff type in the framework of non-equilibrium molecular dynamics. The conductivity of SiC is found to decrease with dose, in very good quantitative agreement with low temperature irradiation experiments. The results are analyzed in view of the amorphization states that are created by the cascade accumulation simulations. The calculated conductivity values at lower doses are close to the smallest measured values after high temperature irradiation, indicating that the decrease of the conductivity observed at lower doses is related to the creation of point defects. A subsequent decrease takes place upon further cascade accumulation. It is characteristic of the amorphization of the material and is experimentally observed for low temperature irradiation only.

Campbell LW, F Gao, R Devanathan, and WJ Weber. 2007. "Model of Plasmon Decay for Electron Cascade Simulation." Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment 579(1):454-457. doi:10.1016/j.nima.2007.04.095 Abstract A dominant energy loss mechanism for electrons in radiation cascades is the excitation of plasmons. Once created, plasmons eventually decay through interband transitions, leading to the production of additional charge carriers that are responsible for much of the signal measured by the detector. The cross section for exciting plasmons and charge carrier pairs follows from the material’s dielectric function. The energy distribution of the resulting secondary particles can be obtained from the one-electron orbitals in the random phase approximation. We apply this method to a nearly free electron crystal for plasmon decays. A Monte Carlo simulation of the resulting decay cascade is carried out for material parameters appropriate to silicon, using the Callaway-Tosatti dielectric function for cascading electrons and holes. Parameters that characterize the detector performance (mean energy to produce a charge carrier pair and the Fano factor) are determined, and are found to be sensitive to the low energy electronic structure.

Bae IT, Y Zhang, WJ Weber, M Higuchi, and L Giannuzzi. 2007. "Electron-beam induced recrystallization in amorphous apatite." Applied Physics Letters 90(2):021912, 1-3. doi:10.1063/1.2430779 Abstract Electron-beam-induced recrystallization of irradiation-induced amorphous Sr2Nd8(SiO4)6O2 is investigated in situ using transmission electron microscopy with 200 keV electrons at room temperature. Epitaxial recrystallization is observed from both the amorphous/crystalline interface and the surface, and the recrystallization is more pronounced with increasing electron-beam flux. Since the temperature increase induced by electron-beam irradiation is estimated to be less than 7 K and maximum energies transferred to target atoms are below the displacement energies, ionization-induced processes are considered to be the primary mechanisms for the solid-phase epitaxial recrystallization observed in the present study.

Bae IT, WJ Weber, M Ishimaru, and Y Hirotsu. 2007. "Effect of ionization rates on dynamic recovery processes during electron-beam irradiation of 6H-SiC." Applied Physics Letters 90(12):121910. Abstract We have investigated the effects of 200 and 300 keV electron-beam irradiations on amorphization in 6H-SiC at 100 and 295 K. Amorphization is induced by the accumulation of defects produced by direct atomic displacements. Dynamic recovery of these defects during irradiation, due to temperature increases and ionization effects, results in increases in the amorphization dose. By comparing with previous data for 2 MeV electron irradiations and 1.5 MeV Xe irradiations, the results demonstrate that ionization-enhanced recovery in 6H-SiC increases linearly with ionization rate above an ionization rate threshold.

Sickafus KE, WJ Weber, and BP Uberuaga. 2006. "" Abstract REI-2005 was the 13th International Symposium in a series of conferences dedicated to research relating to radiation effects in insulators and non-metallic materials. It was held in Santa Fe, New Mexico, USA, from 28th August 2005 to 2nd September 2005. This meeting is convened every other year and has been on-going for the last 26 years. REI-2005 sessions reviewed recent developments in the following topical areas related to radiation effects in insulators and non-metals: • fundamentals, theory and computer simulations; • ion–solid interactions and ion-beam modification; • laser–insulator interactions; • simple and complex oxides; • metal carbide and nitride compounds; • polymers; • nanocomposites and nanostructured materials; • semiconductor materials; • radiation effects in semiconductors; • glasses and silica; • swift heavy ion irradiations; • neutron irradiations; • applications (optoelectronics, optics, polymers, biomaterials, nuclear).

Lindner JK, M Toulemonde, WJ Weber, and BL Doyle. 2006. "" Abstract Electronic Proceedings of MRS Symposium on "Growth, Modification, and Analysis by Ion Beams at the Nanoscale" held as part of the Annual Fall 2005 MRS meeting.

Zhang Y, WJ Weber, V Shutthanandan, and S Thevuthasan. 2006. "Non-linear Damage Accumulation in Au-irradiated SrTiO3." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 251(1):127-132. doi:10.1016/j.nimb.2006.05.018 Abstract Ion-induced damage in strontium titanate (SrTiO3) has been investigated using 1.0 MeV Au ions at 150 K and room temperatures. Rutherford backscattering spectrometry using 2.0 MeV He+ beam was carried out along the <100> direction to study crystalline damage and the accumulation behavior. Damage accumulation has been determined as the relative disorder on the Sr and Ti sublattices at the damage peak as a function of local dose. A disorder accumulation model has been fit to both the Sr and Ti damage accumulation data, and contributions from the amorphous fraction and the crystalline disorder are discussed. The results indicate that defect-stimulated amorphization is the primary amorphization mechanism in SrTiO3. The sigmoidal accumulation behavior in damage accumulation leads to non-linear increase of the width of the damage profile.

Zhang Y, CM Wang, MH Engelhard, and WJ Weber. 2006. "Irradiation behavior of SrTiO3 at temperatures close to the critical temperature for amorphization." Journal of Applied Physics 100(11):113533 (8 pages). doi:10.1063/1.2399932 Abstract Damage accumulation on both the Sr and Ti sublattices in strontium titanate (SrTiO3) has been investigated under 1.0 MeV Au+ irradiation at 360 and 400 K, close to the critical temperature for amorphization (~ 370 K). Under irradiation at 360 K, the relative disorder on both sublattices follows a nonlinear dependence on ion dose. Amorphization starts from the damage peak region (at a depth of 60 nm) and grows toward the surface and into the bulk. At 400 K, evolution of point defects to extended defects occurs as ion fluence increases. The disorder initially peaks at a depth of 60 nm, saturates at disorder level of ~0.75, and then decreases with further irradiation. At an ion fluence of 6.0×1015 cm-2, an amorphous layer of ~ 10 nm thickness is formed at the sample surface. After annealing at 375 K for one hour, the microstructural features indicate that the buried amorphous layer formed during irradiation at 360 K is re-crystallized with planar defects and dislocation loops. The surface amorphous layer formed at 400 K irradiation remains amorphous and less defects are observed at the irradiated region. The irradiation-enhanced recrystallization due high flux electron energy deposition is observed.

Zhang Y, WJ Weber, DA Grove, J Jensen, and G Possnert. 2006. "Electronic Stopping Powers for Heavy Ions in Niobium and Tantalum Pentoxides." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 250(1-2):62-65. doi:10.1016/j.nimb.2006.04.148 Abstract Electronic energy loss is the fundamental mechanism accountable for the response of materials to swift heavy ions that drives many new applications. Due to experimental difficulties in preparing and handling compound targets for energy-loss measurements, stopping data in compounds are very limited. The electronic stopping power of He, Li, Be and O ions in self-supporting foils of niobium and tantalum pentoxides (Nb2O5 and Ta2O5) have been measured in transmission over a continuous range of energies. The measured stopping powers are compared with theoretical predictions based on the SRIM (stopping and range of ions in matter) code. In general, the predicted values are in reasonable agreement with the experimental data. However, significant deviations are observed in some cases, particularly around the stopping maximum.

Zhang Y, WJ Weber, A Razpet, and Gö Possnert. 2006. "Electronic Stopping Powers for Be, Ca and Ti in SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 242(1-2):82-84. Abstract Energy loss of ions in matter is fundamental to many applications dependent on the transport of ions in matter. In spite of a long history of studies, the electronic stopping power is not adequately described over all ranges of ions, energies and targets, particularly in the case of heavy ions or compound targets. In this study, stopping powers for Be, Ca and Ti in SiC have been determined using a Time-of-Flight Elastic Recoil Detection Analysis (ToF ERDA) set-up. In transmission geometry, the energy loss of heavy ions in the self-supporting SiC foil was measured over a continuous range of energies using the ToF data that was tagged by a Si detector with and without the stopping foil. By essentially calibrating the Si detector for each channel over the measured energy region based on TOF spectrometry, measurement uncertainties of less than 4% are achieved. In comparing with the experimental data, both the SRIM (Stopping and Range of Ions in Matter) code and the binary theory provide reasonable predictions.

Zhang Y, J Jensen, G Possnert, DA Grove, DE McCready, BW Arey, and WJ Weber. 2006. "Electronic Stopping Forces of Heavy Ions in Metal Oxides." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 249(1-2):18-21. doi:10.1016/j.nimb.2006.03.013 Abstract Electronic energy loss of charged particles in materials is a fundamental process accountable for the unique response of materials in applications of advanced nuclear power, radiation detectors, and advanced processing of electronic devices. For over a century, the stopping of energetic ions in matter has been a subject of great experimental and theoretical interest. In spite of a long history of studies, the electronic stopping force is not adequately described over all ranges of ions, energies and targets, particularly in the case of heavy ions in compound targets. In this study, stopping powers for ions in ceramic oxides of SiO2, ZrO2, Ta2O5 and Nb2O5 have been determined using a time-of-flight energy elastic recoil detection analysis (ToF-E ERDA) set-up. In transmission geometry, the energy loss of heavy ions in the thin foils was measured over a continuous range of energies from a few 10 keV/nucleon to over a thousand keV/nucleon using the ToF data that was tagged by a Si detector with and without the stopping foils. Comparisons are made with the SRIM-2003 (The Stopping and Range of Ions in Matter) predictions, and deviations are discussed.

Wiss TA, JP Hiernaut, P Damen, S Lutique, R Fromknecht, and WJ Weber. 2006. "Helium Behaviour in Waste Conditioning Matrices during Thermal Annealing." Journal of Nuclear Materials 352(1-3):202-208. doi:10.1016/j.jnucmat.2006.02.055 Abstract Reprocessing of spent fuel produces high level waste including minor actinides and long living fission products that might be disposed in waste conditioning matrices. Several natural mineral phases were proven to be able to incorporate fission products or actinides in their crystalline structure for long periods of time. In this study, synthetic compounds of zirconolite (CaZrTi2O7) and pyrochlores (Gd2Ti2O7 and Nd2Zr2O7) were fabricated and doped with the short-lived alpha-emitter 244Cm to increase the total amount of helium and damage generated in a laboratory time scale. Helium implantations were also used to simulate the damage caused by the alpha-decay and the build-up of helium in the matrix. The samples were annealed in a Knudsen cell, and the helium release profile interpreted in conjunction with radiation damage studies and previous analysis of annealing behaviour. Several processes like diffusion, trapping or phase changes could then be attributed to the helium behaviour depending on the material considered. Despite high damage and large amount of helium accumulated, the integrity of the studied materials was preserved during storage.

Wang Z, X Zu, F Gao, and WJ Weber. 2006. "Atomistic Simulation of Brittle to Ductile Transition in GaN Nanotubes." Applied Physics Letters 89(24):243123, 1-3. doi:10.1063/1.2405879 Abstract Molecular dynamics methods with a Stillinger-Weber potential have been used to investigate the mechanical properties of wurtzite-type single crystalline GaN nanotubes under applied tensile stresses. At lower temperatures, the nanotubes show brittle properties; whereas at higher temperatures, they behave as ductile materials. The brittle to ductile transition (BDT) is systemically investigated, and the corresponding transition temperatures have been determined in GaN. The BDT temperature generally increases with increasing thickness of nanotubes and strain rate.

Wang Z, X Zu, F Gao, and WJ Weber. 2006. "Atomic-Level Study of Melting Behavior of GaN Nanotubes." Journal of Applied Physics 100(06):063503, 1-6. doi:10.1063/1.2345616 Abstract Molecular dynamics simulations with a Stillinger-Weber potential have been used to investigate the melting behavior of wurtzite-type single crystalline GaN nanotubes. The simulations show that the melting temperature of GaN nanotubes is much lower than that of bulk GaN, which may be associated with the large surface-to-volume ratio of the nanotubes. The melting temperature of the GaN nanotubes increases with the thickness of the nanotubes to a saturation value, which is close to the melting temperature of a GaN slab. The results reveal that the nanotubes begin to melt at the surface, and then the melting rapidly extends to the interior of the nanotubes as the temperature increases. The melting temperature of a single-crystalline GaN nanotube with [100]-oriented lateral facets is higher than that with [110]-oriented lateral facets for the same thickness.

Wang CM, V Shutthanandan, Y Zhang, S Thevuthasan, LE Thomas, WJ Weber, and G Duscher. 2006. "Atomic level imaging of Au nanocluster dispersed in TiO2 and SrTiO3 ." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 242(1-2):380-382. doi:10.1016/j.nimb.2005.08.144 Abstract Au nanoclusters dispersed in single crystal TiO2 and SrTiO3 have been prepared by ion implantation at 300 and 975 K and subsequent annealing at 1275 K for 10 hours. High resolution transmission electron microscopy and high-angle-annular-dark-field (HAADF) imaging in an aberration corrected scanning transmission electron microscope (STEM) have been used to characterize the microstructure of the dispersed gold nanoclusters. The results indicate that Au atoms substitute for cations in these systems. Cavities of up to several tens of nanometers are observed in TiO2 and SrTiO3. The nanometer-sized cavities and Au clusters are faceted along the same lattice plane of the matrix, indicating that the interfacial energy (defined by the Au cluster and the matrix) and the surface energy of the matrix (defined by cavity and the matrix ) follow a similar trend of change with respect to different lattice planes of the matrix.

Van Ginhoven RM, A Chartier, C Meis, WJ Weber, and LR Corrales. 2006. "Theoretical Study of Helium Insertion and Diffusion in 3C-SiC." Journal of Nuclear Materials 348(1-2):51-59. Abstract Insertion and diffusion of helium in cubic silicon carbide have been investigated by means of density functional theory. That method has first been assessed by calculating relevant properties for the perfect crystal along with point defect formation energies. Results are consistent with available theoretical as well as experimental data. Helium insertion energies have been calculated to be lower for divacancy and silicon vacancy defects compared to the other mono-vacancies and interstitial sites considered. Migration barriers for helium have been determined by using the nudged elastic band method. Calculated activation energies for migration in and around vacancies (silicon vacancy, carbon vacancy or divacancy) range from 0.6 to 1.0 eV. The activation energy for interstitial migration is calculated to be 2.5 eV. These values are discussed and related to recent experimental activation energies for migration that range from 1.1 to 3.2 eV, depending on the SiC samples used and on helium implantation conditions.

Posselt M, F Gao, and WJ Weber. 2006. "Atomistic Simulations on the Thermal Stability of the Antisite Pair in 3C- and 4H-SiC." Physical Review. B, Condensed Matter 73(12):125206: 1-8. doi:10.1103/PhysRevB.73.125206 Abstract The thermal stability of the first-neighbor antisite pair configurations in 3C- and 4H-SiC is investigated by a comprehensive atomistic study. At first the structure and energetics of these defects is determined in order to check the accuracy of the Gao-Weber interatomic potential used. The results are comparable with literature data obtained by the density-functional theory. Then, the lifetime of the antisite pair configurations is calculated for temperatures between 800 and 2500 K. Both in 3C- and 4H-SiC the thermal stability of the antisite pairs is rather low. In contrast to previous theoretical interpretations, the antisite pair can be therefore not correlated with the DI photoluminescence center that is stable to above 2000 K. The atomic mechanisms during the recombination of the antisite pair in 3C-SiC and of three antisite pair configurations in 4H-SiC is a modified concerted exchange. Due to the different sizes of the silicon and the carbon atoms, this process is not identical with the concerted exchange in Si. Two intermediate metastable configurations found during the recombination are similar to the bond defect in Si. Since the SiC lattice contains two types of atoms, there are also two different types of bond defects. The two bond defects can be considered as the result of the incomplete recombination of a carbon vacancy and a neighboring mixed dumbbell interstitial. For selected temperatures the thermal stability of the antisite pair in 3C-SiC is investigated by molecular dynamics simulations that are based on the density-functional theory. Their results are very similar to those of the atomistic study, i.e. the Gao-Weber potential describes the antisite pair and its recombination reasonably well. The antisite pair in 4H-SiC with the two atoms on hexagonal sites has a slightly different formation energy than the other three antisite pair configurations in 4H-SiC. Its lifetime shows another dependence on the temperature, and its recombination is characterized by a separate motion of atoms.

Lian J, WJ Weber, W Jiang, L Wang, LA Boatner, and RC Ewing. 2006. "Radiation-Induced Effects in Pyrochlore and Nanoscale Materials Engineering." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 250(1-2):128-136. doi:10.1016/j.nimb.2006.04.157 Abstract Pyrochlore materials, A2B2O7, encompass a wide range of compositions and are technological important for energy and environment issues, for example, used as ionic conductor in solid oxide fuel cells and nuclear waste forms for the storage of actinides, particularly Pu. Here, the recent progresses in understanding ion beam irradiation-induced phenomena in pyrochlore compounds are briefly reviewed with the focus of ion beam-induced crystalline-to-amorphous and pyrochlore to fluorite structural transitions. Systematic ion irradiation studies of lanthanide pyrochlores in which B = Ti, Zr, and Sn have suggested that the radiation response of pyrochlore compounds is highly dependent on compositional changes. Both ionic size and the cation electronic configurations (e.g., bond-types) affect the structural distortion from the ideal fluorite structure and the response behavior of pyrochlore-structure types to ion beam irradiation. Ion beam-induced pyrochlore-to-fluorite structural transition occurs in all irradiated pyrochlore compositions, and the independent kinetics of cation and anion disordering processes were discussed. Numerous novel nanostructures have been created by utilizing the ion beam-induced amorphization, order-disorder transition and phase decomposition, such as amorphous and disordered nano-domains, perfectly latticed matched two-dimensional nanolayer, self-organized ripple structure, metallic nanoparticles and nanowires. The potential application of energetic particle irradiation for nano-engineering pyrochlore structured compounds is highlighted.

Jiang W, Y Zhang, V Shutthanandan, S Thevuthasan, and WJ Weber. 2006. "Temperature response of 13C atoms in amorphized 6H-SiC." Applied Physics Letters 89(26):art. no.:261902, (3 pages). doi:10.1063/1.2422892 Abstract Implantation of 13C2+ ions was employed to produce a concentration profile in 6H-SiC at 140 K. In-situ study of 13C implanted species was performed using the resonant reaction of 13C(p,gamma)14N at Ep=1.748 MeV. Significant 13C diffusion in the amorphized SiC does not occur up to 1130 K. The presence of Au implants (1.9 at.%) does not affect the 13C behavior. High-energy H+ irradiation also does not promote the 13C diffusion. The results suggest that C atoms are readily trapped locally in the SiC structure during disordering, which is important to understanding the amorphization processes in SiC.

Jiang W, WJ Weber, JS Young, LA Boatner, J Lian, L Wang, and RC Ewing. 2006. "Irradiation-induced nanostructures in cadmium niobate pyrochlores." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 250(1-2):188-191. Abstract This paper reports the formation processes of crystalline Cd nanostructures (nanoparticles and nanowires) on ion-cut surfaces of cadmium niobate pyrochlores (Cd2Nb2O7). Irradiation with 3 MeV He+ ions has been performed at low temperatures (≤ 295 K) to induce material decomposition and aggregation of host atoms. The irradiation also leads to surface exfoliation due to rupture of gas (He and O2) filled blisters. Nanoparticles and nanowires are observed on the ion-cut surfaces at low and higher doses, respectively. The nanostructures are examined and characterized using a suite of experimental tools. Both the nanoparticles and nanowires are found to be single crystals that primarily consist of metallic Cd.

Jiang W, V Shutthanandan, Y Zhang, S Thevuthasan, WJ Weber, and GJ Exarhos. 2006. "Hydrogen behavior in Mg+-implanted graphite." Journal of Materials Research 21(4):811-815. doi:10.1557/JMR.2006.0121 Abstract A graphite wafer has been implanted with Mg+ to produce a uniform Mg concentration. Subsequent H+ implantation covered both the Mg+-implanted and unimplanted regions. Ion-beam analysis shows a higher H retention in graphite embedded with Mg than in regions without Mg. A small amount of H diffuses out of the H+ implanted graphite during thermal annealing at temperatures up to 300°C. However, significant H release from the region implanted with both Mg+ and H+ ions occurs at 150°C; further release is also observed at 300°C. The results suggest that there are efficient H trapping centers and fast pathways for H diffusion in the Mg+ implanted graphite, which may prove highly desirable for reversible H storage.implanted graphite, which may prove highly desirable for reversible H storage.

Jiang W, and WJ Weber. 2006. "Effect of irradiation temperature on dynamic recovery in gallium nitride." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 242(1-2):431-433. Abstract A single crystal gallium nitride film on sapphire was successively irradiated to a fluence of 4.5 Au3+/nm2 in different areas at varied temperatures ranging from 150 to 800 K. The temperature dependence of disorder on both the Ga and N sublattices has been investigated using a 3.736 MeV He+ backscattering analysis along the <0001>- and <10-11>-axial channeling directions. Significant dynamic recovery of disorder occurs over the applied temperature range. There is a higher degree of disorder on the N sublattice observed along the <10-11> axis. Some of the defects produced during the irradiation in GaN are effectively shielded by the <0001> axis.

Jiang W, Y Zhang, WJ Weber, J Lian, and RC Ewing. 2006. "Direct evidence of N aggregation and diffusion in Au+ irradiated GaN." Applied Physics Letters 89(2):021903-1-3. Abstract A surface amorphized layer and a buried disordered structure were created in gallium nitride (GaN) irradiated using 1.0 MeV Au+ ions to fluences of 25 and 70 Au+/nm2 at room temperature. Bubbles of N2 gas within both the amorphized and disordered GaN are formed. A gradient profile with a lower N concentration in the amorphized region is observed, which provides direct evidence of N loss by diffusion in the Au+ irradiated GaN. These results are important to understanding the amorphization processes in GaN and may have significant implications for the design and fabrication of GaN-based devices.

