X-ray diffraction studies and equation of state of methane at 202 GPa

Solid methane (CH4) was compressed up to 202 GPa at 300 K in a diamond-anvil cell. The crystal structure and equation of state over this entire range were determined from angle dispersive X-ray diffraction results. CH4 undergoes phase transitions from rhombohedral to a simple cubic phase at 19 GPa and from simple cubic to a higher pressure cubic phase at approximately 94 GPa. This higher pressure cubic phase was stable to the maximum pressure investigated. Combined with previous optical measurements, it was found that at room temperature compressed CH4 remains an insulator with cubic structure to 202 GPa.