Single-crystal growth of the high-pressure phase II of methane hydrate and its Raman scattering study

A single crystalline methane hydrate (MH) of high-pressure phase II was synthesized in a diamond anvil cell at P≈1.0 GPa and 50 °C through its decomposition line. Its shape is a hexagonal pillar that suggests MH-II is the hexagonal structure. In situ Raman spectra of the C–H stretching vibrations in the guest CH4 molecules present a broad band at 0.94 GPa and separate into two Raman bands at P≈1.3 GPa and room temperature. By considering the Raman intensities and the pressure dependences of their frequencies, we suggest that there exist two or three CH4 molecules in a large cage if five small cages are fully occupied in the MH-II (sH) phase.