Gao F, R Devanathan, T Oda, and WJ Weber. 2006. "Development of Partial-Charge Potential for GaN." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 250(1-2):50-53. doi:10.1016/j.nimb.2006.04.082 Abstract Partial-charged potentials for GaN are systematically developed that describes a wide range of structural properties, where the reference data for fitting the potential parameters are taken from ab initial calculations or experiments. The present potential model provides a good fit to different structural geometries and high pressure phases of GaN. The high-pressure transition from wurtzite to rock-salt structure is correctly described yielding the phase transition pressure of about 55 GPa, and the calculated volume change at the transition is in good agreement with experimental data. The results are compared with those obtained by ab initio simulations.

Gao F, Y Zhang, M Posselt, and WJ Weber. 2006. "Atomic-Level Simulations of Epitaxial Recrystallization and Amorphous-to-Crystalline Transition in 4H-SiC." Physical Review. B, Condensed Matter 74(10):104108, 1-9. doi:10.1103/PhysRevB.74.104108 Abstract The amorphous-to-crystalline (a-c) transition in 4H-SiC has been studied using molecular dynamics (MD) methods, with simulation times of up to a few hundred ns and at temperatures ranging from 1000 to 2000 K. Two nano-sized amorphous layers, one with the normal of a-c interfaces along the [ -12-10] direction and the other along the [ -1010] direction, were created within a crystalline cell to study expitaxial recrystallization and the formation of secondary phases. The recovery of bond defects at the interfaces is an important process driving the epitaxial recrystallization of the amorphous layers. The amorphous layer with the a-c interface normal along the [-12-10] direction can be completely recrystallized at the temperatures of 1500 and 2000 K, but the recrystallized region is defected with dislocations and stacking faults. On the other hand, the recrystallization process for the a-c interface normal along [-1010] direction is hindered by the nucleation of polycrystalline phases, and these secondary ordered phases are stable for longer simulation times. A general method to calculate activation energy spectra is employed to analyze the MD annealing simulations, and the recrystallization mechanism in SiC consists of multiple stages with activation energies ranging from 0.8 to 1.7 eV.

Gao F, R Devanathan, Y Zhang, M Posselt, and WJ Weber. 2006. "Atomic-Level Simulation of Epitaxial Recrystallization and Phase Transformation in SiC." Journal of Materials Research 21(6):1420-1426. doi:10.1557/JMR.2006.0176 Abstract A nano-sized amorphous layer embedded in a perfect crystal has been created to study the amorphous-to-crystalline (a-c) transition and subsequent phase transformation in 3C-SiC by means of classical molecular dynamics methods. The recovery of bond defects and the rearrangement of atoms at the interfaces are important processes driving the initial epitaxial recrystallization of the amorphous layer, which is eventually hindered by the nucleation and growth of a polycrystalline 2H-SiC phase. A spectrum of activation energies, ranging from about 0.8 eV to 2.0 eV, is associated with these processes. Following formation of the 2H phase, the kink sites and triple junctions formed at the interfaces between 2H- and 3C-SiC provide low-energy paths for 2H-SiC atoms to transform to 3C-SiC atoms, and complete recrystallization back to the 3C structure occurs at 2000 K with an activation energy on the order of 2.3 eV.

Du J, R Devanathan, LR Corrales, WJ Weber, and AN Cormack. 2006. "Short- and medium-range structure of amorphous zircon from molecular dynamics simulations." Physical Review. B, Condensed Matter 74(21):art. no.:214204, (14 pages). doi:10.1103/PhysRevB.74.214204 Abstract We have obtained new insights into the structure of amorphous zircon using classical molecular dynamics simulations with a partial charge model. We present detailed structural characterizations of the simulated high and low density amorphous zircon and compare our results with available neutron diffraction, EXAFS, NMR and other experimental results. The results show that amorphization leads to polymerization of the silicon-oxygen network and the formation of regions rich in zirconium. The average n value of Qn species is 1.6-1.8. A considerable percentage of the oxygen ions (around 20%) have only zirconium in the first coordination shell (free oxygen) in amorphous zircon. The Zr-O bond length (around 2.10Å) is shorter and the oxygen coordination number around zirconium smaller (6-7) than those in crystalline zircon, in good agreement with the EXAFS results. The total structure factors of simulated amorphous zircon also agree well with neutron diffraction results. We have examined the effects of the simulation cell size and relative density on the amorphous structure. The general features such as polymerization of silicon-oxygen network and the formation of clustered zirconium rich regions appear to be independent of system size and volume expansion in the range of 11 to 18%. Based on the obtained amorphous zircon structure, experimentally observed lower chemical durability of amorphous zircon compared to its crystalline form can be explained by the existence of the silicon-oxygen networks and zirconium rich regions in amorphous zircon that provides diffusion channels and eases dissolution processes. Battelle operates PNNL for the USDOE

Devanathan R, LR Corrales, F Gao, and WJ Weber. 2006. "Signal Variance in Gamma Ray Detectors - A Review." Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment 565(2):637-649. doi:10.1016/j.nima.2006.05.085 Abstract Signal variance in gamma ray detector materials is reviewed with an emphasis on intrinsic variance. Phenomenological models of electron cascades are examined and the Fano factor (F) is discussed in detail. In semiconductors F is much smaller than unity and charge carrier production is nearly proportional to energy. Based on a fit to a number of semiconductors and insulators, a new relationship between the average energy for electron-hole pair production and band-gap energy is proposed. In scintillators, the resolution is governed mainly by photoelectron statistics and proportionality of light yield with respect to energy.

Devanathan R, LR Corrales, WJ Weber, A Chartier, and C Meis. 2006. "Molecular Dynamics Simulation of Energetic Uranium Recoil Damage in Zircon." Molecular Simulation 32(12-13):1069-1077. Abstract Defect production and amorphisation due to energetic uranium recoils in zircon (ZrSiO4), which is a promising ceramic nuclear waste form, is studied using molecular dynamics simulations with a partial charge model. An algorithm that distinguishes between undamaged crystal, crystalline defects and amorphous regions is used to develop a fundamental understanding of the primary damage state. The amorphous cascade core is separated from the surrounding crystal by a defect-rich region. Small, chemically inhomogeneous amorphous clusters are also produced around the core. The amorphous regions consist of under-coordinated Zr and polymerised Si leading to amorphisation and phase separation on a nanometer scale into Zr- and Si-rich regions. This separation could play an important role in the experimentally observed formation of nanoscale ZrO2 in ZrSiO4 irradiated at elevated temperatures.

Devanathan R, WJ Weber, SC Singhal, and JD Gale. 2006. "Computer Simulation of Defects and Oxygen Transport in Yttria-Stabilized Zirconia." Solid State Ionics 177(15-16):1251-1258. doi:10.1016/j.ssi.2006.06.030 Abstract We have used molecular dynamics simulations and energy minimization calculations to examine defect energetics and oxygen diffusion in yttria-stabilized zirconia (YSZ). Oxygen vacancies prefer to be second nearest neighbors to yttrium dopants. The oxygen diffusion coefficient shows a peak at 8 mole % yttria consistent with experimental findings. The activation energy for oxygen diffusion varies from 0.6 to 1.0 eV depending on the yttria content. The Y-Vo-Y complex with a binding energy of -0.85 eV may play an important role in any conductivity degradation of YSZ.

Devanathan R, LR Corrales, WJ Weber, A Chartier, and C Meis. 2006. "Atomistic Simulation of Collision Cascades in Zircon." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 250(1-2):46-49. doi:10.1016/j.nimb.2006.04.109 Abstract Defect production in energetic collision cascades in zircon has been studied by molecular dynamics simulation using a partial charge model combined with the Ziegler-Biersack-Littmark potential. Energy dissipation, defect accumulation, Si-O-Si polymerization, and Zr coordination number were examined for 10 keV and 30 keV U recoils simulated in the constant NVE ensemble. For both energies an amorphous core was produced with features similar to that of melt quenched zircon. Disordered Si ions in this core were polymerized with an average degree of polymerization of 1.5, while disordered Zr ions showed a coordination number of about 6 in agreement with EXAFS results. These results suggest that nano-scale phase separation into silica- and zirconia-rich regions occurs in the amorphous core.

Crocombette JP, A Chartier, and WJ Weber. 2006. "Atomistic Simulation of Amorphization Thermokinetics in Lanthanum Pyrozirconate." Applied Physics Letters 88(5):051912 (3). doi:10.1063/1.2171651 Abstract The kinetics of amorphization in La2Zr2O7 pyrochlore is investigated using molecular dynamics simulations. Irradiation damage is simulated by continuous accumulation of cation Frenkel pairs at various temperatures. As observed experimentally, La2Zr2O7 first transitions to the fluorite structure, independent of the temperature, and amorphization occurs at low temperatures. A model fit of the simulated dose–temperature curve reproduces experimental results in the literature, with a low temperature amorphization dose D0 = 1.1 dpc (displacement per cation) and an activation energy Eact = 0.036 eV. Present simulations indicate that point defect recombination can control the temperature dependence of amorphization driven by point defect accumulation.

Chartier A, JP Crocombette, C Meis, WJ Weber, and LR Corrales. 2006. "Radiation Effects in Lanthanum Pyrozirconate." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 250(1-2):17-23. doi:10.1016/j.nimb.2006.04.079 Abstract The present paper reviews recent results on radiation resistance of lanthanum pyrozirconate obtained using empirical potentials molecular dynamic simulations. First, displacement cascades (DCs) with a 6 keV U4+ cation representing the α- recoil nucleus have been performed in the lanthanum pyrozirconate La2Zr2O7. Only point defects are observed after each DC. They represent on average only 10% of the total number of displaced atoms during the cascade, with two times more cation anti-sites than Frenkel pairs. These calculations indicate that amorphization does not occur by a direct impact mechanism in pyrozirconate. Second, consequences of point defects accumulation have been simulated by introducing different types – either cation anti-sites or Frenkel pairs – and concentrations of point defects in pyrochlore. Results show that cation Frenkel pairs accumulation is the driving force for lanthanum zirconate amorphization. Under cation Frenkel pair accumulation, the crystal transits first from the pyrochlore to the disordered fluorite structure, with the oxygen atoms simply rearranging around cations. Amorphization occurs as a second step. These results consequently provide atomic-level interpretation to experimental irradiation observations of a two-step phase transition.

Zhang Y, F Gao, W Jiang, DE McCready, and WJ Weber. 2005. "Studies of Damage Accumulation in 4H Silicon Carbide by Ion-Channeling Techniques." Materials Science Forum 475-479(1-5):1341-1344. Abstract Single crystal 4H-SiC was irradiated with 2 MeV Au ions at 165 K. Ion-induced defect configurations and damage accumulation were studied by ion-channeling techniques along the <0001>, <4403> and <2201> directions. A nonlinear dependence of damage accumulation is observed for both the Si and C sublattices along all three directions, and the relative disorder observed along the <4403> and <2201> directions is much higher than that along the <0001> direction. The damage accumulation can be described by a disorder accumulation model, which indicates that defect-stimulated amorphization is the primary amorphization mechanism in SiC, and the high disorder level for the large off-axis angles is partially attributed to a geometrical effect. Molecular dynamics (MD) simulations demonstrate that most single interstitial configurations are shielded by Si and C atoms on the lattice sites along the <0001> direction, which significantly reduces their contribution to the yield along the <0001> direction.

Zhang Y, J Lian, CM Wang, W Jiang, RC Ewing, and WJ Weber. 2005. "Ion-Induced Damage Accumulation and Electron-Beam-Enhanced Recrystallization in SrTiO3." Physical Review. B, Condensed Matter and Materials Physics 72(9):094112, 1-8. Abstract Damage accumulation in strontium titanate (SrTiO3) from 1.0 MeV Au irradiation has been investigated at temperatures from 150 to 400 K. The relative disorder on the Sr and Ti sublattices at the damage peak has been determined as a function of local dose and temperature. A disorder accumulation model has been fit to data from this study and from the literature, indicating that defect-stimulated amorphization is the primary amorphization mechanism up to ~360 K. High-dose irradiation at 400 K leads to formation of an amorphous surface layer. Analyses of the temperature dependence for amorphization indicate that the amorphization kinetics are consistent with irradiation-enhanced and thermal recovery processes with activation energies of 0.1 eV and 0.7 eV, respectively. Under 200 keV electron-beam irradiation, the epitaxial recrystallization rates are orders of magnitude higher than thermal rates, and an activation energy of 0.1 eV is determined for the e-beam enhanced recrystallization processes.

Zhang Y, WJ Weber, DE McCready, DA Grove, J Jensen, and G Possnert. 2005. "Experimental Determination of Electronic Stopping for Ions in Silicon Dioxide." Applied Physics Letters 87(10):104103. Abstract The electronic energy loss for 4He, 7Li, 9Be, 12C, 16O, 19F and 28Si ions in self-supported SiO2 films has been measured in transmission geometry using a time of flight detection system over a continuous range of energies. SRIM (The Stopping and Range of Ions in Matter) predicts stopping power well for He and C ions within the stated uncertainties. Deviations around the Bragg peak for Li, Be and C ions and different energy dependence for Be, O, F and Si ions, as compared with the SRIM predictions, are observed. The results indicate that the modified Bohr formula is suitable for scaling the stopping number of heavy ions (except He, Li and Be) in the classical interaction regime. The measured electronic stopping powers are parameterized for easily implementation in other applications.

Zhang Y, WJ Weber, A Razpet, and Gö Possnert. 2005. "Electronic Stopping Powers for He, Be and F Ions in Au." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 227(4):479-484. Abstract The stopping powers for He, Be and F ions in Au are determined using a previously developed Time-of-Flight Elastic Recoil Detection Analysis (TOF ERDA) set-up. In transmission geometry, the energy loss of particles in a self-supporting Au foil is measured over a continuous range of recoil energies using the time-of-flight (TOF) spectrometry with uncertainty of less than 5%. The stopping powers are parameterized using a sixth order polynomial and compared with existing experimental data in the literature. In the energy regimes where experimental data exist, the present data exhibit good agreement with most data. Stopping powers predicted by SRIM (The Stopping and Range of Ions in Matter) are in reasonable agreement with much of the experimental data.

Wellman DM, JP Icenhower, and WJ Weber. 2005. "Elemental Dissolution Study of Pu-Bearing Borosilicate Glasses." Journal of Nuclear Materials 340(2-3):149-162. Abstract Single-pass flow-through tests were conducted to study the effects of selfradiation damage from alpha decay on dissolution kinetics of three radiation-aged Pu-bearing (1 mass% PuO2) borosilicate glasses over a pH interval of 9 to 12 at 80o to 88oC. The chemical compositions of the glasses were identical except the 239Pu/238Pu isotopic ratio, which was varied to yield accumulated doses of 1.3 x 1016, 2.9 x 1017, and 2.6 x 1018 -decays/g at the time of testing. Release of Al, B, Cs, Na, Si and U to solution increased with increasing pH, whereas Ca, Pu, and Sr were invariant over the pH interval. Average dissolution rates, based on B release, were identical within experimental uncertainty for all three glass compositions and increased from 0.17 ± 0.07 at pH(23oC) 9 to 10.6 ± 2.7 [g/(m2•d1)] at pH(23oC) 12. Release rates of Pu were 102- to 105-fold slower compared to all other elements, and were not affected by isotopic composition, selfradiation damage sustained by the glass, or pH. These data demonstrate that self - radiation damage does not affect glass dissolution rates, despite exposure to internal radiation doses for >20 years.

Weber WJ, F Gao, R Devanathan, W Jiang, and Y Zhang. 2005. "Defects and Ion-Solid Interactions in Silicon Carbide." Materials Science Forum 475-479(1-5):1345-1350. Abstract Atomic-level simulations are used to determine defect production, cascade-overlap effects, and defect migration energies in SiC. Energetic C and Si collision cascades primarily produce single interstitials, mono-vacancies, antisite defects, and small defect clusters, while amorphous clusters are produced within 25% of Au cascades. Cascade overlap results in defect stimulated cluster growth that produces amorphization. The good agreement of disordering behavior and changes in volume and elastic modulus obtained computationally and experimentally provides atomic-level interpretation of experimentally observed features. Simulations indicate that close-pair recombination activation energies range from 0.24 to 0.38 eV, and long-range migration energies for interstitials and vacancies have been determined.

Wang CM, Y Zhang, V Shutthanandan, DR Baer, WJ Weber, LE Thomas, S Thevuthasan, and G Duscher. 2005. "Self-assembling of nanocavities in TiO2 dispersed with Au nanoclusters." Physical Review. B, Condensed Matter 72(24):245421, 1-5. doi:10.1103/PhysRevB.72.245421 Abstract There has been considerable research effort on tailoring the non-linear optical properties of dielectric materials by dispersing nanometer-sized metallic clusters in them. It has been proposed that the optical response of this type of material is related to the quantum antidots (a vacancy cluster), which is spatially located at the interface between the metal cluster and the dielectric matrix. In order to clarify the vacancy clustering behavior as well as its correlation with Au clustering, single crystal TiO2 has been implanted with Au ions at 975 K and subsequently annealed at 1275 K for 10 hours. A characteristic self-assembling of nano-cavities along the boundary between the region of Au clusters and the free surface has been observed in the present system. These cavities are faceted along TiO2(110) and have a size of ~10 nm. High angle annular dark-field (HAADF) imaging in an aberration corrected scanning transmission electron microscope (STEM) revealed that vacancy clusters of ~ 2 nm in size also exist in the Au populated regions. Formation of cavities in Au-irradiated TiO2 strongly indicates that vacancy clustering processes prevail over Frenkel-pair recombination. Furthermore, the Au atoms substitution for Ti in TiO2 is also directly observed by STEM-HAADF imaging and by channeling Rutherford backscattering spectrometry (RBS).

Sun K, LM Wang, RC Ewing, and WJ Weber. 2005. "Effects of Electron Irradiation in Nuclear Waste Glasses." Philosophical Magazine. Structure and Properties of Condensed Matter 85(4-7):597-608. Abstract This article summarizes recent studies of electron irradiation damage in sodium borosilicate, iron phosphate and aluminophosphate glass waste forms using a modern analytical electron microscope. Three different borosilicate and iron phosphate glasses and an aluminophosphate glass were studied. Results indicate that all these glasses decomposed under the 200 kV electron irradiation. Migration of alkali elements from the irradiated centers to the peripheries under irradiation occurred in the alkali-element containing glasses, which results in the formation of alkali-depleted and -enriched phases. Formation of bubbles was only observed in the alkali-element containing iron phosphate and aluminophosphate glasses, not in the sodium borosilicate glasses when irradiated over a broad of dose rates. Separation of boron-rich phase from silicon-rich phase, iron-rich/aluminum-rich phase from phosphorous-rich domains were observed in the three types of glasses, respectively. Further irradiation resulted in formation of small particles. In the Fe-containing borosilicate glasses, the Fe is associated with the boron-rich phases after phase separation.

Sun K, L Wang, RC Ewing, and WJ Weber. 2005. "Analytical Electron Microscopy of Phase Separations in Borosilicate Glasses." Microscopy and Analysis 106:5-8. Abstract Analytical electron microscopy was used for the study of electron irradiation induced phase separations in three sodium borosilicate glasses. Results directly indicate that after the glasses have been irradiated to different electron doses two different types of phase separations occurred in all the three glasses. In the early stage of irradiation Na migrates to the periphery substrate. Further irradiation induced B-rich phases in the glass without containing Fe, and B-Fe phases in the two Fe-containing glasses separation from Si-rich phases. It demonstrates that AEM is powerful in study of structure change in a glass.

Nachimuthu P, S Thevuthasan, V Shutthanandan, EM Adams, WJ Weber, BD Begg, DK Shuh, DW Lindle, EM Gullikson, and RC Perera. 2005. "Near-Edge X-ray Absorption Fine-Structure Study of Ion-Beam-Induced Phase Transformation in Gd2(Ti1-yZry)2O7." Journal of Applied Physics 97(3):Article 033518. Abstract The structural and electronic properties of Gd2(Ti1-yZry)2O7 (y=0-1) pyrochlores following 2.0 MeV Au2+ ion-beam irradiation (~5.0x1014 Au2+/cm2) have been investigated by Ti 2p and O 1s near-edge x-ray absorption fine structure (NEXAFS). The irradiation of Gd2(Ti1-yZry)2O7 leads to the phase transformation from the ordered pyrochlore structure (Fd3m) to the defect fluorite structure (Fm3m) regardless of Zr concentration. Irradiated Gd2(Ti1-yZry)2O7 with y≤0.5 are amorphous although significant short-range order is present. Contrasting to this behavior, compositions with y≥0.75 retain crystallinity in the defect fluorite structure following irradiation. The local structures of Zr4+ in the irradiated Gd2(Ti1-yZry)2O7 with y≥0.75 determined by NEXAFS are the same as in cubic fluorite-structured yttria-stabilized zirconia (Y-ZrO2), thereby providing conclusive evidence for the phase transformation. The TiO6 octahedra present in Gd2(Ti1-yZry)2O7 are completely modified by ion-beam irradiation to TiOx polyhedra, and the Ti coordination is increased to eight with longer Ti-O bond distances. The similarity between cation sites and the degree of disorder in Gd2Zr2O7 facilitate the rearrangement and relaxation of Gd, Zr, and O ions/defects. This inhibits amorphization during the ion-beam induced phase transition to radiation-resistant defect fluorite structure contrasting to the ordered Gd2Ti2O7. Thus, during the ion-beam induced phase transition, the Gd2(Ti1-yZry)2O7 with y≤0.5 becomes amorphous whereas compositions with y≥0.75 retain crystalline structure.

