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Experimental studies of Fe1−xO wüstite and (Mg0.8Fe0.2)O ferropericlase were performed at room temperature and high pressure using a diamond anvil cell technique. Mössbauer spectroscopy indicated a magnetic ordering transition in FeO at 4.7 GPa, while no evidence of magnetic ordering was detected in ferropericlase up to at least 50 GPa. A structural cubic-to-rhombohedral distortion was observed in both materials using powder X-ray diffraction: at 12 GPa for FeO and at 35 GPa for ferroperi-clase. This suggests that the structural distortion is independent of magnetic ordering in the FeO-MgO system. A complete single-crystal elasticity tensor of FeO was determined up to 10 GPa, and only minor changes of elastic moduli were detected at the magnetic transition pressure, indicating relatively weak magnetoelastic coupling in FeO at high pressures. The existence of a rhombohedral distortion in ferropericlase with mantle composition at high pressures and the absence of magnetic ordering have important implications for the interpretation of seismological data with respect to lower-mantle inhomogeneity.

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