Mössbauer modeling to interpret the spin state of iron in (Mg 5Fe)SiO3 perovskite

Abstract

The properties of (Mg,Fe)SiO3 perovskite at lower mantle conditions are still not well understood, and particular attention has recently been given to determining the Fe spin state. A major challenge in spin states studies is interpretation of Mössbauer spectra to determine the electronic structure of iron under extreme conditions. In this paper ab initio methods are used to predict quadrupole splitting values of high-, intermediate- and low-spin Fe2+ and Fe3+ in perovskite, as a function of pressure and composition. The calculations in (Mg0.75Fe0.25) SiO3 yield quadrupole splitting values in the range of 0.7-1.7 mm/s for all spin and valence states except high-spin Fe2+, which has two possible quadrupole splittings, 2.3 and 3.3 mm/s. The unexpected multiple quadrupole splitting values for highspin Fe2+ are explained in terms of small changes in local structure and d-orbital occupations. The computed results are applied to interpret existing perovskite Mössbauer data for iron's spin state. Copyright 2009 by the American Geophysical Union.