Nachimuthu P, S Thevuthasan, EM Adams, WJ Weber, BD Begg, BS Mun, DK Shuh, DW Lindle, EM Gullikson, and RC Perera. 2005. "Near-edge X-ray Absorption Fine Structure Study of Disordering in Gd₂(Ti1-yZry)₂O₇ Pyrochlores." Journal of Physical Chemistry B 109(4):1337-1339. Abstract Disorder in Gd₂(Ti1-yZry)₂O₇ pyrochlores, for y=0.0-1.0, is investigated by Ti 2p and O 1s near-edge x-ray absorption fine structure spectroscopy. Ti⁴⁺ ions are found to occupy octahedral sites in Gd₂Ti₂O₇ with a tetragonal distortion induced by vacant oxygen sites. As Zr substitutes for Ti, the tetragonal distortion decreases, and Zr coordination increases from 6 to 8. The migration of oxygen ions from 48f or 8b sites to vacant 8a sites compensate for the increased Zr coordination, thereby reducing the number of vacant 8a sites, which further reduces the tetragonal distortion and introduces more disorder around Ti. This is evidence for simultaneous cation disorder with anion migration.

Jiang W, V Shutthanandan, S Thevuthasan, CM Wang, and WJ Weber. 2005. "Nitrogen Analysis Using Energetic Ion Beams." Surface and Interface Analysis 37(4):374-378. Abstract As a special case of nuclear reaction analysis (NRA), nuclear elastic scattering analysis (or non-Rutherford scattering analysis) is one of the important methods in ion-beam analysis, and is the preferred technique to analyze light elements in a heavy matrix. Compared to nuclear reaction, nuclear scattering usually has cross sections several orders of magnitude larger, which allows a quantitative analysis of light elements in a quicker and more convenient manner. Similar to NRA, this method complements the analysis of widely used Rutherford backscattering spectrometry. In this study, the scattering cross-sections for 14N(p,p)14N and 14N(alpha,alpha)14N at a laboratory angle of 150 degrees are measured over energy regions from 2.480 to 3.774 MeV using an amorphous film of Si3N4 on Si wafer. Examples for the analysis of lattice disorder on the N sublattice in Au2+-irradiated GaN single crystals will be demonstrated.

Jiang W, V Shutthanandan, S Thevuthasan, DE McCready, and WJ Weber. 2005. "Erratum to: “Carbon analysis using energetic ion beams” [Nucl. Instr. and Meth. B 222 (2004) 538-546] ." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 227(3):450-451. Abstract This paper corrects the energy calibration for the accelerator system we used for carbon cross section measurements.

Jiang W, WJ Weber, CM Wang, JS Young, LA Boatner, J Lian, L Wang, and RC Ewing. 2005. "Cadmium Nanowire Formation Induced by Ion Irradiation." Advanced Materials 17(13):1602-1606. Abstract One-dimensional nanostructures, such as nanowires, of semiconductors and metals are of great technological interest due to their potential for many advanced technology applications. Utilization of these materials versus their bulk counterparts will not only allow for device miniaturisation, but also may improve device performance or create new functions. Here we report a novel method for the synthesis of crystalline Cd-nanowires without involving either templates or a “seeded” structure. Ion irradiation at low temperatures (≤ 295 K) has been used to induce material decomposition and phase segregation in a cadmium niobate pyrochlore (Cd2Nb2O7) wafer. During the formation and rupture of the gas-filled blisters in the material, soft metallic Cd is extruded/extracted as nanowires through pores in the exfoliated layer. The entire process may be readily controlled by changing the ion irradiation conditions (e.g., ion species, dose and energy) with minimal thermal constraints.

Jiang W, WJ Weber, and LA Boatner. 2005. "Accumulation and Recovery of Disorder in Au2+-Irradiated Cd2Nb2O7." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 241(1-4):372-376. Abstract Cadmium niobate pyrochlore (Cd2Nb2O7) single crystals have been irradiated at 150, 300, 450 and 600 K using 1.0 MeV Au2+ ions over fluences ranging from 0.01 to 3.5 ions/nm2. The relative disorder on the Cd sublattice in the as-irradiated Cd2Nb2O7 has been analyzed based on in-situ 3.0 MeV He+ Rutherford backscattering spectrometry along the <100>-axial channeling direction. The results show that the crystal can be readily amorphized under the Au2+ irradiation at or below 450 K; however, the relative Cd disorder tends to saturate at 600 K, and full amorphization does not occur at doses up to 5 dpa. Isochronal annealing (20 min) also has been performed at temperatures from 180 to 295 K for samples irradiated at 150 K. Thermal recovery of the disorder has been observed below room temperature.

Heinisch HL, and WJ Weber. 2005. "Computational Model of Alpha-Decay Damage Accumulation in Zircon." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 228(1-4):293-298. Abstract Atomic-scale computer simulations are used to study defect accumulation and amorphization due to alpha decay in zircon (ZrSiO4). The displacement cascades, which represent 234U recoil nuclei from alpha-decay of 238Pu in zircon, are generated using a crystalline binary collision model, and the stochastic production of defects in the crystal lattice, recombination of defects, and the identification of amorphous regions are followed within the framework of a kinetic Monte Carlo simulation. Within the model, amorphous regions are identified as those having a critical density of Zr vacancies. The simulation predicts the interstitial content and amorphous fraction as functions of dose that are consistent with experimental data at 300 K for 238Pu-doped zircon, which indicate that the kinetic Monte Carlo model for behavior in zircon at 300 K is reasonable.

Gao F, EJ Bylaska, and WJ Weber. 2005. "Defect Properties in GaN: Ab Initio and Empirical Potential Calculations." Materials Science Forum 475-479(1-5):3087-3090. Abstract The defect properties and atomic configurations in GaN have been comparatively investigated using density functional theory (DFT) and molecular dynamics method with two representative potentials. The DFT calculations show that the relaxation of vacancies is generally small, but the relaxation around antisite defects is large. The N interstitials, starting from any possible configurations, eventually relax into a N+-N<11-20 > split interstitial. In the case of Ga interstitials, the most stable configuration is a Ga octahedral interstitial, but the Ga+-Ga<11-20 > split interstitial can bridge the gap between non-bounded Ga atoms. The formation energies of vacancies and antisite defects obtained using the Stillinger-Weber potential (SW) are in reasonable agreement with those obtained by DFT calculations, whereas the Tersoff-Brenner (TB) potential better describes the behavior of N interstitials.

Gao F, and WJ Weber. 2005. "Atomic-Level Computer Simulation of SiC: Defect Accumulation, Mechanical Properties and Defect Recovery." Philosophical Magazine. Structure and Properties of Condensed Matter 85(4-7):509-518. Abstract Damage accumulation simulated previously has been used to study volume swelling of 3C-SiC, and to calculate the elastic constants, bulk and elastic moduli of the cascade-amorphized SiC. The swelling increases rapidly with dose at low-dose levels, but the rate of increase decreases dramatically at higher dose with a saturation volume change of 8.2% for the cascade-amorphized state. The elastic constants in the cascade-amorphized SiC decrease about 22% for C11 and C12, 43% for C44, and 23% for bulk and elastic moduli. In order to understand defect annealing of damage accumulation, the stable Frenkel pairs created at low energy events have been annealed at different temperatures, using MD methods, to determine the time required for interstitials to recombine with vacancies. The results show that the low activation energies qualitatively overlap with experimental values observed during low temperature recovery. Thus, the present results suggest that low temperature recovery processes are associated with the spontaneous recovery of Frenkel pairs.

Gao F, R Devanathan, Y Zhang, and WJ Weber. 2005. "Annealing Simulations of Nano-Sized Amorphous Structures in SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 228(1-4):282-287. Abstract A two-dimensional model of a nano-sized amorphous layer embedded in a perfect crystal has been developed, and the amorphous-to-crystalline (a-c) transition in 3C-SiC at 2000 K has been studied using molecular dynamics methods, with simulation times of up to 88 ns. Analysis of the a-c interfaces reveals that the recovery of the bond defects existing at the a-c interfaces plays an important role in recrystallization. During the recrystallization process, a second ordered phase, crystalline 2H-SiC, can be nucleated and grow, and is stable for long simulation times. The crystallization mechanism is a two-step process that is separated by a longer period of second-phase stability. The kink sites formed at the interfaces between 2H- and 3C-SiC provide a low energy path for 2H-SiC atoms to transfer to 3C-SiC atoms, which can be defined as a solid-phase epitaxial transformation (SPET). It is observed that the nano-sized amorphous structure can be fully recrystallized at 2000 K in SiC, which is in agreement with experimental observations.

Devanathan R, LR Corrales, WJ Weber, A Chartier, and C Meis. 2005. "Molecular Dynamics Simulation of Defect Production in Collision Cascades in Zircon." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 228(1-4):299-303. Abstract Defect production in collision cascades in zircon has been examined by molecular dynamics simulations using a partial charge model combined with the Ziegler-Biersack-Littmark potential. U, Zr, Si and O recoils with energies ranging from 250 eV to 5 keV were simulated in the NVE ensemble. To obtain good statistics, 5-10 cascades in randomly chosen directions were simulated for each ion and energy. The damage consists of mainly Si and O Frenkel pairs, a smaller number of Zr Frenkel pairs, and Zr on Si antisite defects. Defect production, interstitial clustering, ion beam mixing and Si-O-Si polymerization increase with PKA mass and energy.

Devanathan R, and WJ Weber. 2005. "Insights into the Radiation Response of Pyrochlores from Calculations of Threshold Displacement Events." Journal of Applied Physics 98(8):086110. Abstract We have used molecular dynamics simulations to examine the displacement threshold energy (Ed) surface for cations and anions in Gd2Ti2O7 and Gd2Zr2O7 pyrochlores. In both pyrochlores, the Ed surface is highly anisotropic and it requires less energy to displace anions than cations. Both anion and cation Ed values are higher in the titanate compared to the zirconate. Titanium displacement energies are in excess of 170 eV for all directions examined, because cation exchange is less energetically favorable in Gd2Ti2O7 compared to Gd2Zr2O7. These high energy Ti displacements result in the formation of defect clusters that may prevent efficient defect recovery. This provides an explanation for the difference in susceptibility to amorphization between titanate and zirconate pyrochlores.

Chartier A, C Meis, JP Crocombette, WJ Weber, and LR Corrales. 2005. "Molecular Dynamic Simulation of Disorder Induced Amorphization in Pyrochlore." Physical Review Letters 94:025505. Abstract The defect accumulation of amorphization has been studied for the La2Zr2O7 pyrochlore by means of classical molecular dynamic simulations. Present calculations show that the accumulation of cation Frenkel pairs is the main driving parameter for the amorphization process, while the oxygen atoms simply rearrange around cations. Under Frenkel pair accumulation, the structure follows the pyrochlore-amorphous sequence. Present results consequently provide atomic-level interpretation to previous experimental irradiation observations of the two-step phase transitions.

Jiang W, WJ Weber, CM Wang, L Wang, and K Sun. 2004. "Experimental Studies of Defects, Implants and their Processes in Ion-Irradiated Gallium Nitride Single Crystals." In Defects and Diffusion in Ceramics - an Annual Retrospective - VI, Defect and Diffusion Forum, vol. 226-228, ed. David J. Fisher, pp. 91-111. Trans Tech Publications, Uetikon-Zurich, Switzerland. Abstract This article reviews recent experimental results, obtained by the authors, on disorder accumulation, disorder recovery, and behavior of implanted species in ion-irradiated gallium nitride (GaN) single crystals. The disorder on both the Ga and N sublattices has been studied in situ using Rutherford backscattering spectrometry (RBS) and nuclear resonant scattering along the <0001> axis, while the damage states for as-irradiated and post-annealed specimens have been examined using transmission electron microscopy. The disorder accumulation has been investigated as a function of ion fluence, ion mass and irradiation temperature; disorder annealing has been studied under thermal and dynamic conditions. The behavior of gold implants in GaN during irradiation and thermal annealing also will be discussed.

Zhang Y, and WJ Weber. 2004. "Studies of Electronic Stopping Powers Using Time of Flight Spectrometry." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 219-220:256-262. Abstract Determination of electronic stopping powers using Time of Flight (ToF) spectrometry have been demonstrated by measuring energy loss of He, O, and Al particles based on a ToF Elastic Recoil Detection Analysis (ERDA) set-up. In transmission geometry, the energy loss of the particles in self-supported stopping foils of C, Si and SiC is measured over a continuous range of energies using the ToF spectrometer. This study emphasizes the difference of the stopping power determination with and without dependence on the Si detector calibration over a wide energy range. By calibrating the Si detector for each channel over the measured energy region, the improved approach eliminates much of the error associated with pulsed height defects and measurement uncertainties of less than 4% are achieved. Stopping powers from this study are compared with limited experimental data from the literature and SRIM (The Stopping and Range of Ions in Matter) 2000 and 2003 predictions. In general, the predicted values are in reasonable agreement with the experimental data, and an improved accuracy of SRIM 2003 over SRIM 2000 can be observed in some cases. Furthermore, Bragg’s rule is valid in SiC for O and Al over the energy region studied.

Zhang Y, WJ Weber, and CM Wang. 2004. "Electronic Stopping Powers in Silicon Carbide." Physical Review. B, Condensed Matter and Materials Physics 69(20):205201, 1-9. Abstract The stopping powers in silicon carbide (SiC) of nine different ions, ranging from Be to Au, have been determined using a time-of-flight elastic recoil detection analysis (TOF ERDA) set-up. In transmission geometry, the energy loss of ions in a self-supporting SiC film is measured over a continuous range of recoil energies, from tens to hundreds keV/nucleon, using Time-of-Flight (TOF) spectrometry. By essentially calibrating the Si detector for each channel using the TOF spectrometer, the error resulting from nominal energy calibration is eliminated. An uncertainty of less than 4% is achieved in the stopping measurements. Stopping powers predicted by SRIM (The Stopping and Range of Ions in Matter) code are in reasonable agreement with much of the experimental data, and the SRIM 2003 predictions are in somewhat better agreement than SRIM 2000 for most ions. There are, however, still some discrepancies between SRIM predictions and the experimental data. For Ni, I and Au ions, the predicted values from SRIM 2003 are up to 40% less than the measured values. The stopping data from this study as well as previous work suggest that both the modified Bohr and Bloch formulas are suitable for scaling the stopping number of heavy ions in SiC within the classical interaction regime, and the Bloch formula is more appropriate to use at higher energies.

Zhang Y, WJ Weber, and HJ Whitlow. 2004. "Electronic Stopping Powers for Heavy Ions in Silicon." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 215(1-2):48-56. Abstract The stopping powers in silicon of heavy ions, with atomic numbers ranging from 4 to 29, have been determined using a Time-of-Flight Elastic Recoil Detection Analysis (ToF ERDA) set-up. In transmission geometry, the energy loss of heavy elastic recoils in the self-supporting silicon foil of known thickness is measured over a continuous range of recoil energies using Time-of-Flight (ToF) spectrometry. By eliminating the well-known calibration problem of Si detectors for heavy ions, an accuracy of less than 4% is achieved. The stopping powers are parameterized using a sixth order polynomial and compared with the limited experimental data in the literature. In the energy regimes where experimental data exist, the present data exhibit good agreement with most data. Stopping powers predicted by SRIM (The Stopping and Range of Ions in Matter) are in reasonable agreement with much of the experimental data, and SRIM 2003 predictions are in somewhat better agreement than SRIM 2000. There are, however, still some discrepancies between SRIM predictions and the experimental data.

Zhang Y, WJ Weber, W Jiang, CM Wang, V Shutthanandan, and A Hallen. 2004. "Effects of Implantation Temperature on Damage Accumulation in Al-Implanted 4H-SiC." Journal of Applied Physics 95(8):4012-4018. Abstract Damage accumulation in 4H-SiC under 1.1 MeV Al₂(₂⁺) irradiation is investigated as a function of dose at temperatures from 150 to 450 K. Based on Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA) channeling spectra, the damage accumulation on both the Si and C sublattices have been determined, and a disorder accumulation model has been fit to the data. The model fits indicate that defect-stimulated amorphization is the primary amorphization mechanism in SiC over the temperature range investigated. The temperature dependence of the cross section for defect-stimulated amorphization and the critical dose for amorphization indicate that two different dynamic recovery processes are present, which are attributed to short-range recombination and long-range migration of point defects below and above room temperature, respectively. As the irradiation temperature approaches the critical temperature for amorphization, cluster formation has an increasing effect on disorder accumulation, and ion flux plays an important role on the nature and evolution of disorder. Dislocation loops, which are mostly formed under high ion flux, act as sinks for point defects, thereby reducing the disorder accumulation rate.

Zhang Y, WJ Weber, V Shutthanandan, R Devanathan, S Thevuthasan, G Balakrishnan, and DM Paul. 2004. "Damage Evolution on Sm and O Sublattices in Au-Implanted Samarium Titanate Pyrochlore." Journal of Applied Physics 95(5):2866-2872. Abstract Damage evolution on the Sm and O sublattices in Sm₂Ti₂O₇ single crystals irradiated with 1 MeV Au₂⁺ ions at 170, 300 and 700 K was studied by Rutherford backscattering spectroscopy and 16O(d,p)17O nuclear reaction analysis. The damage accumulation behavior at each irradiation temperature indicates that the relative disorder on the O sublattice is higher than that on the Sm sublattice, and the relative disorder on each sublattice follows a nonlinear dependence on dose that is well described by a disorder accumulation model. While there is little difference in damage accumulation behavior on the Sm sublattice at 170 and 300 K irradiation, the rate of damage accumulation decreases dramatically at 700 K due to dynamic recovery. The critical dose for amorphization at 170 and 300 K is ~0.14 dpa, and a higher dose of ~ 0.22 dpa is observed under irradiation at 700 K. During thermal annealing in an 18O environment, a significant increase in the 18O exchange was observed between 800 and 900 K, which is just below the previously determined critical temperature, 950 K, for amorphization in Sm₂Ti₂O₇, suggesting that the mobility of O vacancies may be important in defining the critical temperature.

Zhang Y, F Gao, W Jiang, DE McCready, and WJ Weber. 2004. "Damage Accumulation and Defect Relaxation in 4H-SiC." Physical Review. B, Condensed Matter and Materials Physics 70(12):125203, 1-7. Abstract A nonlinear dependence of damage disorder on dose is observed for both the Si and C sublattices in 4H-SiC under 2 MeV Au irradiation at 165 K. The relative disorder observed along the <4403> direction is much higher than that along the <0001> direction. Molecular dynamics (MD) simulations demonstrate that most single interstitial configurations are formed on the Si-C dimer rows that are parallel to the <0001> direction. As a result, these interstitials are shielded by the Si and C atoms on the lattice sites, which significantly reduces the contribution of these interstitials to the backscattering/reaction yield along the <0001> direction. During isochronal annealing below room temperature, the relative disorder decreases along the <0001> direction, as expected; however, the disorder is stable on the Si sublattice and increases slightly on the C sublattice along the <4403> direction due to relaxation of some metastable defects to lower energy configurations. As the annealing temperature increases, similar recovery behavior on both the Si and C sublattices along the <0001> direction indicates coupling of Si and C recovery processes; however, slightly higher recovery temperatures on the C sublattice along the <4403> direction suggests some decoupling of the Si and C recovery processes. Based on the structures and energetics of defects from MD simulations, new insights into defect configurations and relaxation processes are described.

Zhang Y, V Shutthanandan, R Devanathan, S Thevuthasan, DE McCready, JS Young, G Balakrishnan, DM Paul, and WJ Weber. 2004. "Damage Accumulation and Amorphization in Samarium Titanate Pyrochlore." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 218:89-94. Abstract Damage accumulation in Sm2Ti₂O₇ single crystals irradiated with Au₂⁺ ions at 170, 300 and 700 K was studied by Rutherford backscattering spectrometry using a 2.0 MeV He⁺ beam along the <001> channeling direction. The relative disorder on the Sm sublattice follows a nonlinear dependence on ion fluence. The nonlinear behavior is described well by a disorder accumulation model that indicates a predominant role of a defect-stimulated amorphization process. The critical dose for amorphization at 300 K is ~0.14 dpa, which is in good agreement with in-situ transmission electron microscopy results for polycrystalline Sm₂Ti₂O₇ irradiated with 600 keV Bi⁺ ions and with Gd₂Ti₂O₇ doped with 244Cm. Despite the six orders of magnitude difference in damage rates, the good agreement between the amorphization doses in Sm2Ti₂O₇ at 300 K and 244Cm-doped Gd₂Ti₂O₇ at 340 K indicates that damage accumulation at these temperatures is relatively independent of dose rate.

Zhang Y, WJ Weber, W Jiang, V Shutthanandan, S Thevuthasan, M Janson, and A Hallen. 2004. "Annealing Behavior of Al-Implantation-Induced Disorder in 4H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 219-220:647-651. Abstract Single crystal 4H-SiC films were implanted at 150 K with 1.1 MeV Al₂₂⁺ and subsequently annealed at elevated temperatures. Rutherford backscattering spectrometry (RBS) results indicate that the relative Si disorder at the damage peak recovers significantly as the annealing temperature increases. However, the residual Si disorder is more resistant to high-temperature annealing in the region of the implanted Al. The maximum concentration of Al profile measured by secondary ion mass spectroscopy (SIMS) is a factor of 1000 lower than the level of the residual Si disorder at the same region. Analysis of these results indicates that the excess residual Si disorder around the implanted Al projected range cannot be accounted for by just the Al interstitials; instead, it appears that each implanted Al stabilizes or inhibits recovery for an equivalent of a few hundred Si interstitials under the current experimental conditions.

Weber WJ, F Gao, R Devanathan, and W Jiang. 2004. "The Efficiency of Damage Production in Silicon Carbide." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 218:68-73. Abstract Molecular dynamics simulations are used to study the statistics of damage production in 3C-SiC due to C, Si and Au primary knock-on atoms (PKAs) over energies from 0.25 to 50 keV. In order to account for the different displacement energies on the Si and C sublattices and accurately assess the damage efficiency, a modified version of the SRIM (Stopping and Range of Ions in Matter) code, with the electronic stopping turned off to duplicate the molecular dynamics conditions, was used to calculate the statistics of damage production for the same PKAs over the energy range from 0.1 to 400 keV under the binary collision approximation using threshold displacement energies of 20 and 35 eV for C and Si, respectively. Using the modified SRIM predictions as a reference, the efficiencies of total damage production are determined for C, Si and Au PKAs as functions of energy. The efficiency for production of C displacements is similar for all PKAs; however, C PKAs have a much lower efficiency for producing stable Si displacements than Si and Au PKAs, which leads to a much higher ratio of C to Si displacements for C PKAs. These results are consistent with the experimental damage production behavior observed in SiC irradiated with C, Si and Au ions at 150 K.

