Structural evolution of methane hydrate under pressures up to 134 GPa

• Published in: The Journal of chemical physics Vol. 152; no. 19; p. 194308
• Main Authors: Kadobayashi, Hirokazu, Hirai, Hisako, Ohfuji, Hiroaki, Ohtake, Michika, Muraoka, Michihiro, Yoshida, Suguru, Yamamoto, Yoshitaka
• Format: Journal Article
• Language: English
• Published: United States 21.05.2020
• ISSN: 1089-7690
• DOI: 10.1063/5.0007511

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 ν and ν 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.