Structural Study on Pressure-Induced Phase Transition from β-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 β-Po type to a bcc structure was observed at about 140 GPa. Relative volume reduction, -ΔV/V 0, at the transition, was less than 0.2%. At 150 GPa, the lattice constant of the bcc phase, a bcc, was 2.7941(3) Å and relative volume, V/V 0, reached 0.4. The transition has been discussed from a crystallographical viewpoint in terms of structural parameters, which indicate the rhombohedral distortion.