Pressure dependence of the Raman modes and pressure-induced phase changes in CuGaS2and AgGaS2

The pressure dependence of the Raman peaks of CuGa${\mathrm{S}}_2$ and AgGa${\mathrm{S}}_2$ (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 ${\Gamma }_5$ phonon, prior to a phase transition, which takes place at 16.5±0.5 GPa in CuGa${\mathrm{S}}_2$ and 4.2±0.5 GPa in AgGa${\mathrm{S}}_2$. In the latter material, discontinuous changes of the Raman frequencies indicate another phase transition at 11.6±0.5 GPa. The transition in CuGa${\mathrm{S}}_2$ is believed to be to the disordered rocksalt structure, and in AgGa${\mathrm{S}}_2$, the high-pressure transition is to the rocksalt, or to the closely related $\alpha$-NaFe${\mathrm{O}}_2$ structure. In AgGa${\mathrm{S}}_2$, the softening of the lowest ${\Gamma }_5$ 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 ${\Gamma }_5$ 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.