Weber WJ, F Gao, R Devanathan, W Jiang, and CM Wang. 2004. "Ion-Beam Induced Defects and Nanoscale Amorphous Clusters in Silicon Carbide." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 216:25-35. Abstract Atomic-level simulations have been employed to study the defects and nanoscale disordering induced in 3C-SiC by C, Si, and Au ions with energies up to 50 keV. Energetic C and Si ions primarily produce interstitials, vacancies, antisite defects, and small defect clusters directly during the collision cascade. The overlap of Si cascades produces nanoscale defect clusters. In the case of energetic Au ions, nanoscale amorphous domains are produced directly within the Au cascade along with point defects and smaller clusters. In about 25% of the 50 keV Au cascades, one or more of the subcascades contain nanoscale clusters that exhibit a structure that is consistent with an amorphous state. Structural image simulations of the subcascade structures produced by energetic Si and Au recoils are consistent with experimental high-resolution transmission electron microscopy images. Simulations on close-pair production and recombination in SiC indicate that the activation energies for recombination of most close pairs range from 0.24 to 0.38 eV.

Thevuthasan S, V Shutthanandan, CM Wang, WJ Weber, W Jiang, AS Cavanagh, J Lian, and LM Wang. 2004. "Ion-Beam Synthesis of Epitaxial Au Nanocrystals in MgO." Journal of Materials Research 19(5):1311-1314. Abstract The formation of Au nanoclusters in MgO using ion implantation and subsequent annealing has been investigated. Approximately 1200 and 1400 Au₂⁺ ions/nm₂ were implanted in MgO(100) substrates at 300 and 975 K, respectively. Subsequent annealing in air for 10 hours at 1275 K promoted the formation of Au nanostructures in MgO. The sample implanted at 300 K showed severe radiation damage. In addition, two-dimensional platelet-like structures with possible composition of Au and MgO were formed during implantation in the sample that was implanted at 300 K. In contrast, Au implantation at 975 K promoted the nucleation of Au nanostructures during implantation. Subsequent annealing of both samples show three-dimensional clusters in MgO. However, the 975 K implanted sample shows clean high quality single crystal Au clusters that have an epitaxial relationship to MgO(100).

Sun K, LM Wang, RC Ewing, and WJ Weber. 2004. "Electron Irradiation Induced Phase Separation in a Sodium Borosilicate Glass." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 218:368-374. Abstract Electron-irradiation-induced phase separation in a sodium borosilicate glass was studied in-situ by analytical electron microscopy. Distinctly separate phases that are rich in boron and silicon formed at electron doses higher than 4.0 � 1011 Gy during irradiation. The separated phases are still in amorphous states even at a much high dose (2.1 � 1012 Gy). It indicates that most silicon atoms remain tetrahedrally coordinated in the glass during the entire irradiation period, except some possible reduction to amorphous silicon. The particulate B-rich phase that formed at high dose was identified as amorphous boron that may contain some oxygen. Both ballistic and ionization processes may contribute to the phase separation.

Smith RJ, Y Zhang, V Shutthanandan, LJ Bissell, S Thevuthasan, W Jiang, and WJ Weber. 2004. "NRA and ERDA Investigation of Helium Retention in SiC as a Function of Irradiation and Annealing." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 219-220:631-635. Abstract Silicon carbide has been proposed for coating applications in advanced reactor designs, so studies of its behavior in the presence of ion irradiation and fission products are of Interest. We investigated the retention of He in single crystal 6H SiC as a function of irradiation dose and annealing temperature using both nuclear reaction analysis (NRA) and time-of-flight elastic recoil detection analysis (ToF ERDA). Ions of 3He⁺ were implanted at 40 keV in SiC to a depth of ~200 nm at room temperature. NRA was performed using 1.0 MeV D⁺ and the 3He(D,α)1H reaction. No change in the He profile was seen for irradiation dose up to 6.8x1017 D⁺/cm₂ at room temperature. Isochronal annealing of the SiC between 300 and 1200 K also showed no significant helium loss. Subsequently, a sample was irradiated with D⁺ at 900 K and again at 1100 K. No loss of 3He associated with irradiation was seen for a dose up to 5x1017 D⁺/cm₂. Annealing the sample above 1200 K resulted in thermally activated loss of He. ToF ERDA measurements were performed using 44 MeV 127I10⁺ for both irradiation and analysis. Depth profiles of the He distribution showed no significant change under I bombardment with an ion fluence up to ~1014/cm₂ at room temperature. NRA was performed on the implanted sample subjected to ERDA. The 3He profiles for regions subjected to I irradiation were similar in shape to those with no I irradiation.

Posselt M, F Gao, WJ Weber, and V Belko. 2004. "A Comparative Study of the Structure and Energetics of Elementary Defects in 3C- and 4H-SiC." Journal of Physics. Condensed matter 16(8):1307-1323. Abstract The potential non-equivalent defects in both 3C- and 4H-SiC are classified by a new method that is based on symmetry considerations. In 4H-SiC, their number is considerably higher than in 3C-SiC, since the hexagonal symmetry leads to diversification. The different theoretical methods hitherto used to investigate defects in 3C-SiC are critically reviewed. Classical MD simulations with a recently developed interatomic potential are employed to investigate the stability, structure and energetics of the large number of potential non-equivalent defects that may exist in 4H-SiC. Most of the potential defects in 4H-SiC are found to be stable. The interstitials between hexagonal and trigonal rings, which do not exist in 3C-SiC, are characteristic for 4H-SiC and other hexagonal polytypes. The structure and energetics of some complex and anisotropic dumbbells depend strongly on the polytype. On the other hand, polytypism does not have a significant influence on the properties of the more compact and isotropic defects, such as vacancies, antisites, hexagonal interstitials, and many dumbbells. The results allow conclusions to be drawn about the energy hierarchy of the defects.

Nachimuthu P, S Thevuthasan, MH Engelhard, WJ Weber, DK Shuh, NM Hamdan, BS Mun, EM Adams, DE McCready, V Shutthanandan, DW Lindle, G Balakrishnan, DM Paul, EM Gullikson, RC Perera, J Lian, LM Wang, and RC Ewing. 2004. "Probing Cation Antisite Disorder in Gd₂Ti₂O₇ Pyrochlore by Site-Specific Near-Edge X-ray-Absorption Fine Structure and X-ray Photoelectron Spectroscopy." Physical Review. B, Condensed Matter and Materials Physics 70(10):100101(R), 1-4. Abstract Disorder in Gd₂Ti₂O₇ is investigated by near-edge x-ray absorption fine structure (NEXAFS) and x-ray photoelectron spectroscopy (XPS). NEXAFS shows Ti⁺⁴ ions occupy octahedral sites with a tetragonal distortion induced by vacant oxygen sites. O 1s XPS spectra obtained with a charge neutralization system from Gd₂Ti₂O₇ (100) and the Gd₂Ti₂O₇ pyrochlore used in Phys. Rev. Lett. 88, 105901 (2002), both yielded a single peak, unlike the previous result on the latter that found two peaks. The current results give no evidence for an anisotropic distribution of Ti and O. The extra features reported in the aforementioned communication resulted from charging effects and incomplete surface cleaning. Thus, a result confirming the direct observation of simultaneous cation–anion antisite disordering and lending credence to the split vacancy model has been clarified.

Jiang W, WJ Weber, V Shutthanandan, L Li, and S Thevuthasan. 2004. "Thermal and Dynamic Responses of Ag Implants in Silicon Carbide ." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 219-220:642-646. Abstract A single crystal wafer of 6H-SiC was sequentially implanted in two different areas at 210 and 873 K, respectively, to the same fluence of 500 Ag₂⁺/nm₂. Rutherford backscattering spectrometry (RBS) has been used in a random orientation to profile the Ag implants and along the <0001>-axial channeling direction to determine the defect concentrations. Additional irradiation at 873 K with 5.4 MeV Si₂⁺ ions does not promote diffusion of the implants in either the crystalline or fully-amorphized SiC. There is no evidence of significant diffusion of the implanted Ag in crystalline SiC during thermal annealing at temperatures up to 1573 K. However, it appears that the Ag tends to diffuse toward the surface in amorphous SiC at 1573 K.

Jiang W, Y Zhang, and WJ Weber. 2004. "Temperature Dependence of Disorder Accumulation and Amorphization in Au-Ion Irradiated 6H-SiC." Physical Review. B, Condensed Matter 70(16):165208, 1-8. Abstract Disorder accumulation and amorphization in 6H-SiC single crystals irradiated with 2.0 MeV Au₂⁺ ions at temperatures ranging from 150 to 550 K have been investigated systematically based on 0.94 MeV D⁺ channeling analyses along the <0001> axis. Physical models have been applied to fit the experimental data and to interpret the temperature dependence of the disordering processes. Results show that defect-stimulated amorphization in Au₂⁺-irradiated 6H-SiC dominates the disordering processes at temperatures below 500 K, while formation of clusters becomes predominant above 500 K. Two distinctive dynamic recovery stages are observed over the temperature range from 150 to 550 K, resulting from the coupled processes of close-pair recombination and interstitial migration and annihilation on both sublattices. These two stages overlap very well with the previously observed thermal recovery stages. Based on the model fits, the critical temperature for amorphization in 6H-SiC under the Au₂⁺ ion irradiation conditions corresponds to 501 +- 10 K.

Jiang W, CM Wang, WJ Weber, MH Engelhard, and LV Saraf. 2004. "Direct Determination of Volume Changes in Ion-Beam-Irradiated SiC." Journal of Applied Physics 95(9):4687-4690. Abstract A single crystal 6H-SiC wafer was sequentially implanted in two areas at 873 and 295 K using 2.0 MeV Au₂⁺ ions under off-axis conditions. Identical Au profiles, as a function of atomic areal density, were produced at 873 and 295 K. The linear expansion in the amorphous state produced at 295 K was measured relative to the slightly damaged state produced at 873 K, using the Au profiles as references. The red-shift of the plasmon-loss peak was also used to directly measure the local density changes. Based on these measurements, the volume expansion of the amorphous state in 6H-SiC at 295 K is 11.51.9%, while that in the slightly damaged state at 873 is 0.9%.

Jiang W, V Shutthanandan, S Thevuthasan, DE McCready, and WJ Weber. 2004. "Carbon Analysis using Energetic Ion Beams." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 222(3-4):538-546. Abstract Both Nuclear reaction analysis and non-Rutherford elastic scattering have been widely used for analysis of light elements in solids; these two ion-beam methods complement more traditional analysis by Rutherford backscattering spectrometry. In this study, the reaction/scattering cross-sections for 12C(p,p)12C, 12C(d,p)13C, and 12C(a,a)12C at an angle of 150 (degrees) are measured over relevant energy regions using thin films of carbon (5.8 ug/cm2) on silicate glass. The results are plotted and tabulated as a function of ion energy, with typical uncertainties of 4% for the cross section data. In addition, the angular distribution of the cross sections for the reaction 12C(d,p)13C at an ion energy of 0.94 MeV has also been determined from 100 (degrees) to 170 (degrees). An example for the application of this reaction to SiC is given.

Jiang W, WJ Weber, LM Wang, and K Sun. 2004. "Amorphization Processes in Au Ion Irradiated GaN at 150 - 300 K." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 218:427-432. Abstract Epitaxial single-crystal gallium nitride (GaN) films on sapphire were irradiated at temperatures between 150 and 300 K using 1.0 MeV Au₂⁺ ions over a range of fluences. The accumulation of disorder on the Ga sublattice has been investigated based on 2.0 MeV He+ RBS along the <0001>-axial channeling direction. In general, the degree of disorder in the irradiated GaN increases at low doses and saturates at intermediate doses; at higher doses, a rapid amorphization process occurs as a result of the ingrowth of surface defects. Results from this study indicate that there may be a dynamic recovery stage on the Ga sublattice in GaN between 250 and 300 K. High-resolution TEM studies show that the microstructure in the disorder saturation stage contains a dense network of planar defects (basal-plane dislocation loops and stacking faults), while the more highly disordered regime includes amorphous domains and small crystalline zones that are randomly oriented.

Heinisch HL, LR Greenwood, WJ Weber, and RE Williford. 2004. "Displacement Damage in Silicon Carbide Irradiated in Fission Reactors." Journal of Nuclear Materials 327(2-3):175-181. Abstract Calculations are performed for displacement damage in SiC due to irradiation in the neutron environments of various types of nuclear reactors using the best available models and nuclear data. The displacement damage calculations use recently developed damage functions for SiC that are based on extensive molecular dynamics simulations of displacement events1. Displacements per atom (DPA) cross sections for SiC have been calculated as a function of neutron energy, and they are presented here in tabular form to facilitate their use as the standard measure of displacement damage for irradiated SiC. DPA cross sections averaged over the neutron energy spectrum are calculated for neutron spectra in the cores of typical commercial reactors and in the test sample irradiation regions of several materials test reactors used in both past and present irradiation testing. Particular attention is focused on a next-generation high-temperature gas-cooled pebble bed reactor, for which the high-temperature properties of silicon carbide fiber-reinforced silicon carbide composites are well suited. Calculated transmutations and activation levels in a pebble bed reactor are compared to those in other reactors.

Gao F, EJ Bylaska, A El-Azab, and WJ Weber. 2004. "Wannier Orbitals and Bonding Properties of Interstitial and Antisite Defects in GaN." Applied Physics Letters 85(23):5565-5567. Abstract Intrinsic interstitial and antisite defects in GaN have been studied using density functional theory (DFT), and their configurations, electronic structures and bonding properties have been characterized using the Wannier function. All N interstitial configurations eventually transform into N-N split interstitials, between which two π orbitals exist. The relaxation of a Ga antisite defect also leads to the formation of a N-N split configuration; however, its local Wannier orbitals are remarkably different from the N-N split interstitial. The different local Wannier orbitals around Ga interstitial configurations demonstrate that Ga interstitials are critical defects in GaN. The electronic orbitals of the Ga octahedral interstitial is, for example, greatly delocalized, and there are no covalent bonds formed between the interstitial and the surrounded atoms. The most striking feature is that Ga-Ga split interstitials can bridge the gap between non-bonded Ga atoms, thereby leading to a chain of four metallic-like-bonded Ga atoms in GaN, which may exhibit novel quantum properties.

Gao F, M Posselt, V Belko, Y Zhang, and WJ Weber. 2004. "Structures and Energetics of Defects: A Comparative Study of 3C- and 4H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 218:74-79. Abstract The structures, formation energies and stable configurations of elementary defects (vacancies, antisite defects and self interstitials) in 3C- and 4H-SiC are studied using classical molecular dynamics simulation with a recently developed interatomic potential. The defect structures in 3C-SiC are relatively simple, but those in 4H-SiC are more complicated. The interstitials between hexagonal and trigonal rings are characteristic for 4H-SiC and other hexagonal polytypes, but not for 3C-SiC. The number of non-equivalent defects in 4H-SiC is much higher than that in 3C-SiC, and a considerable difference is found for some complex and anisotropic defects, in particular for the dumbbells D1Si-Si, D1Si-C and D2Si-C. The lattice deformation beyond the first nearest neighbor shell, which depends strongly on the polytype structure, plays an important role on these effects. However, the polytypism does not have a significant influence on the structure and energetics of the more compact and isotropic defects, such as vacancies and antisite defects. Despite the complexity of defect configurations, the tetrahedral interstitials have very similar properties in 3C- and 4H-SiC because their first, second and third nearest neighbor shells are identical.

Gao F, and WJ Weber. 2004. "Mechanical Properties and Elastic Constants Due to Damage Accumulation and Amorphization in SiC." Physical Review. B, Condensed Matter and Materials Physics 69(22):224108, 1-10. Abstract Damage accumulation due to cascade overlap, which was simulated previously, has been used to study the changes of elastic constants, bulk and elastic moduli as a function of dose. These mechanical properties generally decrease with increasing dose, and the rapid decrease at low-dose level indicates that point defects and small clusters play an important role in the changes of elastic constants rather than topological disorder. The internal strain relaxation has no effect on the elastic constants, C11 and C12, in perfect SiC, but it has a significant influence on all elastic constants calculated in damaged SiC. The elastic constants in the cascade-amorphized (CA) SiC decrease about 19%, 29% and 46% for C11, C12 and C44, respectively. The bulk modulus decrease 23% and the elastic modulus decreases 29%, which is consistent with experimental measurements. The stability of both the perfect SiC and CA-SiC under hydrostatic tension has been also investigated. All mechanical properties in the CA-SiC exhibit behavior similar to that in perfect SiC, but the critical stress at which the CA-SiC becomes structurally unstable is one order of magnitude smaller than that for perfect SiC.

Gao F, EJ Bylaska, and WJ Weber. 2004. "Intrinsic Defect Properties in GaN Calculated By Ab Initio and Empirical Potential Methods." Physical Review. B, Condensed Matter and Materials Physics 70(24):245208-1-245208-8. doi:10.1103/PhysRevB.70.245208 Abstract Density functional theory (DFT) has been used to investigate the formation, properties, and atomic configurations of vacancies, antisite defects and interstitials in GaN, and the DFT results are compared with those calculated by molecular dynamics (MD) simulations using two representative potentials. The DFT calculations reveal that the relaxation of vacancies is generally small, but the relaxation around antisite defects is large, especially for the Ga antisite that is not stable and converts to a N+-N<0001> split interstitial plus a Ga vacancy at the original site. The N interstitials, starting from all possible sites, eventually relax into a N+-N<11-20> split interstitial. In the case of Ga interstitials, the most stable configuration is a Ga octahedral interstitial, but the energy difference among all the interstitials is small. The Ga+-Ga<11-20> split interstitial can bridge the gap between non-bonded Ga atoms, thereby leading to a chain of four Ga atoms along the <11-20> direction in GaN. The formation energies of vacancies and antisite defects obtained using the Stillinger-Weber potential (SW) are in reasonable agreement with those obtained by DFT calculations, whereas the Tersoff-Brenner (TB) potential better describes the behavior of N interstitials. In the case of Ga interstitials, the most stable configuration predicted by the TB-model is a Ga+-N<11-20> split interstitial; while for the SW-model the Ga tetrahedral configuration is more stable, which is in contrast to DFT results.

Gao F, WJ Weber, M Posselt, and V Belko. 2004. "Atomistic Study of Intrinsic Defect Migration in 3C-SiC." Physical Review. B, Condensed Matter and Materials Physics 69(24):245205, 1-5. Abstract Atomic-scale computer simulations, both molecular dynamics (MD) and the nudged-elastic band method, have been applied to investigate long-range migration of point defects in 3C-SiC over the temperature range from 0.36 to 0.95 Tm (melting temperature). A wide set of diffusion characteristics has been obtained, including the self-diffusion coefficient, activation energy and defect correlation factor. Stable C split interstitials can migrate via the first or second neighbor sites, but the relative probability for the later mechanism is very low. Si interstitials migrate directly from one tetrahedral position to another neighboring equivalent position by a kick-in/kick-out process via a split interstitial configuration. Both C and Si vacancies jump to one of their equivalent sites through a direct migration mechanism. The migration energies obtained for C and Si interstitials are consistent with those obtained experimentally for the recovery processes in irradiated SiC. Also, energy barriers for C interstitial and vacancy diffusion are in reasonable agreement with ab initio data.

Gao F, WJ Weber, M Posselt, and V Belko. 2004. "Atomic Computer Simulations of Defect Migration in 3C and 4H-SiC." Materials Science Forum 457-460(2004):457-460. Abstract Knowledge of the migration of intrinsic point defects is crucial to understand defect recovery, various annealing stages and microstructural evolution after irradiation or ion implantation. Molecular dynamics (MD) and the nudged-elastic band method have been applied to investigate long-range migration of point defects in SiC over the temperature range from 0.36 to 0.95 Tm , and the defect diffusion coefficient, activation energy and defect correlation factor have been determined. The results show that the activation energies for C and Si interstitials in 3C-SiC are about 0.74 and 1.53 eV, respectively, while it is about 0.77 eV for a C interstitial in 4H-SiC. The minima energy paths reveal that the activation energies for C and Si vacancies are about 4.1 and 2.35 eV, respectively. Finally, the results are discussed and compared with experimental observations and available ab initio data.

Farnan IE, HM Cho, WJ Weber, RD Scheele, NR Johnson, and AE Kozelisky. 2004. "High-Resolution Solid-State Nuclear Magnetic Resonance Experiments on Highly Radioactive Ceramics." Review of Scientific Instruments 75(12):5232-5236. Abstract A triple containment magic-angle spinning rotor insert system has been developed and a sample handling procedure formulated for safety analyzing highly radioactive solids by high resolution solid state NMR. The protocol and containment system have been demonstrated for magic angle spinning (MAS) experiments on ceramic samples containing 5-10 wt% 239Pu and 238Pu at rotation speeds of 3500 Hz. The technique has been used to demonstrate that MASNMR experiments can be used to measure amorphous atomic number fractions produced during accelerated internal radioactive decay. This will allow incorporated ν-emitters with short half-lives to be used to model the long-term radiation tolerance of potential ceramic radioactive waste forms. It is believed to be the first example of MASNMR spectroscopy on samples containing fissionable isotopes.

