STRUCTURAL STUDY ON PRESSURE-INDUCED PHASE-TRANSITION FROM BETA-PO TYPE TO BCC IN SELENIUM AT 140 GPA

High-pressure X-ray diffraction study of selenium has been carried out in the megabar pressure region. The phase transition from a beta-Po type to a bcc structure was observed at about 140 GPa. Relative volume reduction, - DELTAV/V0, at the transition, was less than 0.2%. At 150 GPa, the lattice constant of the bcc phase, a(bcc), was 2.7941(3) angstrom and relative volume, V/V0, reached 0.4. The transition has been discussed from a crystallographical viewpoint in terms of structural parameters, which indicate the rhombohedral distortion.