High pressure crystal phases of solid CH4 probed by Fourier transform infrared spectroscopy

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Abstract
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 D4h unit cell symmetry. On the contrary, the analysis of the ω1 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 Cs symmetry, while the unit cell symmetry must be chosen among D4h, D6h, Th and Oh groups. Qualitative considerations point to D6h as the more favored unit cell symmetry for phase B.

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