Mössbauer 57Fe spectra exhibiting “ferrous character”

Abstract

The term “ferrous character” reported extensively in the literature for the Mössbauer spectroscopy of octahedrally coordinated iron atoms in chalcogenides and antimonides is critically evaluated. The significance of a formal valence state for the iron is examined. The magnitude of the isomer shift is shown to provide a guide not only to the existence of high-spin versus low-spin states, but also to localized versus itinerant electrons or to extended electrons within Fe X M interactions ( M = transition metal and X = anion). The existence of a quadrupole splitting for iron atoms in intrinsically cubic fields is only possible where the β-spin electron outside a closed α-spin half-shell is localized and Jahn-Teller coupled to lattice vibrations to form vibronic states. This situation is distinguished from quadrupolar fields associated with local-site symmetries deformed from cubic symmetry as well as from the case where the formation of itinerant β-spin electrons inhibits the formation of vibronic states.