High-resolution synchrotron X-ray powder diffraction and Rietveld structure refinement of two (Mg (sub 0.95) , Fe (sub 0.05) )SiO ₃ perovskite samples synthesized under different oxygen fugacity conditions

University of Oxford, Department of Earth Sciences, Oxford, United Kingdom

This paper presents high-resolution synchrotron X-ray powder diffraction data at 290 K on two Fe-bearing, polycrystalline silicate perovskite samples with approximate compositions (Mg (sub 0.95) Fe (sub 0.05) )SiO ₃ synthesized at 25 GPa and 1920 K in a multi-anvil press at different oxygen fugacity conditions. Mossbauer studies have indicated that Fe (super 3+) /Sigma Fe for the samples are 0.09+ or -0.01 and near 0.16+ or -0.03. Rietveld structural refinements confirm that Fe (super 2+) and Fe (super 3+) dominantly substitute for Mg (super 2+) in the 8-fold to 12-fold coordinated A site for both compositions. There appears to be no significant differences in the bond distances for these amounts of Fe (super 3+) and no conclusive structural evidence to support indications from Mossbauer experiments that Fe (super 3+) may occupy both A and B sites. To explore the effect of valence state further, this study also reports the first diffraction patterns of (Mg,Fe)SiO ₃ perovskite collected at a wavelength near the Fe absorption edge.

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