HIGH-PRESSURE CRYSTAL PHASES OF SOLID CH4 PROBED BY FOURIER-TRANSFORM INFRARED-SPECTROSCOPY

High pressure infrared spectra of solid CH4 are reported in the range 0.8-30 GPa at room temperature, coupling a Fourier transform infrared spectrometer to a membrane diamond-anvil cell by means of a high efficiency beam condensing optical system. Two crystal phases, A and B, have been investigated. The phase transition is affected by hysteresis and occurs at 9+/-0.5 GPa during compression and at 7+/-0.5 GPa during expansion. Due to hysteresis, the transition has been studied as a function of time at higher pressures and found to undergo a first-order kinetics, with rate constant increasing with pressure. Since our experimental apparatus allows us to perform high pressure Raman measurements too, structural properties of both A and B phases have been proposed from the analysis of the infrared and Raman data. Within the framework of the widely used three-site model, the A phase structure is consistent with a D-4h unit cell symmetry. On the contrary, the analysis of the omega(1) infrared and Raman multiplets in phase B as a function of pressure suggests quite plausibly a single site, well-ordered crystal structure. By means of group-theoretical arguments it is concluded that CH4 molecules occupy sites of C-s symmetry, while the unit cell symmetry must be chosen among D-4h, D-6h, T-h and O-h groups. Qualitative considerations point to D-6h as the more favored unit cell symmetry for phase B. (C) 1995 American Institute of Physics.