A cellular atomic model for the Mössbauer isomer shift of 197Au in alloys

Abstract

It is demonstrated that the atomic model previously shown to give a semi-quantitative account of the heat of formation of transition metal intermetallic compounds, can also offer a relatively simple interpretation of the isomer shift in metallic alloys. The two main terms in the isomer shift are due to (1) electronic charge transfer between more electropositive and more electronegative metal atoms and (2) the discontinuity in the electron density that exists at the boundary between disimilar atoms if the atoms have similar charge distributions in both alloy and the pure metals. The electron density must become continuous at the cell boundary; this leads to a term in the isomer shift that usually, but not always, opposes the charge transfer term. In solid solutions there is an additional volume (size mismatch) contribution to the isomer shift which, however, is not as important as suggested by some other authors. In addition to experimental results obtained on the isomer shift of solid solutions of Au as an impurity in transition metals hosts, we discuss results on Au-base alloys and gold intermetallic compounds.