HIGH-PRESSURE PHASE-TRANSITION TO BETA-POLONIUM TYPE-STRUCTURE IN SELENIUM

High-pressure x-ray powder diffraction experiments have been carried out in VIb group selenium in the pressure range between 42 GPa and 101 GPa. A new phase transition to beta-Po type structure of space group R3mBAR was observed at 60 GPa. Lattice constants of the high-pressure phase at 60 GPa were a= 3.992(3) A, c= 3.119(2) A, V= 43.04(8) A3 and Z= 3. The beta-Po type phase remained stable to at least 101 GPa. A negative pressure dependence of the lattice constant ratio, c/a, suggests that the post-beta-Po type structure is the bcc one.