Mössbauer isomer shifts in iron borides based on the local structure of the Fe sites

Abstract

An expression for the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer isomer shift in iron borides has been developed, which reproduces accurately the experimental values measured in the nine iron sites existing in FeB, ${\mathrm{Fe}}_2$B, ${\mathrm{Fe}}_3$B (body-centered tetragonal), ${\mathrm{Fe}}_3$B (orthorhombic), as well as in metallic α-Fe and γ-Fe, in terms of their local structure. It is shown that the isomer shift can be described using two terms. The first one depends linearly on the number of boron nearest neighbors of the iron site. The second term is needed because of the volume effects which cannot be neglected in the Fe-B interstitial compounds, and it depends strongly on the average iron-iron interatomic distances. This term gives an account of the important deviations observed from the linear correlation with the number of boron neighbors.