Pressure effect on the electronic structure of iron in (Mg,Fe)(Si,Al)O3 perovskite: a combined synchrotron Mössbauer and X-ray emission spectroscopy study up to 100 GPa |
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| Authors: | J Li W Sturhahn J M Jackson V V Struzhkin J F Lin J Zhao H K Mao G Shen |
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| Institution: | (1) Department of Geology, University of Illinois at Urbana Champaign, 245 Natural History Building, 1301 West Green Street, Urbana, IL 61801, USA;(2) Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA;(3) Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA;(4) Lawrence Livermore National Laboratory, Livermore, CA 94550, USA |
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| Abstract: | We investigated the valence state and spin state of iron in an Al-bearing ferromagnesian silicate perovskite sample with the composition (Mg0.88Fe0.09)(Si0.94Al0.10)O3 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 Fe2+-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 Fe2+-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 Fe3+-like site, Fe3+/∑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 Fe3+-like and Fe2+-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 Fe2+ alone, or concurrently with Fe3+, undergoes pressure-induced spin crossover between 20 and 100 GPa. |
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| Keywords: | Aluminum-bearing perovskite Valence state Spin crossover Synchrotron M?ssbauer spectroscopy X-ray emission spectroscopy Pressure effect |
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