Pressure-induced phase transformations have been found in CuGa${\mathrm{S}}_2$ and AgGa${\mathrm{S}}_2$ by high-pressure x-ray-diffraction studies, using the energy dispersion technique. CuGa${\mathrm{S}}_2$ transforms from the chalcopyrite to the NaCl type near 16 GPa. AgGa${\mathrm{S}}_2$ undergoes three pressure-induced phase changes: (1) from chalcopyrite to a hexagonal lattice near 5 GPa, (2) from chalcopyrite and hexagonal to $\alpha -\mathrm{N}\mathrm{a}\mathrm{F}\mathrm{e}{\mathrm{O}}_2$ type near 12 GPa, and (3) to the NaCl type at pressures above 15 GPa. These results confirm the pressure-induced phase transitions indicated by the phonon behavior in a recent high-pressure Raman study of CuGa${\mathrm{S}}_2$ and AgGa${\mathrm{S}}_2$. From the high-pressure x-ray data on the chalcopyrite phase, the following bulk modulus $B_0$ and $\frac{d{B}_0}{\mathrm{dP}}$ were obtained by fitting the data to the Murnaghan equation of state: CuGa${\mathrm{S}}_2$, $B_0=96\mathrm{}\pm 10 \mathrm{GPa}$, $B_0^{\prime }=6.5\mathrm{}$; $\mathrm{AgGa}{\mathrm{S}}_2$, $B_0=60\mathrm{}\pm 8 \mathrm{GPa}$, $B_0^{\prime }=6\mathrm{}$.

• Received 2 September 1980

DOI:https://doi.org/10.1103/PhysRevB.23.3836