High-pressure experiments were performed to understand the structural evolution of methane hydrate (MH) up to 134 GPa using x-ray powder diffraction (XRD) and Raman spectroscopy with diamond anvil cells. XRD revealed the distinct changes in the diffraction lines of MH owing to phase transition from a guest-ordered state phase [MH-III(GOS)] to a new high-pressure phase (MH-IV) at 33.8–57.7 GPa. MH-IV was found to be stable up to at least 134 GPa without decomposition into solid methane and high-pressure ices. Raman spectroscopy showed the splits in the C–H vibration modes ν3 and ν1 of guest methane molecules in filled-ice Ih (MH-III) at 12.7 GPa and 28.6 GPa, respectively. These splits are caused by orientational ordering of guest methane molecules contained in the hydrate structure, as observed in a previous study. These results suggest that the structural evolution of the filled-ice structure of MH is caused by successive orientational ordering of guest methane molecules, thereby inducing changes in the host framework formed by water molecules.

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