Elastic properties of GaS under high pressure by Brillouin scattering

Brillouin scattering from LA phonons in the layered semiconductor GaS has been measured in a diamond anvil cell up to 17.5 GPa. In the same range, linear compressibilities parallel and perpendicular to the c axis have been determined, as well as the ordinary and extraordinary refractive indices. From these data the variations of the elastic constants C₁₁ and C₃₃ and the sound velocities under pressure are deduced. Brillouin data at ambient pressure give the five independent elastic constants of GaS(D₆ₕ), and the compressibilities computed from this set of Cᵢⱼ are found to coincide with the values observed in the high-pressure measurements. The optical-index anisotropy decreases to negligible values above 14 GPa. Comparison with an anisotropic modification of Murnaghan's equation of state shows that under high pressure, inter- and intra-layer forces still retain their individual behavior, although the former have become comparable in magnitude with the latter. Brillouin scattering in the ethanol-methanol mixture has been measured up to 17.5 GPa and the variation of the elastic constant of the fluid is compared with Tait's and Murnaghan's equations of state.