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High-pressure infrared study of solid methane: Phase diagram up to 30 GPa

1997; American Physical Society; Volume: 55; Issue: 22 Linguagem: Inglês

10.1103/physrevb.55.14800

1095-3795

Roberto Bini, Gabriele Pratesi,

Spectroscopy and Laser Applications

High-pressure infrared spectra of solid methane are reported up to 30 GPa between 50 and 300 K. The symmetric stretching mode (${\ensuremath{\nu}}_{1}$) was successfully used as a probe of the phase transitions. Seven different phases have been identified. Pressure and temperature-dependent studies allowed us to outline all the phase boundaries in this portion of the diagram. A high-pressure phase (HP), stable in all the temperature range analyzed, has been identified. The transition to this phase occurs at about 8 GPa at 50 K, and 25 GPa at 300 K. The wide range of stability of this phase suggests a single site-ordered structure. Group-theoretical and qualitative arguments point to hcp (${\mathrm{D}}_{6\mathrm{h}}$ factor group) as the favored crystal structure of the HP phase. The knowledge of the phase diagram allows us to outline the evolution of the crystal structure and of the site symmetries as the pressure increases. The low-pressure fcc crystalline modifications transform to the fully ordered hcp structure through intermediate tetragonal phases. Competition between molecular and crystalline fields determines a complex site-symmetry evolution. Similarities with analogous fcc-hcp evolution observed in rare gases and atomic systems support our conclusions.