New structure and spin state of iron-rich (Mg,Fe)SiO₃ post-perovskite

There was a discrepancy between the seismic tomography and the elastic property of MgSiO₃ perovskite at near the D" zone and core boundary. Since a discovery of post-perovskite (ppv) of MgSiO₃, many investigations have made to explain the presence of low seismic velocity of the lower mantle and D" zone. However, precise experimental structure analysis of ppv-(Mg_1-xFe_x)SiO₃ has never been reported because of the experimental difficulty. Fe and Mg cation distribution and ordering in ppv-(Mg,Fe)SiO₃ in consideration of spins states are significant subject in lower mantle electronic and magnetic states. The present experiment aims X-ray emission study and structure analysis by Rietveld profile fitting of ppv-(Mg_0.6,Fe_0.4)SiO₃ by the precise powder diffraction measurement. Monte Carlo calculation proposed the reliable structures of iron-rich phase of ppv-(Mg,Fe)SiO₃: Pmmn, Pmma, and Cm2m and Cmcm proposed by. The best-fit structure model with the highest reliability in the Rietveld fitting of ppv-(Mg_0.6Fe_0.4)SiO₃ is the structure of space group Pmma, in which Fe and Mg occupy two different sites of M1 and M2: the site occupancies are (Fe_0.25Mg_0.75) in the larger M1 site and (Fe_0.55, Mg_0.45) in the smaller M2 site. The two-site model is consistence with the previous results of X-ray emission and X-ray Mössbauer experiments.