High-Pressure Phase Transformations in Hexagonal and Amorphous Selenium

Hexagonal selenium transforms to a new phase at $\text{140 ± 10}$ 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.