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

Abstract

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 ${\ensuremath{\Gamma}}_{5}$ phonon, prior to a phase transition, which takes place at 16.5\ifmmode\pm\else\textpm\fi{}0.5 GPa in CuGa${\mathrm{S}}_{2}$ and 4.2\ifmmode\pm\else\textpm\fi{}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\ifmmode\pm\else\textpm\fi{}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 $\ensuremath{\alpha}$-NaFe${\mathrm{O}}_{2}$ structure. In AgGa${\mathrm{S}}_{2}$, the softening of the lowest ${\ensuremath{\Gamma}}_{5}$ 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 ${\ensuremath{\Gamma}}_{5}$ 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.