Pressure dependence of the Raman modes and pressure-induced phase changes in CuGaS₂ and AgGaS₂

Abstract

The pressure dependence of the Raman peaks of CuGaS₂ and AgGaS₂ (chalcopyrite structure) has been studied up to 20 GPa in a gasketed diamond anvil cell. Nonlinear pressure dependence of several phonon frequencies have been noted, including a softening of the lowest Γ₅ phonon, prior to a phase transition, which takes place at 16.5±0.5 GPa in CuGaS₂ and 4.2±0.5 GPa in AgGaS₂. In the latter material, discontinuous changes of the Raman frequencies indicate another phase transition at 11.6±0.5 GPa. The transition in CuGaS₂ is believed to be to the disordered rocksalt structure, and in AgGaS₂, the high-pressure transition is to the rocksalt, or to the closely related α-NaFeO₂ structure. In AgGaS₂, the softening of the lowest Γ₅ phonons can be followed across the 4.2-GPa phase transition, an observation which to our knowledge has been made for the first time in a tetrahedral semiconductor. It is concluded that the first phase transition takes place in all these materials whenever the lowest Γ₅ mode (or its equivalent) is lowered to 0.7 times its zero-pressure value. The effect of pressure on the phonon frequencies is discussed in terms of Born's transverse dynamical charges and their pressure dependence.