Spectrometer: Mossbauer
Quick Specs
- 57Fe focus; 151Eu is available
- Provides information about valence state, coordination number, crystal field strenghts, and magnetic ordering temperatures
- Capable of room and cryogenic temperature measurements
Mössbauer spectroscopy is a type of nuclear spectroscopy involving the resonant emission and absorption of γ-rays (i.e., the Mössbauer effect). The Mössbauer technique provides information about the valence state, coordination number, crystal field strengths (e.g., low spin and high spin Fe(III)), and magnetic ordering temperatures. In contrast to X-ray diffraction (XRD), it also provides information on compounds that do not exhibit long-range order (poorly crystalline or amorphous materials). Common Fe-oxide phases such as magnetite and hematite, are readily distinguished from each other and from Fe in layer silicates and predominantly Fe(II) compounds.
This versatile, highly sensitive, and nondestructive technique has a wide range of applications in various fields including geochemistry, soil science, and materials science. The primary uses of 57Fe-Mössbauer spectroscopy at EMSL are to study mineralization associated with dissimilatory bacterial reduction of Fe(III)-oxides (Kukkadapu et al., 1999, 2001; Dong et al., 2000; Fredrickson et al., 2001; Zachara et al., 2001); to characterize natural and synthetic minerals; and to identify Fe-oxides in soils and sediments, catalysts, and Fe-doped glasses.
System Configuration and Operational Overview
EMSL's Mössbauer spectroscopy system consists of two velocity transducers (a WissEL MVT-1000, and a Ranger Scientific MS-900A). The WissEL transducer is designed to allow simultaneous collection of data with two sources. Both room temperature and cryogenic temperature (ca. 2 K and above) measurements are carried out routinely. A Janis cryostat is employed for the low temperature measurements. The Ranger transducer is mostly used for room temperature measurements. The WissEL transducer can be configured to collect spectra for samples under an external magnetic field at field strengths <12 Tesla (Oxford Instruments SpectroMag 10/12 Tesla superconducting magnet). A third transducer was recently added for conversion-electron Mössbauer spectroscopy, which eliminates the high background of conventional transmission Mössbauer spectroscopy and offers high surface sensitivity. Although our focus is on 57Fe, a source is also available for 151Eu Mössbauer spectroscopy. All work with the Mössbauer spectroscopy system and in EMSL labs must be performed in compliance with EMSL practices and permits.
In addition to the conventional Mössbauer capability here at EMSL, experiments are ongoing with the synchrotron Mössbauer approach in collaboration with E. E. Alp at the Advanced Photon Source. This approach relies on pulsed monochromatic (bandwidth ca. 0.8 meV) X-rays as an excitational source, monitors the intensity of the Mössbauer resonance in the time domain rather than in the energy domain, eliminates the high background associated with the conventional approach to maximize signal-to-noise, and thereby allows collection of complete Mössbauer spectra in periods of seconds or minutes rather than hours or days. Using 57Fe-enriched samples, we have demonstrated the potential of the technique for kinetic studies of Fe redox reactions in solids (Amonette, 1997; Amonette et al., 1998).
Additional Information - Mössbauer Spectroscopy
Mössbauer spectroscopy is a type of nuclear spectroscopy involving the resonant emission and absorption of γ-rays (i.e., the Mössbauer effect). This effect requires a "recoil-free" nuclear transition, i.e., a nuclear transition in which no net change in momentum is imparted to the nucleus. Secondly, the energy of the source photon must be exactly identical to the nuclear transition energy in the absorber. The probability for a recoil-free transition increases with the rigidity of the source and absorber (i.e., the sample being analyzed) and thus is highest for solids at low temperatures. In conventional instruments, the energy of the source photon is varied over a small range (tens of neV) using the Doppler effect. The source is repetitively accelerated through a range in velocities (from a few to hundreds of mm s-1) to add or subtract energy to the photons being emitted. When a match in the energy of the source photon and the absorber transition energy is achieved, resonant absorption occurs. Because subsequent emission of the absorbed photon has no directional probability, in contrast to the source photon directed at the detector, a decrease in the intensity of the background signal is observed at the energies (velocities) where resonant absorption occurs, thus giving rise to a Mössbauer spectrum (Fig. 1).
Although more than half the elements in the periodic table have isotopes exhibiting the Mössbauer effect, the 57Fe isotope is the most favorable isotope for Mössbauer spectroscopy. This is because (a) the recoil energy associated with absorption of the γ-rays of 14.41 keV (I = 3/2 to I = 1/2 transition) is low, (b) the half-width of the resonant line is narrow (3 x 10-13 times the energy of the γ-rays), and (c) the natural abundance of 57Fe is high (2.14%). The main advantage of 57Fe Mössbauer spectroscopy is that it is an Fe-specific technique with greater sensitivity than XRD. For example, Fe oxidation states and local environments are identifiable for samples with Fe contents as low as 0.5 wt.%.
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