Isomer Shifts in Solid State Chemistry

Abstract

The isomer shift of the Mössbauer resonance is a rather unique quantity that cannot be obtained by any of the other techniques used for measuring hyperfine interactions in solids, such as NMR or perturbed angular correlations (TDPAC). It shifts the resonance pattern as a whole without affecting the magnetic dipole and electric quadrupole hyperfine splittings. Methods that measure only these hyperfine splittings are insensitive to the isomer shift. The magnitude of the observed shift is proportional to the product of a nuclear parameter, the change hr 2 i of the nuclear radius that goes along with the Mössbauer transition, and to an electronic property of the material, the electron density .0/ at the Mössbauer nucleus or, more precisely, to the difference .0/ of the electron densities at the Mössbauer nuclei in the materials of which the source and the absorber are made. The electron density at the nucleus is due to s-electrons and, to a lesser extent and mainly in heavy nuclei, to relativistic p 1=2 -electrons. All the other electrons have a vanishing density inside the nucleus and do not contribute. Thus, to a very good accuracy, the Mössbauer isomer shift enables one to obtain information on the s-electron density at the Mössbauer nuclei in solids.The isomer shift thus yields important insights into the chemical bonding in solids. For instance, it often allows an easy distinction between different oxidation states of an element, e.g., between divalent and trivalent iron or divalent and tetravalent tin. The s-electron densities are also sensitive to covalency in chemical compounds and to effects of band structure in metals. For this reason, the isomer

Mössbauer and Raman spectroscopic characterization of iron and carbon in iron-loaded Japanese cypress charcoal

“…In this study, it is assumed that the oxidation number of Fe in Fe 3 C is zero; in other words, the carbide is assigned to Fe 0 species, because the IS values of Fe 0 species range from −0.25 to 0.63 mm/s according to Ref. [14]. The CTdependence of the relative yields for the four species are illustrated in Fig.…”