Ewing RC, WJ Weber, and J Lian. 2004. "Nuclear Waste Disposal—Pyrochlore (A₂B₂O(7)): Nuclear Waste Form for the Immobilization of Plutonium and "Minor" Actinides." Journal of Applied Physics 95(11 ):5949-5971 Part 1. Abstract During the past half-century, the nuclear fuel cycle has generated approximately 1,400 metric tons of plutonium and substantial quantities of the “minor” actinides, such as Np, Am and Cm. The successful disposition of these actinides has an important impact on the strategy for developing advanced nuclear fuel cycles, weapons proliferation and the geologic disposal of high-level radioactive waste. During the last decade, there has been substantial interest in the use of the isometric pyrochlore structure-type, A₂B₂O₇, for the immobilization of actinides. Most of the interest has focused on titanate-pyrochlore because of its chemical durability; however, these compositions experience a radiation-induced transition from the crystalline-to-aperiodic state due to radiation damage from the alpha-decay of actinides. Depending on the actinide concentration, the titanate pyrochlore will become amorphous in less than 1,000 years of storage. Recently, systematic ion beam irradiations of a variety of pyrochlore compositions has revealed that zirconate pyrochlore does not become amorphous, but remains crystalline as a defect fluorite structure due to disordering of the A- and B-site cations. The zirconate pyrochlore will remain crystalline even to very high doses, greater than 100 displacements per atom. Systematic experimental studies of actinide-doped and ion beam-irradiated pyrochlore, studies of natural U-bearing pyrochlore, and simulations of the energetics of the disordering process now provide a rather detailed understanding of the structural and chemical controls of the pyrochlore structure on its response to radiation. These results provide a solid basis for predicting the behavior and durability of pyrochlore used to immobilize plutonium.

Devanathan R, LR Corrales, WJ Weber, A Chartier, and C Meis. 2004. "Molecular Dynamics Simulation of Disordered Zircon." Physical Review. B, Condensed Matter and Materials Physics 69(6):064115, 1-9. Abstract The melting of zircon and the amorphous state produced by quenching from the melt were simulated by molecular dynamics using a new partial charge model combined with the Ziegler-Biersack-Littmark potential. The model has been established for the description of the crystalline and aperiodic structures of zircon in order to be used for the simulation of displacement cascades. It provides an excellent fit to the structure, and accounts with convenient precision the mechanical and thermodynamic properties of zircon. The calculated melting temperature is about 2100 K. The activation energy for self-diffusion of ions in the liquid state was determined to be 190-200 kJ/mole. Melt quenching was employed to produce two different disordered states with distinct densities and structures. In the high density disordered state, the zircon structure is intact but the bond angle distributions are broader, 4% of the Si units are polymerized, and the volume swelling is about 8%. In the low density amorphous state, the Zr and Si coordination numbers are lower, and the Zr-O and Si-O bond lengths are shorter than corresponding values for the crystal. In addition, a highly polymerized Si network, with average connectivity of two, is observed in the low density amorphous state. These features have all been experimentally observed in natural metamict zircon. The present findings, when considered in light of experimental radiation effects studies, suggest that the swelling in zircon arises initially from disorder in the zircon crystal, and at high doses the disordered crystal is unable to accommodate the volume expansion and transforms to the amorphous state.

Devanathan R, F Gao, and WJ Weber. 2004. "Amorphization of Silicon Carbide by Carbon Displacement." Applied Physics Letters 84(19):3909-3911. Abstract We have used molecular dynamics simulations to examine the possibility of amorphizing silicon carbide (SiC) by exclusively displacing C atoms. At a defect generation corresponding to 0.2 displacements per atom, the enthalpy surpasses the level of melt-quenched SiC, the density decreases by about 15%, and the radial distribution function shows a lack of long-range order. Prior to amorphization, the surviving defects are mainly C Frenkel pairs (67%), but Si Frenkel pairs (18%) and anti-site defects (15%) are also present. The results indicate that SiC can be amorphized by C sublattice displacements. Chemical short-range disorder, arising mainly from interstitial production, plays a significant role in the amorphization.

Jiang W, and WJ Weber. 2003. "Experimental Studies of Defects, Implants, and Their Processes in Ion-Irradiated Silicon Carbide Single Crystals." Chapter 19 in Recent Research Developments in Applied Physics, vol. 6, part II, ed. S.G. Pandalai, pp. 451-495. Transworld Research Network, Trivandrum, India. Abstract This chapter reviews the experimental results, obtained by the authors, on the disorder accumulation, disorder recovery, and behavior of implants in ion-irradiated 6H silicon carbide (6H-SiC) single crystals. The disorder on both the Si and C sublattices has been studied using a combination of Rutherford backscattering spectrometry and nuclear reaction analysis in channeling geometry. Damage states have been analyzed based on multiaxial channeling along different orientations. The surface morphologies and damage states have been examined using electron microscopy. Damage accumulation has been investigated as a function of ion fluence, irradiation temperature, ion species, and dose rate. The results of both isochronal and isothermal annealing, as well as dynamic recovery induced by energetic ion beams are summarized. In addition, the behavior of noble-metal implants and gas species in SiC are discussed.

Heinisch HL, LR Greenwood, WJ Weber, and RE Williford. 2003. "Total DPA Cross Sections for SiC as a Function of Neutron Energy." In Fusion Materials: Semi-Annual Progress Report Ending December 31, 2002, vol. 33, ed. Ron Klueh and Renetta Godfrey, pp. 44-48. DOE Office of Fusion Energy Sciences, Washington DC. Abstract Total DPA cross sections for SiC as a function of neutron energy have been calculated using the latest and best knowledge about damage production in SiC. We encourage the adoption of these cross sections as the standard to be used for calculating radiation damage production in DPA for all neutron-irradiated SiC samples, including those in past irradiations.

Ewing RC, A Meldrum, LM Wang, WJ Weber, and LR Corrales. 2003. "Radiation Effects in Zircon." Chapter 14 in Zircon, Reviews in Mineralogy and Geochemistry, vol. 53, ed. J. M. Hanchar and P. W. O. Hoskin, pp. 387-425. Mineralogical Society of America, Washington, DC. Abstract The widespread distribution of zircon in the continental crust, its tendency to concentrate trace elements, particularly lanthanides and actinides, its use in age-dating, and its resistance to chemical and physical degradation have made zircon the most important accessory mineral in geologic studies. Because zircon is highly refractory, it also has important industrial applications, including its use as a lining material in high-temperature furnaces. However, during the past decade, zircon has also been proposed for advanced technology applications, such as a durable material for the immobilization of plutonium or, when modified by ion-beam irradiation, as an optic waveguide material. In all of these applications, the change in properties as a function of increasing radiation dose is of critical importance. In this chapter, we summarize the state-of-knowledge on the radiation damage accumulation process in zircon.

Zhang Y, and WJ Weber. 2003. "Validity of Bragg's Rule for Heavy-Ion Stopping in Silicon Carbide." Physical Review. B, Condensed Matter and Materials Physics 68(23):235317, 1-7. Abstract The stopping powers for O, Al, Cr, Mn, Co and Cu in a self-supported SiC film have been measured in transmission geometry over a continuous range of energies using a time of flight elastic recoil detection analysis (ToF-ERDA) system. These stopping data, along with the stopping data in Si and C obtained previously using the same ions and measurement technique, are used to assess the validity of the Bragg additivity rule for stopping powers in SiC over a range of ions and energies. Within the experimental uncertainties (±4%), the results indicate that Bragg's rule is valid in SiC for the ion species and energy region studied. The measured stopping powers in C, Si and SiC are also compared with the stopping power predictions of the two most recent versions of the SRIM (Stopping and Range of Ions in Matter) codes. While both versions of SRIM show varying degrees of agreement with the measured stopping data, there are significant deviations of the SRIM predictions for some ions and energy regions.

Zhang Y, and WJ Weber. 2003. "Electronic Stopping of He, B, N and Al in SiC." Applied Physics Letters 83(8):1665-1667. Abstract Silicon carbide (SiC) is a wide-band gap semiconductor that has attracted extensive investigations for a wide range of device applications and structural components in harsh nuclear environments. Accurate knowledge of stopping powers in SiC, especially for B, N and Al ion for doping purposes and He ions for ion-beam analysis applications, is highly desirable. In the present study, the electronic energy loss of these ions in a self-supported SiC foil is directly measured in transmission geometry using a ToF ERDA set-up over the energy region of interest. The measured electronic stopping powers are parameterized for easily implementation in any other applications, and compared with the predictions of the SRIM (Stopping and Range of Ions in Matter) code.

Zhang Y, WJ Weber, W Jiang, CM Wang, A Hallen, and G Possnert. 2003. "Effects of Implantation Temperature and Ion Flux on Damage Accumulation in Al-Implanted 4H-SiC." Journal of Applied Physics 93(4):1954-1960. Abstract The effects of implantation temperature and ion flux on damage accumulation on both the Si and C sublattices in 4H-SiC are investigated under 1.1 MeV Al₂₂⁺ irradiation at temperatures from 150 to 450 K. The rate of damage accumulation decreases dramatically and the damage profile sharpens due to significant dynamic recovery at temperatures close to the critical temperature for amorphization. At 450 K, the relative disorder and the density of planar defects increase rapidly with the increasing ion flux, exhibiting saturation at high ion fluxes. Planar defects are generated through the agglomeration of excess Si and C interstitials during irradiation and post-irradiation annealing at 450 K. Termination of (0001) planes is attributed to the accumulation of vacancies. A volume expansion of ~8% is observed for the peak damage region.

Weber WJ, F Gao, W Jiang, and Y Zhang. 2003. "Fundamental Nature of Ion-Solid Interactions in SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 206:1-6. Abstract The integration of experimental and computer simulation studies is providing atomic-level understanding of the nature of ion-beam modification in SiC. Density functional theory calculations and molecular dynamics simulations are used to determine stable defect configurations, defect production, and cascade-overlap effects. These studies show that ion implantation in SiC results primarily in the creation of interstitials, vacancies, antisite defects, and small defect clusters that interact to produce long-range structural disorder. The accumulation of damage and defect configurations on both the Si and C sublattices have been determined for a range of ions using multi-axial channeling measurements. The consistent agreement of the experimental and computational results provides atomic-level insights into the interpretation of experimentally observed features.

Wang CM, S Thevuthasan, V Shutthanandan, AS Cavanagh, W Jiang, LE Thomas, and WJ Weber. 2003. "Microstructure of Precipitated Au Nanoclusters in MgO." Journal of Applied Physics 93(10):6327-6333. Abstract Gold nanoclusters dispersed in single crystal MgO have been prepared by ion implantation at 975 K and subsequent annealing at 1275 K for 10 hours. The morphological features, size, and crystallographic orientation of the Au nanoclusters with respect to the MgO matrix, as well as the interface structure between the Au nanoclusters and MgO, have been investigated using transmission electron microscopy. During annealing, the Au clusters nucleate coherently in the MgO lattice, leading to an epitaxial orientation relationship of [010]MgO//[010]Au and (200)MgO//(200)Au that is maintained for all the Au clusters. Above a critical size of ~5 to 8 nm, a coherent-semicoherent interface transition is observed for the Au clusters in MgO. This critical cluster size is larger than the critical size, ~3 nm, based on energetic consideration. This discrepancy is discussed with respect to the point and extended defect structures at the interface between the Au clusters and the MgO matrix. The Au clusters larger than this critical size exhibit faceting on the {001} planes and internal dislocations. It is further suggested that the density of quantum antidot should depend on the size of the Au clusters.

Wang CM, Y Zhang, WJ Weber, W Jiang, and LE Thomas. 2003. "Microstructrual Features of Al-Implanted 4H-SiC." Journal of Materials Research 18(4):772-779. Abstract The microstructural features of highly-damaged 4H-SiC implanted with Al₂₂⁺ ions at 450 K have been studied using transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). Conventional TEM images reveal that the crystalline SiC domains are highly strained/distorted when the relative disorder on the Si sublattice ranges between about 0.4 and 0.8, as determined by Rutherford backscattering spectrometry in channeling geometry (RBS/C). As the relative disorder approaches to 1.0, the high strain contrast appears to be relieved and localized amorphized domains are observed. Plasmon-loss energy shows a red-shift following the implantation, and the magnitude of the red-shift increases with increasing relative disorder. Based on the red-shift, the estimated volume expansion is ~ 8% for highly-damaged crystalline SiC and ~ 16% for the amorphous state. Energy-loss near-edge-structure (ELNES) of both the C and Si K-edge reveals the existence of Si-Si and C-C bonding in the Al₂₂⁺implanted SiC.

Thevuthasan S, W Jiang, V Shutthanandan, and WJ Weber. 2003. "Accumulation of Ion Beam Induced Disorder in Stronium Titanate." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 206:162-165. Abstract Damage accumulation has been investigated in single crystal SrTiO₃ (100) irradiated with 1.0 MeV Au₂þ by using in situ Rutherford backscattering spectrometry in channeling geometry (RBS/C). Samples were irradiated at temperatures of 170, 300 and 350 K with ion .uences ranging from 0.05 to 0.60 Au₂þ/nm₂. The in situ RBS/C analysis indicates that the relative disorder in both Sr and Ti sublattices shows a strong sigmoidal dependence on ion dose. After an ion .uence of 0.30 Au₂þ/nm₂ at 170 K, the buried region at the damage peak (_60 nm) becomes fully amorphous, which corresponds to a dose of _0.39 dpa. For irradiation at 300 and 350 K, ion .uences of 0.40 Au₂þ/nm₂ (_0.52 dpa) and 0.45 Au₂þ/nm₂ (_0.59 dpa) are necessary to achieve an amorphous state at the damage peak, respectively.

Jiang W, V Shutthanandan, S Thevuthasan, DE McCready, and WJ Weber. 2003. "Oxygen Analysis Using Energetic Ion Beams." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 207(4):453-461. Abstract Using a thin amorphous layer of SiO₂ (5.2 mg/cm2) on Si, cross sections for the nuclear reactions 16O(d,p1)17O, 16O(d,a)14N and 16O(a,a)16O at a laboratory angle of 150° are determined over energies ranging from 0.70 to 1.06 MeV for D⁺ ions and from 2.95 to 3.05 MeV for He⁺ ions. The results are plotted and tabulated as a function of ion energy. An example for the analysis of atomic displacements on the O sublattice in a Au₂⁺-irradiated SrTiO₃ single crystal is given

Jiang W, and WJ Weber. 2003. "Irradiation-Induced Recovery of Disorder in Gallium Nitride." Applied Physics Letters 83(3):458-460. Abstract Gallium nitride has been irradiated to two fluences with energetic Au₂⁺ ions at 300 K. Two different damage levels and depth profiles were produced that are characterized by a near-surface peak and a deeper damage saturation state. Thermal annealing at 873 K resulted in disorder recovery only in the near-surface region at low fluence. However, simultaneous irradiation with 5.4 MeV Si₂⁺ ions during annealing at 873 K induced significant recovery over the entire damage profile at both low and high fluence. The irradiation-enhanced recovery is primarily attributed to defect-stimulated recovery and epitaxial recrystallization processes due to the creation of mobile Frenkel pairs.

Jiang W, WJ Weber, and CM Wang. 2003. "Ion-Beam-Irradiation Induced Defects in Gallium Nitride." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 206:1037-1041. Abstract Epitaxial single-crystal gallium nitride (GaN) films on sapphire were irradiated at low and room temperatures using O⁺ ions over a range of fluences. The accumulation of disorder on the Ga sublattice has been investigated based on He⁺ Rutherford backscattering analysis along the <0001>-axial channeling direction. The degree of disorder in the O⁺ irradiated GaN increases at low doses and saturates above 10 displacements per atom (dpa). The microstructures of two O⁺-irradiated specimens are characterized using high-resolution transmission electron microscopy. Similar planar defect structures at the saturated disorder level are observed for as-irradiated and thermally annealed GaN. The growth and annihilation of these defects during ion irradiation and thermal annealing are believed to contribute to the saturation and stability of defect concentrations.

Jiang W, WJ Weber, Y Zhang, S Thevuthasan, and V Shutthanandan. 2003. "Ion Beam Analysis of Irradiation Effects in 6H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 207(1):92-99. Abstract Irradiation in 6H-SiC single crystal wafers has been performed at temperatures ranging from 150 to 550 K using 2.0 MeV Au₂⁺ or at 300 K using 50 keV He⁺ ions. Additional irradiation for the He⁺-irradiated specimen was carried out near room temperature using 50 MeV I10⁺ ions to ~0.1 ions/nm₂. In-situ isothermal annealing for 6H-SiC irradiated at 500 K to 2.0 Au₂⁺/nm₂ was also conducted up to 90 min at the irradiation temperature. The lattice disorder in the irradiated samples has been determined using either 2.0 MeV He⁺ or 0.94 MeV D⁺ channeling analysis along the <0001> axis. Results show that there is a substantial diffusion of the Si defects into a greater depth during the Au₂⁺ irradiation at 500 and 550 K. Complete amorphization at 550 K does not occur up to a maximum fluence of 15 Au₂⁺/nm₂ in this study. Significant thermal recovery of the Si defects produced at 150 K was not observed during the subsequent thermal annealing at 500 K. Following the I10⁺ irradiation in the He⁺-irradiated specimen near room temperature, remarkable recrystallization at the amorphous-crystalline interfaces around the damage profile is observed.

Jiang W, WJ Weber, S Thevuthasan, and LA Boatner. 2003. "Effect of Ion Irradiation in Cadmium Niobate Pyrochlores." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 207(1):85-91. Abstract Irradiation experiments have been performed for cadmium niobate pyrochlore (Cd₂Nb2O₇) single crystals at both 150 and 300 K using 1.0 MeV Au₂⁺ ions over fluences ranging from 0.01 to 0.10 ions/nm₂. In-situ 3.0 MeV He⁺ Rutherford backscattering spectrometry along the <100>-axial channeling direction (RBS/C) has been applied to study the damage states ranging from small defect concentrations to a fully amorphous state. Results show that the crystal can be readily amorphized under the irradiation conditions. Room-temperature recovery of the defects produced at 150 K has been observed, while the defects produced at 300 K are thermally stable at room temperature. Results also indicate that the RBS/C analysis used in this study induced negligible damage in the near-surface regime. In addition, irradiation at and below room temperature using He⁺ and C3⁺ ions leads to surface exfoliation at the corresponding damage peaks.

Gao F, and WJ Weber. 2003. "Recovery of Close Frenkel Pairs Produced by Low Energy Recoils in SiC." Journal of Applied Physics 94(7):4348-4356. Abstract The analysis of defects generated by displacement cascades in silicon carbide shows that most defects are point defects that are displaced at a small distance from their original sites. These defects play important roles in the recovery processes observed experimentally. Only 20% of total the interstitials produced become freely migrating defects that can contribute to microstructural evolution during irradiation of SiC. The stable defect configurations in this study were created with low-energy recoils using molecular dynamics (MD) methodology, with most defects being C Frenkel pairs. These stable Frenkel pairs have been annealed at different temperatures, using MD methods, to determine the time required for interstitials to recombine with vacancies. The MD data have been analyzed using an Arrhenius relation, and the estimated activation energies for defect recovery is between 0.22 and 1.6 eV for C Frenkel pairs and between 0.28 and 0.9 eV for Si Frenkel pairs. The low activation energies qualitatively overlap with experimental values observed during Stage I recovery. Thus, the present results suggest that the Stage I is associated with the spontaneous recovery of Frenkel pairs. Based on the data obtained, the spontaneous recombination distance is determined to be 0.66 and 0.70 ao for the C and Si sublattices, respectively.

Gao F, GA Henkelman, WJ Weber, LR Corrales, and H Jonsson. 2003. "Finding Possible Transition States of Defects in Silicon Carbide and α-Iron using the Dimer Method." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 202:1-7. Abstract Energetic primary recoil atoms from ion implantation or fast neutron irradiation produce isolated point defects and clusters of both vacancies and interstitials. The migration mechanisms and mobility of these defects are crucial to the successful multiscale modeling of microstructural evolution during ion-implantation, thermal annealing, or under irradiation over long periods of time. In this paper, the dimer method is employed to search the possible transition states of interstitials and small interstitial clusters in SiC and alpha-Fe. The method uses only the first derivatives of the potential energy to find saddle points without knowledge of the final state of the transition. In SiC, the possible migration pathway for the C interstitial is found to consist of the first neighbor jump via a Si site or second neighbor jump, but the relative probability for the second neighbor jump is very low. In alpha-Fe, the possible transition states are studied as a function of interstitial cluster size, and the lowest energy barriers correspond to defect migration along <111> directions, as seen in molecular dynamics simulations. However, this paper addresses whether migrating interstitial clusters can thermally change their direction. The activation energies for changing the direction of these clusters are determined, and the corresponding mechanisms are discussed in detail. Finally, the results are discussed in terms of modeling the long time scale dynamics of defects under irradiation or thermal annealing.

Gao F, and WJ Weber. 2003. "Atomic-Scale Simulations of Cascade Overlap and Damage Evolution in Silicon Carbide." Journal of Materials Research 18(8):1877-1883. Abstract In a previous computer simulation experiment, the accumulation of damage in SiC from the overlap of 10 keV Si displacement cascades at 200 K was investigated, and the damage states produced following each cascade were archived for further analysis. In the present study, interstitial clustering, system energy, and volume changes are investigated as the damage states evolve due to cascade overlap. An amorphous state is achieved at a damage energy density of 27.5 eV/atom (0.28 displacements per atom). At low dose levels, most defects are produced as isolated Frenkel pairs, with a small number of defect clusters involving only 4 to 6 atoms; however, after the overlap of 5 cascades (0.0125 displacements per atom), the size and number of interstitial clusters increases with increasing dose. The average energy per atom increases linearly with increasing short-range (or chemical) disorder. The volume change exhibits two regimes of linear dependence on system energy and increases more rapidly with dose than either the energy or the disorder, which indicate a significant contribution to swelling of isolated interstitials and anti-site defects. The saturation volume change for the cascade-amorphized state in these simulations is 8.2%, which is in reasonable agreement with the experimental value of 10.8% in neutron-irradiated SiC.

