Phase changes induced by guest orientational ordering of filled ice Ih methane hydrate under high pressure and low temperature

18th APS-SCCM; 24th AIRAPT

Hirai, H.^1 ; Tanaka, T.^1 ; Matsuoka, T.^2 ; Ohishi, Y.^3 ; Yagi, T.^1 ; Ohtake, M.^4 ; Yamamoto, Y.^4

Geodynamics Research Center, Ehime University, Ehime, Matsuyama

790-8577, Japan^1

KYOKUGEN, Osaka University, Osaka, Toyonaka

560-8531, Japan^2

Japan Synchrotron Radiation Research Institute, Harima

679-5198, Japan^3

National Institute of Advanced Industrial Science and Technology, Tsukuba

305-8568, Japan^4

关键词: Diamond-anvil cell;    High-pressure experiment;    High-pressure phase;    Low temperature regions;    Methane molecules;    Orientational orderings;    Pressure and temperature;    Situ X-ray diffractometry;   

Others  :  https://iopscience.iop.org/article/10.1088/1742-6596/500/19/192006/pdf
DOI  :  10.1088/1742-6596/500/19/192006

来源: IOP

【 摘 要 】

Low-temperature and high-pressure experiments were performed with filled ice Ih structure of methane hydrate under pressure and temperature conditions of 2.0 to 77.0 GPa and 30 to 300 K, respectively, using diamond anvil cells and a helium-refrigeration cryostat. Distinct changes in the axial ratios of the host framework were revealed by In-situ X-ray diffractometry. Splitting in the CH vibration modes of the guest methane molecules, which was previously explained by the orientational ordering of the guest molecules, was observed by Raman spectroscopy. The pressure and temperature conditions at the split of the vibration modes agreed well with those of the axial ratio changes. The results indicated that orientational ordering of the guest methane molecules from orientational disordered-state occurred at high pressures and low temperatures, and that this guest ordering led to the axial ratio changes in the host framework. Existing regions of the guest disordered-phase and the guest ordered-phase were roughly estimated by the X-ray data. In addition, above the pressure of the guest-ordered phase, another high pressure phase was developed at a low-temperature region. The deuterated-water host samples were also examined and isotopic effects on the guest ordering and phase changes were observed.