HIGH-PRESSURE RAMAN-SCATTERING AND OPTICAL-ABSORPTION STUDY OF BETA-GES2

We have measured Raman scattering and optical absorption spectra oi la yered beta-GeS2 at pressures up to 15 GPa. Frequency shifts of selecte d internal bond bending and stretching modes are discussed, and three external (rigid-layer) modes are identified. Changes in the Raman spec tra near 9 GPa clearly indicate a structural phase transition. The pre ssure-induced red shift of the optical absorption edge shows a change in slope at about the same pressure. Samples turn opaque near 15 GPa, indicating a second transition to a disordered phase. The pressure-ind uced phase changes are reversible.