SINGLE-CRYSTAL X-RAY-DIFFRACTION OF WUSTITE TO 30 GPA HYDROSTATIC-PRESSURE

Phase transition, pressure-volume relations, twinning, and internal st rains of a single-crystal wustite (Fe0.947O) have been studied in a he lium pressure medium to 30 GPa with a novel synchrotron X-ray diffract ion technique. The long-standing enigma of the high-pressure wustite h as been solved; its crystallographic structure has been proven unequiv ocally to be rhombohedral. At 18 GPa, the wustite crystal (cubic) tran sforms to four twin domains of the rhombohedral phase, with each of th e four body diagonals <111> of the original cubic crystal correspondin g to a unique c-axis of the rhombohedral phase. Lifting the fourfold d egeneracy at the transition causes mismatch at the domain boundaries a nd produces internal strains. The phase transition is sharp, reversibl e, and displacive. The P-V relation of the low pressure cubic phase ag rees with previous polycrystalline results of Fe0.92O and Fe0.98O wust ites in a neon medium; the P-V relation of the high-pressure rhombohed ral phase is reported for the first time.