Isothermal compression behavior of (Mg,Fe)O using neon as a pressure medium
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We present isothermal volume compression behavior of two polycrystalline (Mg,Fe)O samples with FeO = 39 and 78 mol% up to ~90 GPa at 300 K using synchrotron X-ray diffraction and neon as a pressure-transmitting medium. For the iron-rich (Mg0.22Fe0.78)O sample, a structural transition from the B1 structure to a rhombohedral structure was observed at 41.6 GPa, with no further indication of changes in structural or compression behavior changes up to 93 GPa. In contrast, a change in the compression behavior of (Mg0.61Fe0.39)O was observed during compression at P ≥ 71 GPa and is indicative of a spin crossover occurring in the Fe2+ component of (Mg0.61Fe0.39)O. The low-spin state exhibited a volume collapse of ~3.5%, which is a larger value than what was observed for a similar composition in a laser-heated NaCl medium. Upon decompression, the volume of the high-spin state was recovered at approximately 65 GPa. We therefore bracket the spin crossover at 65 ≤ P (GPa) ≤ 77 at 300 K (Mg0.61Fe0.39)O. We observed no deviation from the B1 structure in (Mg0.61Fe0.39)O throughout the pressure range investigated.
Keywords(Mg,Fe)O Lower mantle Spin crossover Phase transition
We thank E. Hamecher (Caltech) for help with conducting experiments, S. Mackwell (Lunar & Planetary Institute, TX) for synthesizing and providing the (Mg0.22Fe0.78)O sample. The powdered (Mg0.61Fe0.39)O sample was synthesized with the help of Y. Fei (Carnegie Institution of Washington). I. Kantor and an anonymous reviewer provided helpful suggestions that improved the manuscript. This work was supported by the National Science Foundation EAR Geophysics 0711542 (JMJ). The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Portions of this work were supported by COMPRES under NSF Cooperative Agreement EAR 06-49658.
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