Temperature and pressure dependence of Raman scattering in amorphous GeS2

The evolution of the Raman spectrum of amorphous GeS2 is reported over wide temperature (300 to 1075 K) and pressure (0 to 10.8 GPa) ranges. The tempe rature dependence shows that a-GeS2 turns gradually crystalline above T-g, first to the SD-phase and then to the layered (2D)-phase, with the latter o ne being retained up to the melting point and upon normal cooling to 300 K. The evolution of the A(1)(c) companion band of a-GeS2 into the crystalline phases indicates that this band is due to the symmetric stretch of S atoms in bridges of edge-sharing Ge(S-1/2)(4) tetrahedra. The Raman bands of a-G eS2 harden with increasing pressure, indicating a continuous decrease of th e Ge-S bond length, while the material remains amorphous throughout the pre ssure range. Above 10 GPa, the material becomes almost opaque, thus making difficult the observation of Raman scattering. All these pressure induced e ffects are reversible after bringing the sample to ambient pressure.