Single-crystal X-ray diffraction of wustite to 30 GPa hydrostatic pressure

Phase transition, pressure-volume relations, twinning, and internal strains of a single-crystal wustite (Fe0.947O) have been studied in a helium pressure medium to 30 GPa with a novel synchrotron X-ray diffraction technique. The long-standing enigma of the high-pressure wustite has been solved; its crystallographic structure has been proven unequivocally to be rhombohedral. At 18 GPa, the wustite crystal (cubic) transforms to four twin domains of the rhombohedral phase, with each of the four body diagonals <111> of the original cubic crystal corresponding to a unique c-axis of the rhombohedral phase. Lifting the fourfold degeneracy at the transition causes mismatch at the domain boundaries and produces internal strains. The phase transition is sharp, reversible, and displacive. The P-V relation of the low pressure cubic phase agrees with previous polycrystalline results of Fe0.92O and Fe0.98O wustites in a neon medium; the P-V relation of the high-pressure rhombohedral phase is reported for the first time.