Raman scattering under pressure and the phase transition in ε-GaSe

Raman scattering in epsilon-GaSe has been investigated at room temperature in a diamond anvil cell by using helium gas as a pressure transmitting medium. The first observation of the solid-solid phase transition under pressure from a hexagonal structure at ambient pressure to a NaCl-type at around 29.2 GPa is reported. A photoinduced change in the pressurised crystal was observed. The transition under pressure was found to be irreversible at room temperature. It was shown that the pressurised form of epsilon-GaSe at ambient conditions most probably belongs to the gamma-type as characterised by an increased intensity of the Raman modes at 234 cm(-1) and 250 cm(-1).