Gao F, and WJ Weber. 2003. "Atomic-Level Study of Ion-Induced Nanoscale Disordered Domains in Silicon Carbide." Applied Physics Letters 82(6):913-915. Abstract Atomic-level simulations have been employed to study the nanoscale disordering induced in 3C-SiC by energetic Si and Au ions (up to 50 keV). Topologically disordered nanoscale domains are generated with low frequency in the cascades produced by Au ions, whereas Si ions create only a few small defect clusters, with most defects being single interstitials and monovacancies. The structural image simulations of the nanoscale domains provide for atomic-level insights into disordered states. The simulations suggest that it is possible to design and fabricate nanoscale optoelectronic devices based on SiC using ion-beam-induced order-disorder transformation.

Gao F, and WJ Weber. 2003. "Atomic Simulation of Ion-Solid Interaction in Ceramics." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 207(1):10-20. Abstract Understanding dynamic processes during ion irradiation, as well as irradiation-induced microstructural changes, requires fundamental knowledge on defect properties, defect generation in atomic collision processes, multiple ion-solid interactions and defect migration. The multiple scale simulation methods are presented in this paper, and in particular, an application on SiC is discussed in detail. Density functional theory (DFT) has been employed to determine defect energetics and the most favorable interstitial configurations in SiC. Based on DFT calculations, a new empirical potential has been developed in order to carry out large-scale simulations of microstructural evolution. Multimillion atom systems (up to 6 million) have been used to study defect production, defect clustering, multiple ion-solid interactions and structural evolution in SiC. The defect-stimulated growth and coalescence of clusters represents an important mechanism for irradiation-induced crystalline-to-amorphous (c-a) transformation. The relative disordering and swelling behavior, as well as HRTEM image simulations, based on molecular dynamics results provide atomic-level interpretations of experimentally observed features in SiC.

Corrales LR, and WJ Weber. 2003. "State of Theory and Computer Simulations of Radiation Effects in Ceramics." Current opinion in solid state & materials science 7(1):35-40. Abstract This article presents opinions based on the presentations and discussions at a Workshop on Theory and Computer Simulations of Radiation Effects in Ceramics held in August 2002 at Pacific Northwest National Laboratory in Richland, WA, USA. The workshop was focused on the current state-of-the-art of theory, modeling and simulation of radiation effects in oxide ceramics, directions for future breakthroughs, and creating a close integration with experiment.

Corrales LR, WJ Weber, A Chartier, C Meis, and JP Crocombette. 2003. "Comment on 'Large Swelling and Percolation in Irradiated Zircon'." Journal of Physics. Condensed matter 15(37):6447-6456. Abstract A recent model for the large radiation-induced swelling behavior in irradiated zircon (ZrSiO4) is partially based on results of molecular dynamics simulations of the partial overlap of two collision cascades that predict a densified boundary of polymerized silica and the scattering of the second cascade away from the densified boundary (Trachenko K, Dove M T and Salje E K H 2003 J. Phys.: Condens. Matter 15 L1). These MD simulations are based on an atomic interaction potential for zircon (Trachenko K, Dove MT and Salje EKH 2001 J. Phys.: Condens. Matter 13 1947) for which, according to our analysis, only reproduces some of the crystallographic properties at equilibrium and do not adequately describe the scattering physics for zircon, and on simulation methodologies for which the standard procedures for boundary conditions of energetic events are ill-defined. In fact, the interatomic potential model used by Tranchenko et al yields a significantly more rigid structure, with very high Frenkel defect formation energies and extremely low entropy and specific heat capacity. The synergy of all these unphysical properties for zircon, naturally leads to highly localized collision cascades. Consequently, the reported results of the cascade simulations, which are events far from equilibrium, may be artifacts of both the potential model and simulation methodologies employed. Thus, the structural changes predicted by the simulations must be viewed cautiously, and these simulations results cannot be taken as confirmation of a new scattering physics process that is the basis for the proposed swelling model. In this comment, the deficiencies in the atomic interaction potential and methodologies employed by these authors are critically reviewed, and the validity of the cascade overlap simulations and proposed physics is discussed.

Chartier A, C Meis, JP Crocombette, LR Corrales, and WJ Weber. 2003. "Atomistic Modeling of Displacement Cascades in La2Zr2O7 Pyrochlore." Physical Review. B, Condensed Matter and Materials Physics 67(17):174102, 1-13. Abstract An empirical potentials molecular dynamics method was used to simulate the a-recoil effects in the lanthanum zirconate pyrochlore La2Zr2O7, at 350 K, where a tetravalent uranium ion was used as the primary knock-on atom with a kinetic energy of 6 keV. The displacement cascades simulations have been carried out along four different crystallographic directions. A detailed analysis indicates that the primary damage state associated with the cascades remains crystalline and consists of point defects, such as cations antisites configurations, various interstitials, and vacancies. There is no evidence for direct amorphization within the cascades. The results are consistent with experimental evidence as well as with previous theoretical work based on static calculations.

Muller I, WJ Weber, ER Vance, GG Wicks, and DG Karraker. 2002. "Glass, Ceramics, and Composites." Chapter 10 in Advances in Plutonium Chemistry 1967-2000, ed. Darleane Hoffman, pp. 260-307. American Nuclear Society, La Grange Park, IL. Abstract Many studies of plutonium in glass and ceramics have taken place in the thirty years covered by this book. These studies have led to a substantial understanding, arising from fundamental research of actinides in solids and research and development in three technical fields: immobilization of the high level wastes (HLW) from commercial nuclear power plants and processing of nuclear weapons materials, environmental restoration in the nuclear weapons complex and, most recently, the immobilization of weapons-grade plutonium as a result of disarmament activities.

Zhang Y, G Possnert, and WJ Weber. 2002. "Measurement of Electronic Stopping Power of Swift Heavy Ions using High-Resolution Time-of-Flight Spectrometer." Applied Physics Letters 80(24):4662-4664. Abstract Using only Time of Flight (ToF) information to determine energy loss, a new procedure based on a modified ToF-Energy Elastic Recoil Detection Analysis (ToF-E ERDA) set-up has been developed to simultaneously measure the electronic stopping powers of swift heavy ions over a continuous range of energies. This novel approach is demonstrated by measuring the stopping powers of Be, C, Si and Br in amorphous C over a continuous energy range from ~50 to ~900 keV per nucleon. In this procedure, the energy prior to entering the stopping foil and the exit energy are both measured by the ToF section. The Si detector, normally present in the ToF-E ERDA configuration, is only used to screen out the extraneous components. This procedure eliminates the well-known calibration problems of Si detectors when used with heavy ions. Consequently, the stopping powers and the energy dependence are determined with high precision. New stopping power data are determined in some energy regimes, and the overlapping of the present data with existing literature data in other energy ranges exhibits good agreement. However, the SRIM stopping powers exhibit some deviation from the measured values, up to 10%, in the energy range of 300-500 keV per nucleon. The modified set-up can be easily applied to measure the stopping power in any self-supporting foil.

Zhang Y, WJ Weber, W Jiang, A Hallen, and G Possnert. 2002. "Evolution and Recrystallization of Buried Amorphous Layers in Al₂₂⁺ Implanted 4H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 195(3-4):320-328. Abstract Epitaxial 4H-SiC has been irradiated at 150 K with 1.1 MeV Al₂ molecular ions to fluences ranging from 1.35 x 1013 to 8.00 x 1014 Al⁺ cm-₂. The damage accumulation on both the C and Si sublattices of the as-implanted samples indicates a sigmoidal dependence on ion fluence, and a buried amorphous layer is formed at 2.00 x 1014 Al⁺ cm-₂. The width of the damage peak is unchanged until the local dose at the damage peak exceeds a critical amorphization value (0.12 dpa). The evolution and isochronal recovery of the buried amorphous layers at higher fluences were investigated by in-situ Rutherford backscattering spectrometry rapidly at 2.00 x 1014 Al⁺ cm-₂ and eventually saturated at a thickness of about 500 nm at high fluences. Recovery processes occurred at both the rear interface and in the surface region, and relative amount of recovery decreased with increasing fluence for temperatures up to 870 K. No recovery is observed at or near the surface at the highest fluence, which supports the absence of any residual crystallinity in the surface region.

Zhang Y, WJ Weber, W Jiang, A Hallen, and G Possnert. 2002. "Damage Evolution and Recovery on both Si and C Sublattices in Al-implanted 4H-SiC Studied by Rutherford Backscattering Spectroscopy and Nuclear Reaction Analysis." Journal of Applied Physics 91(10):6388-6395. Abstract Damage evolution in 4H-SiC epitaxial layers irradiated with 1.1 MeV Al₂ molecular ions at 150 K to ion fluences from 0.15 to 2.25 Al/nm₂ and subsequent damage recovery following isochronal annealing at temperatures up to 870 K were studied by Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA). Using a 0.94 MeV D⁺ beam in channeling geometry, disorder on both the Si and C sublattices was characterized simultaneously by measuring scattering/reaction yields from RBS combined with 12C(d,p)13C NRA analysis. The relative disorder on the as-implanted sublattices follows a sigmoidal dependence on ion fluence and the relative disorder on the C sublattice is higher than that on the Si sublattice at low ion fluences. Isochronal recovery on both the Si and C sublattices exhibits similar nonlinear dependence on annealing temperature, with several step-like recovery regimes that are associated with different recovery processes.

Weber WJ, W Jiang, F Gao, and R Devanathan. 2002. "Ion-Solid Interactions and Defects in Silicon Carbide." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 190(1-4):261-265. Abstract Experimental and computational approaches are used to study the defects, defect clusters and long-range structural disorder produced by interaction of energetic ions with SiC. The accumulation and recovery of disorder on the Si and C sublattices are determined by ion-beam analysis methods in channeling geometry. Ab initio calculations have determined the most stable interstitial configurations, which are consistent with multi-axial channeling measurements. Molecular dynamics methods have been used to study both the energy dependence of defect production and the dynamic processes of cascade overlap. The experimental measurements of damage accumulation and the results of molecular dynamics are consistent with each other. Thus, the integration of experimental and computational studies is providing atomic-level understanding of irradiation-induced defects and disordering processes in SiC.

Weber WJ, W Jiang, Y Zhang, and A Hallen. 2002. "Damage Evolution and Recovery in 4H and 6H Silicon Carbide Irradiated with Aluminum Ions." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 191(1-4):514-518. Abstract Damage evolution and isochronal recovery have been studied in single crystal 4H and 6H SiC irradiated with 1.1 MeV Al₂ molecular ions at 150 K to ion fluences ranging from 0.15 to 2.85 Al+/nm₂. The damage evolution and recovery on both the Si and C sublattices were determined using a 0.94 MeV deuterium beam in ion channeling geometry by simultaneously measuring the scattering/reaction yield from Rutherford backscattering spectrometry combined with 12C(d,p)13C nuclear reaction analysis. The rate of damage evolution at 150 K is higher for 4H-SiC than for 6H-SiC. At low doses, the rate of C disordering is higher than that for Si, which is consistent with the lower displacement energy for C. Both 4H and 6H SiC exhibit only minor damage recovery below 300 K. Above 300 K, damage recovery on the Si and C sublattices is similar for both 4H and 6H SiC. Three distinct recovery stages are observed on each sublattice in 4H-SiC, and at high doses, where a buried amorphous layer is produced, an additional recovery stage is observed above 800 K.

Wang CM, W Jiang, WJ Weber, and LE Thomas. 2002. "Defect Clustering in GaN Irradiated with O⁺ Ions." Journal of Materials Research 17(11):2945-2952. Abstract Transmission electron microcopy (TEM) is used to study microstructures formed in GaN irradiated with 600 keV O⁺ ions at room temperature. Three types of defect clusters are identified in the irradiated GaN: (1) basal-plane stacking faults with dimensions ranging from 5 to 30 nm, (2) Pyramidal dislocation loops, and (3) local regions of highly-disordered material. High resolution TEM imaging clearly reveals that one type of the basal-plane stacking faults corresponds to insertion of one extra Ga-N basal-plane in the otherwise perfect GaN lattice. The interpretation of these results indicates that interstitials of both Ga and N preferentially condense on the basal plane to form a new layer of Ga-N under these irradiation conditions. The formation of these extended defects and their interactions with the point defects produced during irradiation contribute to a dramatic increase in the dynamic recovery of point defects in GaN at room temperature.

Meldrum A, LA Boatner, WJ Weber, and RC Ewing. 2002. "Amorphization and Recrystallization of the ABO(3) Oxides." Journal of Nuclear Materials 300(2-3):242-254. Abstract Single crystals of the ABO3 phases CaTiO3, SrTiO3, BaTiO3, LiNbO3, KNbO3, LiTaO3, and KTaO3 were irradiated by 800 keV Kr+, Xe+, or Ne+ ions over the temperature range from 20 to 1100 K. The critical amorphization temperature, Tc, above which radiation-induced amorphization does not occur varied from approximately ~450 K for the titanate compositions to more than 850 K for the tantalates. While the absolute ranking of increasing critical amorphization temperatures could not be explained by any simple physical parameter associated with the ABO3 oxides, within each chemical group defined by the B-site cation (i.e., within the titanates, niobates, and tantalates), Tc tends to increase with increasing mass of the A-site cation. Tc was lower for the Ne+ irradiations as compared to Kr+, but it was approximately the same for the irradiations with Kr+ or Xe+. Thermal recrystallization experiments were performed on the ion-beam-amorphized thin sections in situ in the transmission electron microscope (TEM). In the high vacuum environment of the microscope, the titanates recrystallized epitaxially from the thick areas of the TEM specimens at temperatures of 800 to 850 K. The niobates and tantalates did not recrystallize epitaxially, but instead, new crystals nucleated and grew in the amorphous region in the temperature range 825 to 925 K. These new crystallites apparently retain some "memory" of the original crystal orientation prior to ion-beam amorphization.

Jones RH, L Giancarli, A Hasegawa, Y Katoh, A Kohyama, B Riccardi, LL Snead, and WJ Weber. 2002. "Promise and Challenges of SiCf/SiC Composites for Fusion Energy Applications." Journal of Nuclear Materials 307(Pt B):1057-1072. Abstract Silicon carbide fiber/silicon carbide matrix composites are a promising structural material for fusion energy applications. They have been specified in several recent fusion power plant design studies because of their high operating temperature (1000-1100?C) and hence high energy conversion efficiencies. Radiation resistance of the b-phase of SiC, excellent high-temperature fracture, creep, corrosion and thermal shock resistance and safety advantages arising from low induced radioactivity and afterheat are all positive attributes favoring the selection of SiCf/SiC composites. With the promise of these materials comes a number of challenges such as their thermal conductivity, radiation stability, gaseous transmutation rates, hermetic behavior and joining technology. Recent advances have been made in understanding radiation damage in SiC at the fundamental level through molecular dynamics simulations of displacement cascades. Radiation stability of composites made with the advanced fibers of Nicalon Type S and the UBE Tyranno SA, where no change in strength was observed up to 10 dpa at 800?C, in the development of materials with improved thermal conductivity, modeling of thermal conductivity, joining techniques and models for life-prediction. High transmutation rates of C and Si to form H, He, Mg, and Al continue to be a concern.

Jiang W, WJ Weber, JS Young, and LA Boatner. 2002. "Irradiation-Induced Formation of Nanoparticles in Cadmium Niobate Pyrochlore." Applied Physics Letters 80(4):670-672. Abstract Sequential irradiation with 3 MeV He⁺ and 10 MeV C3⁺ ions performed at T = 150 K produces two separate amorphous buried layers in cadmium niobate pyrochlore (Cd₂Nb₂O₇) single crystals. Further irradiation at room temperature results in the formation of nanometer-scale particles in the amorphized regions. An ion-cleaving technique was used to facilitate the observation of these nanoparticles by using scanning electron microscopy (SEM). Complete granulation with particle sizes ranging from 30 to 150 nm was observed. X-ray energy-dispersive spectrometry (EDS) analysis indicates that the numerically large population of smaller particles (~50 nm) contains a high Cd content (~70%) and the numerically smaller population of larger particles (>100 nm) contains negligible Nb with a Cd-to-O ratio of about 1:0.54.

Jiang W, WJ Weber, CM Wang, and Y Zhang. 2002. "Disordering Behavior and Helium Diffusion in He⁺ Irradiated 6H-SiC." Journal of Materials Research 17(2):271-274. Abstract Single crystal 6H-SiC wafers were irradiated at 300 K with 50 keV He⁺ ions to fluences ranging from 7.5 to 250 He⁺/nm₂. Ion-channeling experiments with 2.0 MeV He⁺ Rutherford backscattering spectrometry (RBS) were performed to determine the depth profile of Si disorder. The measured profiles are consistent with SRIM-97 simulations at and below 45 He⁺/nm₂, but remarkably higher than the SRIM-97 prediction at both 100 and 150 He⁺/nm₂. Cross-sectional transmission electron microscopy (XTEM) study indicates a homogeneous-amorphization process in the material under the irradiation conditions. Results from elastic recoil detection analysis (ERDA) suggest that the implanted He atoms diffuse in a high-damage regime toward the surface.

Jiang W, WJ Weber, S Thevuthasan, and V Shutthanandan. 2002. "Deuterium Channeling Study of Disorder in Al-₂(₂⁺)-Implanted 6H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 190(1-4):636-640. Abstract Single crystal 6H-SiC wafers have been irradiated 60? off normal at 150, 190, 250 and 295 K using 1.1 MeV Al₂₂⁺ ions over fluences from 0.15 to 2.85 ions/nm₂. The accumulation and recovery of disorder on both the Si and C sublattices have been measured simultaneously using in situ 0.94 MeV D⁺ Rutherford backscattering 28Si(d,d)28Si and nuclear reaction 12C(d,p)13C along the <0001>-axial channeling direction. The behavior of disorder accumulation and recovery on the Si and C sublattices is similar. The data suggest that a dynamic recovery stage occurs between irradiation temperatures of 190 and 250 K. At intermediate doses, isochronal annealing (20 min) results show that significant thermal recovery occurs between 420 and 720 K. Complete recovery is not observed by thermal annealing up to the highest temperature (870 K) used in this study.

Jiang W, WJ Weber, S Thevuthasan, and V Shutthanandan. 2002. "Channeling Study of Lattice Disorder and Gold Implants in Gallium Nitride." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 191(1-4):509-513. Abstract Irradiation experiments have been performed 60? off normal for a GaN single crystal film at 300 K using 3 MeV Au3+ ions over fluences ranging from 0.88 to 86.2 ions/nm2. The accumulation of disorder on both the Ga and N sublattices has been simultaneously investigated using 3.8 MeV He+ non-Rutherford backscattering spectrometry along the <0001> and <10 1> axial channeling directions. The accumulated disorder at the damage peak increases with dose below 10 dpa, and saturates at a relative level of ~0.7 between 10 and 60 dpa. Complete amorphization starts at the surface and grows into the damage peak regime. A higher rate of disordering on the N sublattice is observed at low damage levels, which suggests a lower threshold displacement energy on the N sublattice in GaN. Isochronal annealing (20 min) at temperatures up to 1000 K has been used to follow the thermal response of the Ga disorder and Au implants. Some disorder recovery occurs at the intermediate doses. A fraction of Au occupancy on the Ga lattice site is observed in the as-implanted GaN, and the substitutional fraction of the implanted Au increases with increasing temperature.

Hess NJ, BD Begg, SD Conradson, DE McCready, PL Gassman, and WJ Weber. 2002. "Spectroscopic Investigations of the Structural Phase Transition in Gd2(Ti1-yZry)(2)O-7 Pyrochlores ." Journal of Physical Chemistry B 106(18):4663-4677. Abstract The Gd2(Ti1-yZry)2O7 pyrochlore series undergoes a structural phase transition from pyrochlore (Fd?3m) to defect fluorite (Fm?3m) that can be driven compositionally by increasing the Zr content or thermally by sintering Zr-rich compositions at temperatures above 1550 ?C. Our results demonstrate that ion-beam irradiation can also drive the structural phase transition for Zr-rich compositions. In an effort to understand the effects of composition and ion-beam irradiation on this phase transition, powder X-ray diffraction, polarized Raman, reflection infrared, and X-ray absorption spectroscopy experiments were conducted on Gd2(Ti1-yZry)2O7 pyrochlores prior to and following irradiation with 2 MeV Au2+ ions to a fluence of 5 ions/nm2. Analysis of the vibrational and X-ray absorption data suggests that the structural integrity of the pyrochlore structure is based on distorted corner-shared TiO6 or ZrO6 octahedra. The vibrational spectra indicate that both anion and cation disorder precede the compositionally-driven phase transition, while cation disorder appears to dominate the irradiation-driven transition. Analyses of the extended X-ray absorption fine structure of the Ti and Zr K-edges and the Gd LIII-edges reveal a significant change in the Gd local environment upon irradiation and with increasing Zr content. The Ti and Zr local environments are less affected by irradiation or compositional change, but show evidence of increasing disorder that can be attributed to rotations about shared polyhedral edges and corners.

Heinisch HL, LR Greenwood, WJ Weber, and RE Williford. 2002. "Displacement Damage Cross Sections for Neutron-irradiated Silicon Carbide." Journal of Nuclear Materials 307-311(Part 2):895-899. Abstract Displacements per atom (DPA) is a widely used damage unit for displacement damage in nuclear materials. Calculating the DPA for SiC irradiated in a particular facility requires a knowledge of the neutron spectrum as well as specific information about displacement damage in that material. In recent years significant improvements in displacement damage information for SiC have been generated, especially the energy required to displace an atom in an irradiation event and the models used to describe electronic and nuclear stopping. Using this information, numerical solutions for the displacement functions in SiC have been determined from coupled integro-differential equations for displacements in polyatomic materials and applied in calculations of spectral-averaged displacement cross sections for SiC. This procedure has been used to generate spectrally averaged displacement cross sections for SiC in a number of reactors used for radiation damage testing of fusion materials, as well as the ARIES-IV conceptual fusion device.

