Mössbauer Spectrometry

AbstractMössbauer spectrometry is based on the quantum‐mechanical “Mössbauer effect,” which provides a nonintuitive link between nuclear and solid‐state physics. Mössbauer spectrometry measures the spectrum of energies at which specific nuclei absorb γ rays. Curiously, for one nucleus to emit a γ ray and a second nucleus to absorb it with efficiency, the atoms containing the two nuclei must be bonded chemically in solids.Most applications of Mössbauer spectrometry in materials science utilize “hyperfine interactions,” in which the electrons around a nucleus perturb the energies of nuclear states. Hyperfine interactions cause very small perturbations of 10−9to 10−7eV in the energies of Mössbauer γ rays.Interpretations of Mössbauer spectra have few parallels with other methods of materials characterization. A Mössbauer spectrum looks at a material from the “inside out,” where “inside” means the Mössbauer nucleus. Hyperfine interactions are sensitive to the electronic structure at the Mössbauer atom, or at its nearest neighbors. The important hyperfine interactions originate with the electron density at the nucleus, the gradient of the electric field at the nucleus, or the unpaired electron spins at the nucleus. These three hyperfine interactions are called the “isomer shift,” “electric quadrupole splitting,” and “hyperfine magnetic field,” respectively.The viewpoint from the nucleus is sometimes too small to address problems in the microstructure of materials. In general, it is found that Mössbauer spectrometry is best for identifying the electronic or magnetic structure at the Mössbauer atom itself, such as its valence, spin state, or magnetic moment. The Mössbauer effect is sensitive to the arrangements of surrounding atoms, however, because the local crystal structure will affect the electronic or magnetic structure at the nucleus. Different chemical and structural environments around the nucleus can often be assigned to specific hyperfine interactions.Other applications of the Mössbauer effect utilize its sensitivity to vibrations in solids, its timescale for scattering, or its coherence. To date these phenomena have seen little use outside the international community of a few hundred Mössbauer spectroscopists. Nevertheless, some new applications for them have recently become possible with the advent of synchrotron sources for Mössbauer spectrometry.This article is not a review of the field, but an instructional reference that gives the working materials scientist a basis for evaluating whether or not Mössbauer spectrometry may be useful for a research problem.A few representative applications to materials studies are presented.