Phase transitions in ReO₃ studied by high-pressure X-ray diffraction

ReO₃ has been studied at pressures up to 52 GPa by X-ray powder diffraction. The previously observed cubic Im3 high-pressure phase was shown to transform to a monoclinic MnF₃-related phase at about 3 GPa. All patterns recorded above 12 GPa could be indexed on rhombohedral cells. The compressibility was observed to decrease abruptly at 38 GPa. It is therefore proposed that the oxygen ions are hexagonally close packed above this pressure, giving rise to two rhombohedral phases labelled I and II. The zero-pressure bulk moduli B_o of the observed phases were determined and the rhombohedral phase II was found to have an extremely large value of 617 (10) GPa. It was found that ReO₃ transforms back to the Pm3m phase found at ambient pressure.