Phase transitions of (Mg,Fe)O at megabar pressures

An in situ X-ray diffraction study of magnesiowüstite at high pressures and high temperatures was performed using a high temperature diamond anvil cell and synchrotron radiation. A rhombohedral distortion starts in (Mg0.20,Fe0.80)O, (Mg0.10,Fe0.90)O, (Mg0.05,Fe0.95)O and FeO at the same pressure of 16 GPa. The rhombohedral phase was observed even after laser annealing at megabar pressures. At high temperatures above 1500 °C and megabar pressures, a B1 (rock salt) structured phase was observed to be stable in all samples. A B8 (NiAs) structured phase was not found in magnesiowüstite and was only observed in the end member FeO. The diffraction pattern of the B8 phase was consistent with the calculated intensities for the inverse B8 structure. The results indicate that the stable phase of magnesiowüstite would have a B1 structure under lowermost mantle conditions.