Gao F, and WJ Weber. 2002. "Empirical Potential Approach for Defect Properties in 3C-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 191(1-4):504-508. Abstract Defect energetics in SiC have been widely studied using Tersoff potentials, but these potentials do not provide a good description of interstitial properties. In the present work, an empirical many-body interatomic potential is developed by fitting to various equilibrium properties and defect formation in bulk SiC, using a lattice relaxation fitting approach. This parameterized potential has been used to calculate defect formation energies and to determine the most stable configurations for interstitials using the molecular dynamics (MD) method. Although the formation energies of vacancies are smaller than those obtained by ab initio calculations, the properties of antisite defects and interstitials are in good agreement with the results calculated by ab initio methods. It is found that the most favorable configurations for C interstitials are <100> and <110> dumbbells on both Si and C sites, with formation energies from 3.04 and 3.95 eV. The most favorable Si interstitial is the tetrahedral interstitial site, surrounded by four C atoms, with formation energy of 3.97 eV. The present results will be discussed and compared with those obtained by others using various empirical potentials in SiC.

Gao F, WJ Weber, and R Devanathan. 2002. "Defect Production, Multiple Ion-Solid Interactions and Amorphization in SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 191(1-4):487-496. Abstract Recent progress in the atomic-scale simulations of fundamental damage production processes in SiC is reviewed, which includes the displacement threshold energy surface, the primary damage state and statistics of defect production, multiple ion-solid collision events and structural evolution in SiC. The threshold energy surface, Ed, appears to be highly anisotropic, and the results of molecular dynamics (MD) simulations, in conjunction with experimental studies, suggest that Ed values of 20 eV for C and 35 eV for Si should be used in Kinchin-Pease calculations. The Si displacement cascades with energies up to 50 keV show that the surviving defects are dominated by C interstitials and vacancies, consistent with experimental observations. The defect production efficiency decreases with increasing recoil energy, but the number and size of clusters or complex domains formed at the end of cascades are very small, independent of cascade energy. A large number of 10 keV displacement cascades were randomly generated in a model crystal to simulate multiple ion-solid interaction and damage accumulation. The coalescence of clusters represents an important mechanism leading to the complete amorphization of SiC, and the relative disorder and swelling behavior show an excellent agreement with experimental observations. HRTEM images simulated from the MD cell reveal the microstructural evolution of multiple ion-solid collision events, and provide atomic-level interpretations of experimentally observed features in SiC.

Gao F, and WJ Weber. 2002. "Cascade Overlap and Amorphization in 3C-SiC: Defect Accumulation, Topological Features and Disordering ." Physical Review. B, Condensed Matter 66(2):024106, 1-10. Abstract Molecular dynamics (MD) simulations with a modified Tersoff potential have been used to investigate cascade overlap, damage accumulation and amorphization processes in 3C-SiC over dose levels comparable to experimental conditions. A large number of 10 keV displacement cascades were randomly generated in a model crystal to produce damage and cause amorphization. At low dose, the damage state is dominated by point defects and small clusters, and their concentration increases sigmoidally with increasing dose. The coalescence and growth of clusters at intermediate and higher doses is an important mechanism leading to amorphization in SiC. The homogenous nucleation of small clusters at low dose underpins the homogenous-like amorphization observed in SiC. A large increase in the number of antisite defects at higher dose indicates that both interstitials and antisite defects play an important role in producing high-energy states that lead to amorphization in SiC. The topologies (such as total pair correlation function, bond-angle and bond-length distributions) of damage accumulation in the crystal suggest that a crystalline-to-amorphous (c-a) transition occurs at about 0.28 dpa, which is in reasonable agreement with the experimental value of 0.27 dpa under similar irradiation conditions. After the model crystal transforms to the fully amorphous state, the long-range order is completely lost, while the short-range order parameter saturates at a value of about 0.43.

Gao F, and WJ Weber. 2002. "Atomic-Scale Simulations of Multiple Ion-Solid Interactions and Structural Evolution in Silicon Carbide." Journal of Materials Research 17(2):259-262. Abstract Molecular dynamics (MD) have been employed in atomic-level simulations of fundamental damage production processes due to multiple ion-solid collision events in cubic SiC. Isolated collision cascades produce single interstitials, vacancies, antisite defects, and small defect clusters. As the number of cascades (or equivalent dose) increases, the concentration of defects increases and the collision cascades begin to overlap. The coalescence of defects and clusters with increasing dose is an important mechanism leading to amorphization in SiC and is consistent with the homogeneous amorphization process observed experimentally in SiC. The driving force for the crystalline-amorphous (c-a) transition is the accumulation of both interstitials and antisite defects. High-resolution transmission electron microscopy (HRTEM) images of the defect accumulation process and loss of long-range order in the MD simulation cell are consistent with experimental HRTEM images and disorder measurements. Thus, the MD simulations are providing atomic-level insights into the interpretation of experimentally observed features associated with multiple ion-solid collision events in SiC.

Chartier A, C Meis, WJ Weber, and LR Corrales. 2002. "Theoretical Study of Disorder in Ti-Substituted La2Zr2O7." Physical Review. B, Condensed Matter 65(13):134116, 1-11. Abstract In this work, computer simulation studies on Ti substituted La2Zr2O7 are used to investigate the defect mechanisms that can lead to the disordered fluorite structure. A detailed study of defect formation and migration activation energies is carried out by a combination of ab-initio and classical methods. It is found that La2Zr2O7 has a tendency to transform to the disordered fluorite structure, whereas, substitution of Zr with Ti has a tendency to prevent this transition and could instead facilitate radiation-induced amorphization.

Williford RE, and WJ Weber. 2001. "Computer Simulation of Pu3+ and Pu4+ Substitutions in Gadolinium Zirconate." Journal of Nuclear Materials 299(2):140-147. Abstract Atomistic computer simulations have been used to determine the energetics of a variety of defect reactions related to the incorporation of Pu3+ and Pu4+ into the pyrochlore and the fluorite-type structures of Gd2Zr2O7. The lowest energy states were found for Pu3+ substitutions on Gd sites in the pyrochlore (-1.00 eV/Pu) and the fluorite-type (-1.55 eV/Pu) structures, so these defect reactions are the most likely configurations under reducing conditions that favor Pu3+ ions. Slightly higher, but still exothermic, energies (-0.26 to ?0.45 eV) were calculated for Pu4+ substitutions on Zr sites for several fluorite-type cases, indicating that oxidizing conditions should favor Pu4+ incorporation on Zr sites in Gd2Zr2O7 hosts. Defect reactions involving cation vacancies or interstitials exhibited significantly higher energies, and are therefore not expected to occur. Mean field calculations indicated that the increases in crystal volume associated with Pu incorporation are minimized by the excess free volume associated with the Gd site in the pyrochlore structure. Volume changes upon thermal phase transformation from fluorite to pyrochlore are smaller for the material incorporating Pu by substitution than for the virgin material, with a slight advantage for the reducing conditions associated with Pu3+ substitutions on Gd sites

Weber WJ, W Jiang, and S Thevuthasan. 2001. "Accumulation, Dynamic Annealing and Thermal Recovery of Ion-Beam-Induced Disorder in Silicon Carbide." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 175-177:26-30. Abstract Ion-beam-induced disordering in single crystals of 6H-SiC has been investigated for a wide range of ion species (from H⁺ to Au2⁺) using in situ ion-channeling methods. Silicon carbide is readily amorphized below room temperature with all ions. The rate of ion-beam-induced disordering decreases with decreasing ion mass and with increasing temperature. Analysis of limited data suggests that the activation energy for dynamic recovery during irradiation below 300 K is on the order of 0.1 eV. Thermal annealing indicates similar three-stage recovery on both the Si and C sublattices, which suggests similar recovery processes and activation energies. The activation energies for thermal recovery on the Si sublattice are estimated to be 0.3 ? 0.15 eV (Stage I), 1.3 ? 0.25 eV (Stage II), and 1.5 ? 0.3 eV (Stage III).

Thevuthasan S, W Jiang, and WJ Weber. 2001. "Cleaving Oxide Films Using Hydrogen Implantation." Materials Letters 49(6):313-317. Abstract Precise cleaving of oxide films with known thickness using hydrogen implantation and subsequent annealing was investigated using strontium titanate (SrTiO₃) as a model material. Rutherford backscattering in channeling geometry (RBS/C), nuclear reaction analysis (NRA), and scanning electron microscopy (SEM) have been used to characterize this process.

Thevuthasan S, W Jiang, V Shutthanandan, and WJ Weber. 2001. "Accumulation and Thermal Recovery of Disorder in Au₂⁺-Irradiated SrTiO₃." Journal of Nuclear Materials 289(1-2):204-209. Abstract Damage accumulation and thermal recovery processes have been investigated in single crystal SrTiO₃ (100) irradiated with 1.0 MeV Au₂⁺ using in-situ Rutherford Backscattering Spectroscopy in channeling geometry (RBS/C). Samples were irradiated at temperatures of 170 K and 300 K with ion fluences ranging from 0.10-0.40 Au₂⁺/nm₂. The in situ RBS/C analysis indicates that the relative disorder shows a strong sigmoidal dependence on ion dose. After an ion fluence of 0.30 Au₂⁺/nm₂ at 170 K, the buried region at the damage peak (~60 nm) becomes fully amorphous, which corresponds to a dose of ~ 0.8 dpa. For irradiation at 300 K, an ion fluence of 0.40 Au₂⁺/nm₂(~1.1 dpa) is necessary to achieve an amorphous state at the damage peak. An analysis of the defects dechanneling factor suggests that the irradiation regions consist mostly of interstitial atoms or amorphous clusters. In situ thermal annealing experiments were performed to study damage recovery processes up to a maximum temperature of 870 K. The thermal recovery processes occur over a broad temperature range, and the disorder created by low ion fluences, 0.10-0.27 Au₂⁺/nm₂, is almost completely recovered after annealing at 870 K.

Shutthanandan V, S Thevuthasan, JS Young, TM Orlando, and WJ Weber. 2001. "Hydrogen-Damage Interactions in Yttrium-Stabilized Zirconia." Journal of Nuclear Materials 289(1-2):128-135. Abstract Hydrogen diffusion and accumulation in oxidized Zr and ZrO2/Zr interfaces lead to hydrogen induced cracking in Zr-based alloys that are extensively used in nuclear reactors. In this study, the interaction of hydrogen with irradiation damage in (001) single crystals of yttrium-stabilized zirconia, Y-ZrO2, has been investigated as a function of damage accumulation and annealing temperature. Samples were irradiated with 40 keV hydrogen ions at a temperature of 1220 K to ion fluences of 5x10 16 and 1x10 17 ions/cm2, and isochronal annealing experiments were performed in the temperature range from 300 to 770 K in 100 K steps. Damage accumulation and hydrogen profile measurements indicate unusual damage recovery behavior, pinning of hydrogen by damage, and surface deformation due to hydrogen blisters and bubbles.

Park B, WJ Weber, and LR Corrales. 2001. "Molecular Dynamics Simulation Study of Threshold Displacements and Defect Formation in Zircon." Physical Review. B, Condensed Matter 64(17):174108, 1-16. Abstract Unavailable/originally entered 9/26/2000 and somehow was deleted during the PM2 Clean up. Re-entered.

Muller I, and WJ Weber. 2001. "Plutonium in Crystalline Ceramics and Glasses." MRS Bulletin 26(9):698-706. Abstract Many studies of plutonium in glass and ceramics have been conducted since its discovery and have been partly motivated by the desire to immobilize and safely store this long-lived radioactive material. These studies have led to a substantial understanding, arising from fundamental research on actinides in solids and research and development in three technical fields: immobilization of the high level wastes (HLW) from commercial nuclear power plants and processing of nuclear weapons materials, environmental restoration in the nuclear weapons complex and, most recently, the immobilization of weapons-grade plutonium as a result of disarmament activities.

Jiang W, and WJ Weber. 2001. "Multiaxial Channeling Study of Disorder Accumulation and Recovery in Gold-Irradiated 6H-SiC." Physical Review. B, Condensed Matter 64(12):125206, 1-11. Abstract Single crystal 6H-SiC has been irradiated 60 degrees off normal with 2 MeV Au₂⁺ ions at 300 K to fluences of 0.029, 0.058 and 0.12 ions/nm₂, which produced relatively low damage levels. The disorder profiles as a function of ion fluence on both the Si and C sublattices have been determined simultaneously in situ using Rutherford backscattering and nuclear reaction analysis with 0.94 MeV D+ ions in channeling geometry along the <0001>, <1 02> and <10 1> axes. Along the <0001> axis at these low doses, similar levels of Si and C disorder are observed, and the damage accumulation is linear with dose. However, along <1 02> and <10 1>, the disorder accumulation is larger and increases sublinearly with dose. Furthermore, a higher level of C disorder than Si disorder is observed along the <1 02> and <10 1> axes, which is consistent with a smaller threshold displacement energy on the C sublattice in SiC. The mean lattice displacement, perpendicular to each corresponding axis, ranges from 0.014 to 0.037 nm for this range of ion fluences. A steady accumulation of small displacements due to lattice stress is observed along the <10 1> axis, and a detectable reduction of the lattice stress perpendicular to the <0001> axis occurs at 0.12 Au₂⁺/nm₂. There is only a moderate recovery of disorder, produced at and below 0.058 Au₂⁺/nm₂, during thermal annealing at 570 K; more significant recovery is observed for 0.12 Au₂⁺/nm₂ along both the <0001> and <1 02> axes.

Jiang W, WJ Weber, and S Thevuthasan. 2001. "Ion Implantation and Thermal Annealing in Silicon Carbide and Gallium Nitride." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 178:204-208. Abstract Ion-beam-induced disordering in single crystals of 6H-SiC and single-crystal films of GaN has been investigated using ion-channeling methods. Amorphization in GaN requires a dose that is about 30 and 100 times higher than in SiC at 180 and 300 K, respectively. Dynamic defect-recovery processes in both materials increase with increasing irradiation temperature. Amorphization in SiC is consistent with a combined direct-impact and defect-stimulated process. Three recovery stages are observed on both the Si and C sublattices under isochronal annealing in Au₂⁺-irradiated 6H-SiC. In GaN, an intermediate saturation state is observed for disordering at the damage peak, which suggests enhanced defect annihilation processes. Implanted Au diffuses towards the surface during implantation at 300 K and undergoes further diffusion into the amorphous surface layer during post-implantation annealing at 870 K.

Jiang W, WJ Weber, and S Thevuthasan. 2001. "Damage Accumulation and Recovery in Gold-Ion-Irradiated Barium Titanate." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 175:610-614. Abstract Single-crystal barium titanate (BaTiO₃) wafers were irradiated 60? off the surface normal at 170 and 300 K using 1.0 MeV Au₂⁺ ions over a fluence range from 0.03 to 0.19 ions/nm₂. Disorder on both the Ba and Ti sublattices has been studied in situ using Rutherford backscattering spectrometry along the <110> axial direction. At these irradiation temperatures, the temperature dependence of disordering is small. The dose for amorphization under these conditions is on the order of 0.5 dpa, which is 50% of that required to amorphize SrTiO₃ under similar conditions. At low damage levels, recovery of disorder is observed at room temperature, suggesting at least one lower temperature recovery stage. For more highly damaged states, two distinct recovery stages have been identified between 420 and 570 K and between 720 and 870 K. The recovery stage between 420 and 570 K is associated with the critical temperature for full amorphization (~550 K) in BaTiO₃. The higher temperature recovery stage is most likely associated with epitaxial recrystallization.

Jiang W, WJ Weber, S Thevuthasan, and V Shutthanandan. 2001. "Accumulation and Recovery of Disorder on Silicon and Carbon Sublattices in Ion-Irradiated 6H-SiC." Journal of Nuclear Materials 289(1-2):96-101. Abstract Irradiation experiments have been performed at 100, 170 and 300 K for 6H-SiC single crystals using Au₂⁺and He+ ions over a range of fluences. The evolution of disorder on the both Si and C sublattices has been simultaneously investigated using 0.94 MeV D⁺ Rutherford backscattering spectrometry in combination with 12C(d,p) nuclear reaction analysis in a <0001> axial channeling geometry. The results show that the dependence of disorder on dose is consistent with a combined direct-impact / defect-stimulated model. At low doses, a slightly higher rate of C disordering is observed, which is consistent with molecular dynamics simulations that suggest a smaller threshold displacement energy on the C sublattice. At higher doses, the rate of C disordering decreases more rapidly than the rate of Si disordering, which suggests a higher rate of dynamical recovery on the C sublattice under the irradiation conditions. Three distinct recovery stages are observed on both the Si and C sublattices in the Au₂⁺-irradiated 6H-SiC. However, complete recovery of irradiation-induced disorder does not occur during isochronal annealing at temperatures up to 970 K.

Gao F, JR, R Devanathan, JR, and WJ Weber. 2001. "Si Displacement Cascades Revealed by Atomic-Scale Simulation in 3C-SiC." Fusion Technology 39(2):574-578. Abstract The primary damage by displacement cascades in 3C-SiC at 300 K has been studied by molecular dynamics (MD). A large number of cascades, with energies from 0.2 to 50 keV, have been simulated in order to investigate the effects of energy in defect production and clustering. The surviving defects are dominated by C interstitials and vacancies. The number of Frenkel pairs increases with increasing cascade energy, but the efficiency of their production declines with increasing energy in a similar fashion to that found in metals. Although the number of antisite defects is smaller than that of Frenkel pairs, their production also increases with increasing cascade energy. Most surviving defects are single interstitials and vacancies, and the tendency of interstitials to form clusters is very week. The size of the interstitial clusters is very small, which shows significantly different behavior than obtained by MD simulations in metals. The current results provide the statistics of the primary damage states in SiC as a function of primary knock-on energy, which are important in upscaling these results to model behavior over longer time and length scales.

Gao F, WJ Weber, and W Jiang. 2001. "Primary Damage States Produced by Si and Au Recoils in SiC: A Molecular Dynamics and Experimental Investigation." Physical Review. B, Condensed Matter 63(21):214106, 1-6. Abstract Molecular dynamics (MD) simulations, experimental studies and a theoretical model have been combined in an investigation of the disordering and amorphization processes in SiC irradiated with Si and Au ions. In MD simulations, large disordered domains, consisting of interstitials and antisite defects, are created in the cascades produced by Au primary knock-on atoms (PKAs); whereas Si PKAs generate only small interstitial clusters, with most defects being single interstitials and vacancies distributed over a large region. The data for cluster spectrum obtained from MD simulations have been used to calculate the relative cross sections for in-cascade amorphization (or clustering)and in-cascade defect-stimulated amorphization. The ratio of these cross sections for Si and Au is in excellent agreement with thhose derived from the experimental data based from a fit of the direct-impact/defect-stimulated model. This suggests that the observed higher disordering rate and the residual disorder after thermal annealing at 300 K for irradiation with Au2+ are associated with a higher probability for the in-cascade amorphization or large disordered cluster formation. The observed different behavior for the accumulation and recovery of disorder in SiC irradiated by Si and Au ions is quantatatively consistent the present MD simulations and direct-impact/defect-stimulated model.

Gao F, EJ Bylaska, WJ Weber, and LR Corrales. 2001. "Native Defect Properties in Beta-SiC: Ab Initio and Empirical Potential Calculations." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 180(1-4):286-292. Abstract There is considerable ambiguity about the formation of native defects and their clusters in SiC, since different empirical potential gives different results, particular related to the stability of interstitial configurations. Ab intio pseudopotential methods are used to study the formation and properties of native defects in beta-SiC. The results are compared with those calculated by molecular dynamics (MD) using a Tersoff potential, where the various cut-off distances found in the literature are employed. The formation energy of vacancies and antisite defects obtained by ab initio calculations are in good agreement with those given by the Tersoff potential, regardless of the cut-off distances, but there is a disparity for interstitials between the two methods, depending on the cut-off distances used in the Tersoff potential. The present results, however, provide guidelines for evaluating the quality and fit of empirical potentials for large-scale simulations of irradiation damage (displacement cascades) and point defect migration (recombination or annealing) in SiC.

Gao F, JR, and WJ Weber. 2001. "Computer Simulation of Disordering and Amorphization by Si and Au Recoils in 3C-SiC." Journal of Applied Physics 89(8):4275-4281. Abstract Molecular dynamics (MD) has been employed to study the disordering and amorphization processes in SiC irradiated with Si and Au ions. The large disordered domains, consisting of interstitials and antisite defects, are created in the cascades produced by Au primary knock-on atoms (PKAs); whereas Si PKAs generate only small interstitial clusters, with most defects being single interstitials and vacancies distributed over a large region. No evidence of amorphization is found at the end of the cascades created by Si recoils. However, the structure analysis indicates that the large disordered domains generated by Au recoils can be defined as an amorphous cluster lacking long range order. The driving force for amorphization in this materials is due to the local accumulation of Frenkel pairs and antisite defects. These results are in good agreement with experimental evidences, i.e. the observed higher disordering rate and the residual disorder after annealing for irradiation with Au2+ are associated with a higher probability for the in-cascade amorphization or formation of large disordered cluster.

Gao F, WJ Weber, and R Devanathan. 2001. "Atomic-Scale Simulation of Displacement Cascades and Amorphization in Beta-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 180(1-4):176-186. Abstract Molecular Dynamics methods with a modified Tersoff potential have been used to simulate Si displacement cascades with energies up to 50 keV and to compare clustering behavior for Si and Au recoils in beta-SiC. The results show that the lifetime of thermal spike is very short (0.7 ps) compared to metals and that the surviving defects are dominated by C interstitials and vacancies. Only 19% of the interstitial population is contained in clusters, with the the largest cluster containing only 4 interstitial atoms. The energy dependence of stable defect formation exhibits a power-law relationship. The high energy recoils generate multiple cascades and form dispersed defect configurations. These results suggest that the amorphization in SiC does not occur during the cascade lifetime. On the other hand, large disordered domains are created in cascades produced by 10 keV Au recoils. Structure analysis indicates that these highly disordered regions have amorphous characteristics. The data for the cluster spectra have been used to calculate relative cross sections for in-cascade amorphization and defect-stimulated amorphization. The ratios of these cross sections for Si and Au are in excellent agreement with those derived from a fit of the direct-impact/defect-stimulated model to experimental data.

