LOCAL-STRUCTURE AROUND FE IN MG0.9FE0.1SIO3 PEROVSKITE - AN X-RAY-ABSORPTION SPECTROSCOPY STUDY AT FE-K EDGE

A polycrystalline Mg0.9Fe0.1SiO3 perovskite, synthesized at 260 kbar a nd 1973 K in a multi-anvil apparatus, and the enstatite starting mater ial have been probed by X-ray absorption fine structure (XAFS) spectro scopy at the Fe K-edge under ambient conditions. XAFS analysis of the local structure around Fe in perovskite shows evidence for (1) the pre sence of major amounts of divalent iron, (2) a higher average Fe-O dis tance in perovskite compared with enstatite, and (3) a wide distributi on of the individual Fe-O distances, suggesting that a simple harmonic model to fit XAFS data may not be appropriate. Although a classic Gau ssian model of the O-environment is consistent with the presence of 6 x Fe2+-O correlations at 2.16 (2) angstrom, a double-harmonic (e.g., 2 x Gaussian) fit reveals the presence of 2.8 +/- 0.5 additional Fe-0 f irst-neighbours correlations at a mean Fe-0 distance of = 2.30 (2) ang strom, yielding 8.4 +/- 1 Fe-0 first-neighbours correlations around Fe with a mean Fe-0 distance of 2.23 (3) angstrom. An anharmonic fit (cu mulant expansion method) leads to a similar conclusion. These data com pare favourably with those for the Mg-site in the same perovskite (8 x Mg-0 at 2.20 suggesting that Fe substitutes preferentially for Mg in the perovskite sample.