STRUCTURAL STUDIES OF PRESSURE-INDUCED PHASE-TRANSITIONS IN SELENIUM UP TO 150-GPA

High-pressure x-ray-diffraction experiments on selenium have been carried out over a wide range of pressures up to 150 GPa and five structural phase transitions have been found: from a hexagonal-Se (SE-I) to an intermediate (Se-II) phase at 14 GPa, to a base-centered monoclinic (Se-III) phase at 23 GPa, to a base-centered orthorhombic (Se-IV) phase at 28 GPa, to a beta-Po-type primitive rhombohedral (Se-V) phase at 60 GPa, and to a body-centered-cubic (Se-VI) phase at 140 GPa. This structural-transition sequence corresponds to a change of the system toward higher symmetry and exhibits a similarity to that of Te. These transitions are discussed from a crystallographic viewpoint.