Gao F, EJ Bylaska, WJ Weber, and LR Corrales. 2001. "Ab Initio and Empirical Potential Studies of Defect Properties in 3C-SiC." Physical Review. B, Condensed Matter 64(24):245208, 1-7. Abstract Density functional theory (DFT) is used to study the formation and properties of native defects in 3C-SiC. The extensive calculations have been carried out to determine the formation of point defects and the stability of self interstitials. Although there is a good agreement in the formation of vacancies and antisite defects between the present study and previous calculations, a large disparity appears in the formation of self interstitials. It is found that the most stable configuration of interstitials is the C-C dumbbell along the <100> direction at a C site, with a formation energy of 3.16 eV. The present DFT results are also compared with those calculated by molecular dynamics (MD) simulations using the Tersoff potentials, with parameters obtained from the literature. The formation energy of vacancies and antisite defects obtained by MD calculations are in good agreement with those obtained by DFT calculations. However, the MD calculations yield different results for interstitials energies and structures that depend on the cut-off distances used in the Tersoff potentials. The results provide guidelines for evaluating the quality and fit of empirical potentials for large-scale simulations of irradiation damage and defect migration processes in SiC.

Devanathan R, WJ Weber, and F Gao. 2001. "Atomic Scale Simulation of Defect Production in Irradiated 3C-SiC." Journal of Applied Physics 90(5):2303-2309. Abstract Molecular dynamics simulations using a modified Tersoff potential have been used to study the primary damage state and statistics of defect production in displacement cascades in 3C-SiC. Recoils with energies from 0.25 to 30 keV were simulated at 300 K. The results indicate that: 1) the displacement threshold energy surface is highly anisotropic; 2) the dominant surviving defects are C interstitials and vacancies; 3) the defect production efficiency decreases with increasing recoil energy; 4) defect clusters are much smaller and more sparse compared to those reported in metals; and 5) a small fraction of the surviving defects are anti-sites.

Begg BD, NJ Hess, WJ Weber, R Devanathan, JP Icenhower, S Thevuthasan, and BP McGrail. 2001. "Heavy-Ion Irradiation Effects on Structures and Acid Dissolution of Pyrochlores." Journal of Nuclear Materials 288(2-3):208-216. Abstract The temperature dependence of the critical dose for amorphization, using 0.6 MeV Bi+ ions, for A₂Ti₂O₇ pyrochlores, in which A=Y, Sm, Gd and Lu, exhibits no significant effect of A-site ion mass or size. The room temperature dose for amorphization was found to be ~ 0.18 dpa in each case. After irradiation with 2 MeV Au₂⁺ ions glancing-incidence XRD revealed that each pyrochlore underwent an irradiation-induced structural transformation to fluorite in conjunction with amorphization. The effect of amorphization on the dissolution rates of fully dense pyrochlores, at 90oC and pH 2 (nitric acid) varied from a factor of 10-15 increase for Gd₂Ti₂O₇ to none for Y₂Ti₂O₇. Significant differences were observed in the A-site dissolution rates from the crystalline pyrochlores, indicating differences in the manner in which the A-site cations are incorporated into the pyrochlore structure. These indications were supported by Raman spectroscopy.

Begg BD, NJ Hess, DE McCready, S Thevuthasan, and WJ Weber. 2001. "Heavy-Ion Irradiation Effects in Gd2(Ti2-xZrx)O7 Pyrochlores." Journal of Nuclear Materials 289(1-2):188-193. Abstract Gd2(Ti2-xZrx)O7 samples with 0 = x = 1.5 were single-phase and pyrochlore structured after sintering at 1600oC in air. The Gd2Zr2O7 (x=2) end member was predominantly fluorite-structured. Raman spectroscopy indicated that the level of short-range fluorite-like disorder in the unirradiated Gd2(Ti2-xZrx)O7 samples increased significantly as Zr was substituted for Ti, despite the retention of a long-range pyrochlore structure for samples with 0 = x = 1.5. Glancing-incidence X-ray diffraction indicated that pyrochlores with an ionic radii ratio rA/rB = 1.52 (x=1.5) were transformed into a radiation resistant fluorite-structure after irradiation at room temperature with 2 MeV Au2+ to a fluence of 5 ions/nm2. As the ionic radii ratio of the pyrochlore increased beyond rA/rB > 1.52, the fluorite structure became increasingly unstable with respect to the amorphous state under identical irradiation conditions.

Williford RE, BD Begg, WJ Weber, and NJ Hess. 2000. "Computer Simulation of Pu3+ and Pu4+ Substitutions in Zircon." Journal of Nuclear Materials 278(2-3):207-211. Abstract Energy minimization methods were used in atomistic computer simulations to determine the energetics of Pu(3+) and Pu(4+) substitutions and interstitials in zircon, including the effect of ion size. The lowest energy was found for Pu(4+) on Zr sites. The lowest energy for Pu(3+) substitutions was found for the defect cluster 2Pu(3+) on Zr sites plus an oxygen vacancy. Mean field calculations of unit-cell volumes for 8% Pu substitutions were in agreement with XRD data.

Weber WJ, and RC Ewing. 2000. "Plutonium Immobilization and Radiation Effects." Science 289(5487):2051-2052. Abstract Sickafus et al. in "Radiation Tolerance of Complex Oxides" (4 August, p. 748) note "that compounds with the fluorite structure are especially stable to a displacive radiation damage environment" and propose that fluorite structure-types, such as Er2Zr2O7, have important applications in nuclear waste management. The most obvious application is plutonium immobilization. We earlier proposed Gd2Zr2O7 for plutonium immobilization ( ) based on our discovery of the radiation resistance of compositions in the gadolinium-zirconate system (1,2) and the high neutron cross-section for Gd. This letter clarifies the need for radiation-resistance and comments on the stability of fluorite structure types.

Weber WJ. 2000. "Models and Mechanisms of Irradiation-Induced Amorphization in Ceramics." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 166-167:98-106. Abstract A number of models have been developed to describe the various amorphization processes and the effects of temperature on the kinetics of amorphization. These models are reviewed and in some cases further developed. In general, these models contain a number of parameters relating to irradiation-assisted and thermal recovery processes, which make their application to existing data sets challenging. Nonetheless, general aspects of the models yield insights into the rate-limiting processes controlling the kinetics of amorphization within a given temperature regime. Several examples are used to illustrate features of the models and to highlight differences in behavior.

Weber WJ, W Jiang, and S Thevuthasan. 2000. "Defect Annealing Kinetics in Irradiated 6H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 166-167:410-414. Abstract Isochronal and isothermal annealing of ion-irradiation damage on the Si sublattice in 6H-SiC has been investigated experimentally by in-situ Rutherford Backscattering Spectrometry in channeling geometry (RBS/C). At low ion fluences corresponding to dilute concentrations of irradiation-induced defects, complete recovery of disorder on the Si sublattice can occur below room temperature. The implantation of helium impedes the defect recovery processes at low temperatures. Below room temperature, the thermal recovery of defects on the Si sublattice has an activation energy on the order of 0.25 eV. Recovery of disorder on the Si sublattice above 570 K has an activation energy on the order of 1.5 eV.

Wang LM, SX Wang, RC Ewing, A Meldrum, RC Birtcher, PP Newcomer Provencio, WJ Weber, and H Matzke. 2000. "Irradiation-Induced Nanostructures." Materials Science and Engineering. A. Structural Materials: Properties, Microstructure and Processing 286(1):72-80.

Thevuthasan S, W Jiang, JS Young, and WJ Weber. 2000. "Investigation of Thermal Recovery Behavior in Hydrogen-Implanted SrTiO₃ Using High Energy Ion Beam Techniques." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 161-163:544-548. Abstract Perovskite materials, such as SrTiO₃, are of broad interest for applications ranging from electronic devices to the immobilization of high-level nuclear wastes. As a result, there is renewed interest in developing a fundamental understanding of irradiation and implatation effects on the properties and performance of SrTiO₃. In this study, low-energy (40 keV) H⁺ was implanted into single crystal SrTiO₃ at ~ 120 K over a range of ion fluences. Rutherford Backacattering Spectroscopy in channeling geometry (RBS/C) was used to investigate the ion-beam-induced disordering and recovery processed in the irradiated SrTiO₃. The resonant nuclear reaction, 1H(19F, alpha-gamma)16O, was used to profile the implanted H. For annealing temperatures up to ~570 K, isochronal annealing results indicate increasing disorder on the Sr, Ti, and O sublattices with temperature in the vicinity of the implanted hydrogen, possibly as a result of reaction of the implated hydrogen with the lattice. This increased disorder showed evidence for recovery at 570 K and decreased with further increases in annealing temperature up to 770 K. isochronal annealing at 870 K resulted in a uniform increase in the backscattering yield across the penetration depth of the implant hydrogen. Scanning electron microscopy (SEM) showed that blisters (or bubble-like platlets) of several microns diameter formed below the surface of the SrTiO₃ after the 870 K anneal; thus deforming the surface above the blister. The blisters most likely contain H₂. Such blistering is a well-known phenomenon in gas-implanted metals and was extensively investigated in the 1970's. This may be the first reported observation for blistering in a crystalline oxide. Secondary ion mass spectroscopy (SIMS), and Infrared spectroscopy (IR) measurements are planned to charaterize the nature of the chemical bonding in the implanted region. These results will be discussed in detail.

Park B, WJ Weber, and LR Corrales. 2000. "Molecular Dynamics Study of the Threshold Displacement Energy in MgO." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 166-167:357-363. Abstract Molecular dynamics (MD) simulations were used to obtain threshold displacement energies (Ed) of atoms along specific directions in MgO. In the simulation, an atom in the center of the cell was given kinetic energy along a specific crystallographic direction. The positions and site potential energies of the primary knock-on atom (pka) and all neighboring atoms were monitored as a function of time.

Jiang W, WJ Weber, S Thevuthasan, and SX Wang. 2000. "Irradiation Effects and Thermal Annealing Behavior in H2(+)-Implanted 6H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 166-167:374-378. Abstract RBS/Channeling has been used to study the accumulation and isochronal recovery of disorder on the Si sublattice in 6H-SiC single crystals irradiated with 100 keV H₂⁺ ions at 100 and 300 K. The disorder at the damage peak shows a sigmoidal dependence on ion fluence for both irradiation temperatures. Dramatic simultaneous recovery is observed for the irradiation at 300 K. At low fluences, isochronal recovery occurs gradually over a wide temperature range. At high fluences, a near amorphous state is produced, and significant recovery does not occur. At intermediate fluences, damage recovery occurs more rapidly between 300 and 670 K. Further annealing at 1070 K results in the formation of blisters. Hydrogen depth profiles at 100 and 300 K are comparable and are well predicted by SRIM-97 simulations. Hydrogen release of about 30% is observed for SiC irradiated at 100 K and subsequently annealed at 1070 K for 20 min.

Jiang W, WJ Weber, and S Thevuthasan. 2000. "In-situ Ion Channeling Study of Gallium Disorder and Gold Profiles in Au-implanted GaN." Journal of Applied Physics 87(11):7671-7678. Abstract Disorder accumulation and annealing behavior on the Ga sublattice in GaN implanted with 1.0 MeV Au ions (60? off surface normal) at 180 or 300 K have been studied using in-situ Rutherford Backscattering Spectrometry in a <0001>-channeling geometry (RBS/C). Complete amorphization in GaN is attained at 6.0 Au ions/nm₂ and 20 Au ions/nm₂ for irradiation at 180 and 300 K, respectively. A saturation in the Ga disorder at and behind the damage peak was observed at intermediate ion fluences at both 180 and 300 K. No measurable thermal recovery was found at 300 K for full range of damage produced at 180 K. However, distinct epitaxial regrowth in the bulk and Ga reordering at surface occurred after annealing at 870 K. The implanted Au readily diffuses into highly damaged regions at elevated temperatures, and the redistribution of the Au atoms in the implanted GaN varies with the damage profiles. A double-peak Au profile developed with the maxima located in the amorphous surface region and near the Au mean projected range. The result is interpreted as Au atom diffusion into the amorphous regime at surface and trapping at the defects in the crystal structure. This trapping effect is also evidenced in this study by the suppressed recovery of the Au-decorated disorder in GaN.

Jiang W, S Thevuthasan, WJ Weber, and R Grotzschel. 2000. "Deuterium Channeling Analysis for He(+)-Implanted 6H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 161-163:501-504. Abstract Deuterium ion channeling is applied to study accumulated disorder on Si and C sublattices in 6H-SiC crystals irradiated with 50 keV He(+) ions at 100 and 300 K. The relative disorder on both sublattices follows sigmoidal dependence on dose. Carbon disorder is higher at low doses, suggesting a smaller C displacement energy. Isochronal annealing data show that the recovery behavior on the Si and C sublattices is similar. Annealing of a buried amorphous SiC layer, produced at 100 K, exhibits an epitaxial growth rate of ~0.154 nm/K in the temperature range from 370 to 870 K.

Hess NJ, FJ Espinosa-Faller, SD Conradson, and WJ Weber. 2000. "Beta Radiation Effects in 137Cs-Substituted Pollucite." Journal of Nuclear Materials 281(1):22-33. Abstract The effect of high energy beta radiation on the long-range and local structure of 137 Cs-substituted CsAlSi2O6 (pollucite) was studied with X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) experiments at the Cs K-edge...

Gao F, and WJ Weber. 2000. "Atomic Scale Simulation of 50 keV Si Displacement Cascades in Beta-SiC." Physical Review. B, Condensed Matter 63(5):054101, 1-7. Abstract Molecular dynamics (MD) methods with a modified Tersoff potential have been used to simulate high-energy (50 keV) displacement cascades in b-SiC. The results show that the lifetime of the thermal spike is on the order of 0.7 ps, 10 times shorter than that in metals, and the surviving defects are dominated by C interstitials and vacancies, which is similar to behavior for 10 keV cascades in SiC. Antisite defects are generated on both sub-lattices. Although the total number of antisite defects remaining at the end of the cascade is smaller than that of Frenkel pairs, the number of Si antisites is larger than the number of Si interstitials. Most surviving defects are single interstitials and vacancies, and only 19% of the interstitial population is contained in clusters. The size of the interstitial clusters is small, and the largest cluster found, among all the cascades considered, contained only 4 interstitial atoms, which is significantly different behavior than obtained by MD simulations in metals. It is observed that all clusters are created by a quenched-in mechanism directly from the collisional phase of the cascade due to the very short lifetime of the thermal spike. The initial Si recoil traveled about 65 nm on average, generating multiple sub-cascades and forming a dispersed arrangement in the cascade geometry. These results suggest that in-cascade or direct-impact amorphization in SiC does not occur with any high degree of probability during the cascade lifetime of Si cascades, even with high-energy recoils, consistent with previous experimental and MD observations.

Devanathan R, and WJ Weber. 2000. "Displacement Energy Surface in 3C and 6H SiC." Journal of Nuclear Materials 278(2-3):258-265. Abstract The phase stability of 3C-SiC upon heating and the threshold displacement energy (Ed) surfaces for C and Si primary knock-on atoms (PKAs) in 3C-SiC and 6H-SiC have been investigated using molecular dynamics simulations. A recently optimized Tersoff potential is used in conjunction with an ab initio repulsive potential to represent the interactions between atoms. The simulations provide important insights into phase separation of SiC upon heating, and indicate a strong anisotropy in the Ed surface for both Si and C PKAs. The two polytypes show many similarities in the nature of the Ed surface. The average displacement energy is separately determined by simulating 30 different 500 eV cascades in 3C-SiC. The minimum displacement energies of 21 eV for C and 35 eV for Si are in excellent agreement with interpretation of experimental observations and the simulations of 500 eV cascades.

Begg BD, NJ Hess, WJ Weber, SD Conradson, MJ Schweiger, and RC Ewing. 2000. "XAS and XRD Study of Annealed Pu-238- and Pu-239-Substituted Zircons (Zr0.92Pu0.08SiO4)." Journal of Nuclear Materials 278(2-3):212-224.

Williford RE, WJ Weber, R Devanathan, and AN Cormack. 1999. "Native Vacancy Migrations in Zircon." Journal of Nuclear Materials 273:164-170.

Williford RE, WJ Weber, R Devanathan, and JD Gale. 1999. "Effects of Cation Disorder on Oxygen Vacancy Migration in Gd2Ti2O7." Journal of Electroceramics 3(4):409-424.

Williford RE, and WJ Weber. 1999. "Cation Vacancy Energetics in the Gadolinium Titanate/Zirconate System." Journal of the American Ceramic Society 82(11):3266-3268.

Wang LM, and WJ Weber. 1999. "Transmission Electron Microscopy Study of Ion Beam Induced Amorphization of Ca2La8(SiO4)O2." Philosophical Magazine A 79(1):237-253. Abstract N/A

Wang SX, BD Begg, LM Wang, RC Ewing, WJ Weber, and KV Govidan Kutty. 1999. "Radiation Stability of Gadolinium Zirconate: A Waste Form for Plutonium Disposition." Journal of Materials Research 14(12):4470-4473.

Thevuthasan S, CHF Peden, MH Engelhard, DR Baer, GS Herman, W Jiang, Y Liang, and WJ Weber. 1999. "The Ion Beam Materials Analysis Laboratory at the Environmental Molecular Sciences Laboratory." Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment 420(1-2):81-89. Abstract Describes the equipment capabilities at EMSL with special emphasis on the accelerator, ion beam analysis and ion beam modification.

Jiang W, WJ Weber, S Thevuthasan, and DE McCready. 1999. "Rutherford Backscattering Spectrometry Channeling Study of Ion-Irradiated 6H-SiC." Surface and Interface Analysis 27(4):179-184. Abstract Studies damage accumulation and defect annealing (up to 1170 K) using in-situ 2.0 MeV He⁺ Rutherford Backscattering Spectrometry combined with ion channeling methods. Observes that the defect concentration at the damage peak increases sigmoidally with increasing ion fluence during irradiation at low temperatures and that the isochronal recovery of the damage induced at low temperatures follows an exponential dependence on temperature.

Jiang W, S Thevuthasan, WJ Weber, and F Namavar. 1999. "Ion-Channeling Study of the SiC/Si/SiO₂/Si Interface." Applied Physics Letters 74(23):3501-3503. Abstract Ion channeling has been used in a detailed study of 3C-SiC films grown by chemical vapor deposition on a Si/SiO₂/Si substrate. For a 160-nm-thick (100)-oriented SiC film, the results show a minimum yield (Xmin) of ~28% at the SiC-Si interface, while a SiC film with a thickness of ~2.4 microns, grown under identical conditions, was almost defect free (Xmin=5.3%) in the surface region. Angular scans around the (110) axis revealed the existence of a superlattice structure at the SiC-Si interface. The strain-induced angular shift ws determined to be 0.16 degree plus or minus 0.05 degrees, indicating a kink between the SiC and Si layers along the inclined (110) axis. A modified model is suggested to interpret the experimental observations.

Jiang W, WJ Weber, S Thevuthasan, and DE Mccready. 1999. "Displacement Energy Measurements for Ion-Irradiated 6H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 148(1-4):557-561.

Jiang W, WJ Weber, S Thevuthasan, and DE Mccready. 1999. "Damage Formation and Recovery in C+ Irradiated 6H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 148(1-4):562-566.

Gorreta KC, ML Burdt, MM Cuber, LA Perry, D Singh, AS Wagh, JL Routbort, and WJ Weber. 1999. "Solid-Particle Erosion of Portland Cement and Concrete." Wear - an international journal on the science and technology of friction, lubrication and wear 224(1):106-112. Abstract There is no abstract currently available for this item

Weber WJ, LM Wang, N Yu, and NJ Hess. 1998. "Structure and Properties of Ion-Beam-Modified (6H) Silicon Carbide." Materials Science and Engineering. A. Structural Materials: Properties, Microstructure and Processing 253(1-2):62-70.

Wang LM, SX Wang, WL Gong, RC Ewing, and WJ Weber. 1998. "Amorphization of Ceramic Materials by Ion Beam Irradiation." Materials Science and Engineering. A. Structural Materials: Properties, Microstructure and Processing 253(1-2):106-113. Abstract N/A

Jiang W, WJ Weber, S Thevuthasan, and DE McCready. 1998. "Damage Accumulation and Annealing in 6H-SiC Irradiated with Si+." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 143(3):333-341. Abstract Damage accumulation and annealing in 6H-silicon carbide (alpha-SiC) single crystals have been studied in situ using 2.0 MeV He⁺RBS in a <0 0 0 1>-axial channeling geometry (RBS/C). The damage was induced by 550 keV Si⁺ ion implantation (30 degrees off normal) at a temperatur of -110 degrees C, and the damage recovery was investigated by subsequent isochromal annealing (20 min) over the temperature range from -110 degrees C to 900 degress C. At ion fluences below 7.5 X 10 13 Si⁺/cm₂ (0.04 dpa in the damage peak), only point defects appear to be created. Futhermore, the defects on the Si sublattice can be completely recovered by thermal annealing at room temperature (RT), and recovery of defects on the C sublattice is suggested. At higher fluences of 6.6 x 10 15 Si⁺/cm₂ (-90 degrees C), an amorphous layer is created from the surface to a depth of 0.6 mu-m. Because of recovery processes at the buried crystalline-amorphous interface, the apparent thickness of this amorphous layer decreases slightly (<10%) with increasing temperature over the range from -90 degrees C to 600 degrees C.

Jiang W, WJ Weber, S Thevuthasan, and DE Mccready. 1998. "Accumulation and Recovery of Irradiation Damage in He+ Implanted Alpha-SiC." Journal of Nuclear Materials 257(3):295-302.

Hess NJ, WJ Weber, and SD Conradson. 1998. "U and Pu L(III) XAFS of Pu-Doped Glass and Ceramic Waste Forms." Journal of Alloys and Compounds 271-273:240-243.

Devanathan R, JN Mitchell, KE Sickafus, WJ Weber, and M Nastasi. 1998. "Radiation Response of FeTiO₃ MgTiO₃ and Alpha-Al₂O₃." Materials Science and Engineering. A. Structural Materials: Properties, Microstructure and Processing 253(1-2):131-134. Abstract There is no description available for this journal article due to the date it was entered into the system.