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Published November 2006 | public
Journal Article

Pressure effect on the electronic structure of iron in (Mg,Fe)(Si,Al)O_3 perovskite: a combined synchrotron Mössbauer and X-ray emission spectroscopy study up to 100 GPa

Abstract

We investigated the valence state and spin state of iron in an Al-bearing ferromagnesian silicate perovskite sample with the composition (Mg_(0.88)Fe_(0.09))(Si_(0.94)Al_(0.10))O_3 between 1 bar and 100 GPa and at 300 K, using diamond cells and synchrotron Mössbauer spectroscopy techniques. At pressures below 12 GPa, our Mössbauer spectra can be sufficiently fitted by a "two-doublet" model, which assumes one ferrous Fe^2+-like site and one ferric Fe3+-like site with distinct hyperfine parameters. The simplest interpretation that is consistent with both the Mössbauer data and previous X-ray emission data on the same sample is that the Fe^2+-like site is high-spin Fe2+, and the Fe3+-like site is high-spin Fe3+. At 12 GPa and higher pressures, a "three-doublet" model is necessary and sufficient to fit the Mössbauer spectra. This model assumes two Fe2+-like sites and one Fe3+-like site distinguished by their hyperfine parameters. Between 12 and 20 GPa, the fraction of the Fe^3+-like site, Fe^3+/∑Fe, changes abruptly from about 50 to 70%, possibly due to a spin crossover in six-coordinate Fe2+. At pressures above 20 GPa, the fractions of all three sites remain unchanged to the highest pressure, indicating a fixed valence state of iron within this pressure range. From 20 to 100 GPa, the isomer shift between the Fe^3+-like and Fe^2+-like sites increases slightly, while the values and widths of the quadruple splitting of all three sites remain essentially constant. In conjunction with the previous X-ray emission data, the Mössbauer data suggest that Fe^2+ alone, or concurrently with Fe^3+, undergoes pressure-induced spin crossover between 20 and 100 GPa.

Additional Information

© 2006 Springer-Verlag. Received: 21 February 2006; Accepted: 24 July 2006. We thank Ross Angel at Virginia Tech for insightful discussions on the site occupancy of iron in perovskites, and for trying to perform a single-crystal X-ray diffraction measurement on our Al-PV sample. We thank Holger Hellwig at U. Illinois for helpful comments and suggestions on the early drafts of the manuscript. We thank Jinfu Shu at the Geophysical Laboratory for helping with the gasket preparation. We thank Catherine MaCammon at the Bayerisches Geoinstitut for helpful discussions on the effect of spin crossover on the Mössbauer characteristics of iron. We thank Dane Morgan at the University of Wisconsin–Madison for fruitful discussions on data interpretation. The manuscript benefited significantly from an anonymous review. This work is supported by National Science Foundation grant EAR-0337612. This work at LLNL was performed under the auspices of the US DOE at the UC/LLNL under contract no. W-7405-Eng-48. Support for the study was also provided by the Lawrence Livermore Fellowship to J. F. Lin.

Additional details

Created:
August 22, 2023
Modified:
October 23, 2023