Optical studies of methane under high pressure

Abstract

Physical properties of solid methane have been studied at room temperature under high pressure in a diamond-anvil cell. Internal modes have been followed up to 20 GPa by Raman scattering. The refractive index has been measured by a newly developed method up to 12 GPa and the elastic properties determined by Brillouin scattering up to 32 GPa. In the plastic phase I, the experimental equation of state and the elastic properties were analyzed with reference to rare-gas solids, i.e., with use of self-consistent harmonic calculations with various pair potentials. The best agreement is obtained with Aziz's Hartree-Fock+dispersion-C krypton-krypton potential. From analogies with rare-gas and ${\mathrm{NH}}_{3}$ solids, a hcp structure is proposed for phase IV. The IV-VII phase transition is shown to be orientational in nature.