A methane hydrate (MH) single crystal was synthesized in a diamond anvil cell to investigate its intrinsic high-pressure properties. With increasing pressure, the cubic sI phase of MH changed to the MH-II phase at P ) 0.9 GPa and room temperature, and this phase remains stable up to P ) 1.9 GPa, which was visually observed by optical microscopy. In situ Raman spectra for CH 4 molecules encaged in different cages of MH-II show two vibrational bands; the higher frequency band shows a remarkable increase in its frequency versus pressure (17.0 cm -1 /GPa), and the lower band shows a progressive increase in frequency with pressure (6.3 cm -1 /GPa). These results are interpreted on the basis of two different structures recently reported for MH-II. Above P ) 1.9 GPa, MH-II crystals visually decomposed and the O-H stretching Raman band of host cages became unobservable, indicating no more existence of the cage structure. Raman spectra of CH 4 molecules in MH-III show almost the same behavior as those of pure solid methane up to at least 5.2 GPa, which may be consistent with the existence of a new type of MH.
Acoustic velocities and adiabatic elastic constants of structure I of methane hydrate (MH) have been determined as a function of pressure up to 0.6 GPa at 23 • C by the high-pressure Brillouin spectroscopy developed for a single molecular crystal. The pressure dependence of the acoustic velocities of MH is very similar to that of ice-Ih except for the longitudinal acoustic (LA) velocity. The value of the LA velocity along the 100 direction of MH at 0.02 GPa is 3.63 km s −1 which is about 7% lower than the average of the LA velocities in the ice-Ih phase at −35.5 • C and atmospheric pressure.
The elastic properties of structure I (sI) methane hydrate (MH) determined by high-pressure Brillouin spectroscopy up to 0.6 GPa at 23℃ are reviewed by comparing them with those of ice-Ih. The pressure dependence of adiabatic elastic moduli of sI-MH is similar to that of ice-Ih except for C 11. Elastic moduli and bulk modulus indicate that sI-MH is slightly more compressive than ice-Ih, and acoustic velocities show nearly isotropic behaviors with respect to the crystal orientation. The C 44 of sI-MH is less sensitive to pressure and smaller than C 11 and C 12 , which implies the sI-MH is becoming less stable against the shear stress under high pressures. These results are useful to investigate the dynamic stability and the estimated amount of MH in the deep-sea sediments.
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