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.
Structural evolution of methane hydrate under pressures up to 134 GPa
Hirokazu Kadobayashi, Hisako Hirai, Hiroaki Ohfuji, Michika Ohtake, Michihiro Muraoka, Suguru Yoshida, Yoshitaka Yamamoto; Structural evolution of methane hydrate under pressures up to 134 GPa. J. Chem. Phys. 21 May 2020; 152 (19): 194308. https://doi.org/10.1063/5.0007511
Download citation file: