Structural, electronic, and dynamical properties of methane under high pressure.

The electronic structure and lattice dynamical properties of solid methane under high pressure have been studied based on density functional theory. We identify a cubic structure with space group of I[formula]3m below 14 GPa, the Pmn21 structure in the range of 14-21 GPa, and the P21/c structure from 21 to 65 GPa. Our obtained Raman spectra of the P21/c structure agree well with the typical Raman active modes in the available experimental data. At 65 GPa, methane undergoes a phase transition from P21/c to Pnma. The structures with P21/c and Pnma symmetries are insulating, and under any pressure studied methane always remains in molecular form. For Pnma phase, the orientational ordering of CH4 molecules varies significantly at 79, 88, and 92 GPa, and by further increasing pressure the rotation of the molecules freezes and orientational ordering remains unchanged. [ABSTRACT FROM AUTHOR]