Scientific Publications 2005
2005. "Many-body Effects in the 4f X-ray Photoelectron Spectroscopy of the U5+ and U4+ Free Ions." Physical Review. B, Condensed Matter 71(19):195121. doi:10.1103/PhysRevB.71.195121 Abstract A strict ab initio many-electron theory was used to calculate the 4f XPS of the free U⁵⁺ and U⁴⁺ ions. The calculations, based on relativistic Dirac-Fock self-consistent field (DF-SCF) and Dirac configuration interaction (DCI) WF’s, indicate that the atomic spectra have a considerable multiplet structure. However, the multiplet splitting, which is mainly manifest as a broadening of the 4f (5/2 and 7/2) lines, is not as strong as for the first row transition metals. As expected, the U⁴⁺ primary peaks are broader and have more associated satellite structure than does U⁵⁺. A comparison of a synthetic spectrum for U⁴⁺ with the observed XPS of UO₂ indicates that inter-atomic effects may decrease the multiplet and spin-orbital splitting, relative to the free ion. Notably, the 7 eV satellite characteristic of UO₂ is absent from the calculated XPS of U⁴⁺.
2005. "Mica Surfaces Stabilize Pentavalent Uranium." Inorganic Chemistry 44(9):2986-2988. Abstract We used high-resolution x-ray photoelectron spectroscopy to demonstrate that reduction of aqueous U6+ at ferrous mica surfaces at 25oC preserves U5+ as the dominant sorbed species over a broad range of solution compositions. Polymerization of sorbed U5+ with sorbed U6+ and U4+ is identified as a possible mechanism for how mineral surfaces circumvent the rapid disproportionation of aqueous U5+. The general nature of this mechanism suggests that U5+ could play an important, but previously unidentified, role in the low–temperature chemistry of uranium in reducing, heterogeneous aqueous systems.
2005. "Charge Transport in Metal Oxides: A Theoretical Study of Hematite α-Fe2O3 ." Journal of Chemical Physics 122(14):144305. Abstract Transport of conduction electrons and holes through the lattice of Fe2O3 (hematite) is modeled as a valence alternation of iron cations using ab initio electronic structure calculations and electron transfer theory. Experimental studies have shown that the conductivity along the (001) basal plane is four orders of magnitude larger than the conductivity along the  direction. In the context of the small polaron model, a cluster approach was used to compute quantities controlling the mobility of localized electrons and holes, i.e. the reorganization energy and the electronic coupling matrix element that enter Marcus’ theory. The calculation of the electronic coupling followed the Generalized Mulliken-Hush approach using the complete active space self-consistent field (CASSCF) method. Our findings demonstrate an approximately three orders of magnitude anisotropy in both electron and hole mobility between directions perpendicular and parallel to the c-axis, in good accord with experimental data. The anisotropy arises from the slowness of both electron and hole mobility across basal oxygen planes relative to that within iron bi-layers between basal oxygen planes. Interestingly, for elementary reaction steps along either of the directions considered, there is only approximately one order of magnitude difference in mobility between electrons and holes, in contrast to accepted classical arguments. Our findings indicate that the most important quantity underlying mobility differences is the electronic coupling, albeit the reorganization energy contributes as well. The large values computed for the electronic coupling suggest that charge transport reactions in hematite are adiabatic in nature. The electronic coupling is found to depend on both the superexchange interaction through the bridging oxygen atoms and the d-shell electron spin coupling within the FeFe donor-acceptor pair, while the reorganization energy is essentially independent of the electron spin coupling.