Local structure around Fe in Mg0.9Fe0.1SiO3 perovskite: An X-ray absorption spectroscopy study at Fe-K edge

A polycrystalline Mg 0.9 Fe 0.1 SiO 3 perovskite, synthesized at 260 kbar and 1973 K in a multi-anvil apparatus, and the enstatite startinp material have been probed by X-ray absorption fine structure (XAFS) spectroscopy at the Fe K-edge under ambient conditions. XAFS analysis of the local structure around Fe in perovskite shows evidence for (1) the presence of major amounts of divalent iron, (2) a higher average Fe-O distance in perovskite compared with enstatite, and (3) a wide distribution of the individual Fe-O distances, suggestinp that a simple harmonic model to fit XAFS data may not be appropriate. Although a classic Gaussian model of the 0-environment is consistent with the presence of 6 × Fe 2+ -O correlations at 2. 16 (2) A, a double-harmonic (e.g., 2 × Gaussian) fit reveals the presence of 2.8 ± 0.5 additional Fe-O first-neighbours correlations at a mean Fe-O distance of ≃ 2.30 (2) A, yielding 8.4 ± 1 Fe-O first-neighbours correlations around Fe with a mean Fe-O distance of 2.23 (3) A. An anharmonic fit (cumulant expansion method) leads to a similar conclusion. These data compare favourably with those for the Mg-site in the same perovskite (8 × Mg-O at 2.20 A), suggestinp that Fe substitutes preferentially for Mg in the perovskite sample