Pressure-induced metallization and structural transition of orthorhombic Se

Powder X-ray diffraction experiments and electrical resistance measurements were performed at pressures up to 36 GPa for orthorhombic Se consisting of cyclic Se-7 molecules. A successive phase transformation was observed to occur in three stages under pressure. The forth phase (final phase) was assigned to a base-centered orthorhombic lattice, which was the same as the Se-IV phase, the high pressure phase of trigonal Sc. Pressure-induced metallization was observed at pressure around 25 GPa, which corresponded to the second stage of transition. The crystal lattices of the second phase and the third phase have not been clarified yet.