High-Pressure Phase Transformations in Hexagonal and Amorphous Selenium

Hexagonal selenium transforms to a new phase at 140 ± 10 kbar. The c/a ratio of 1.42 at the transformation corresponds to the same c/a ratio for tellurium at its transformation at 40 kbar. It has not been possible to decide unambiguously a unit cell from the diffraction pattern of the new phase. The high-pressure phase is irreversible being retained upon releasing the pressure. Amorphous Se slowly transforms to the crystalline state under pressure and eventually develops the same unknown high-pressure structure as the hexagonal form. The pressure-cycled amorphous material retains its crystalline structure upon releasing the pressure. This structure appears to be identical to the irreversible hexagonal phase. It is thought that the high-pressure structure is superconducting.