Local structure around Fe in Mg (sub 0.9) Fe (sub 0.1) SiO (sub 3) perovskite; an X-ray absorption spectroscopy study at Fe-K edge

A polycrystalline Mg (sub 0.90) Fe (sub 0.10) SiO (sub 3) perovskite, synthesized at 260 kbar and 1973 K in a multi-anvil apparatus, and the enstatite starting material have been examined by X-ray absorption fine structure (XAFS) spectroscopy at the Fe-K edge under ambient conditions. Evidence was found for: 1) the presence of major amounts of Fe (super 2+) , 2) a higher average Fe-O distance in perovskite than in enstatite and 3) a wide distribution of the individual Fe-O distances, suggesting that a simple harmonic model to fit XAFS data may not be appropriate. Although a classic Gaussian model of the O environment is consistent with the presence of 6 X Fe (super 2+) -O correlations at 2.16 Angstrom, a double harmonic (e.g. 2 X Gaussian) fit reveals the presence of 2.8 +- 0.5 additional Fe-O first-neighbours correlation at a mean Fe-O distance of approx 2.30 Angstrom, yielding 8.4 +- 1 Fe-O first-neighbours correlation around Fe with a mean Fe-O distance of 2.23 Angstrom . These data compare favourably with those for the Mg site in the same perovskite (8 X Mg-O at 2.20 Angstrom), suggesting that Fe substitutes preferentially for Mg in this perovskite.