Pressure-induced metallization and structural transition of α-monoclinic and amorphous Se

Electrical resistance measurements and x-ray-diffraction experiments have been carried out under high pressure for {α}-monoclinic and amorphous and trigonal Se. {α}-monoclinic Se showed a transition to the metallic state at 12 GPa, which was considerably lower than 23 GPa of trigonal Se. Corresponding to the metallization, a structural transition to a high-pressure phase which could not be explained by any metallic phase of Se reported previously was observed in {α}-monoclinic Se. In amorphous Se, crystallization was observed at 12 GPa. The diffraction pattern after crystallization was explained as the coexistence of the high-pressure phase and trigonal Se. These results have established the double nature of the sequence of the structure in the pressure-induced phase transition of Se.