Phase changes of filled ice Ih methane hydrate under low temperature and high pressure

Abstract

Low-temperature and high-pressure experiments were performed with filled ice Ih structure of methane hydrate under 2.0-77.0 GPa and 30-300 K using diamond anvil cells and a helium-refrigeration cryostat. In situ X-ray diffractometry revealed distinct changes in the compressibility of the axial ratios of the host framework with pressure. Raman spectroscopy showed a split in the C-H vibration modes of the guest methane molecules, which was previously explained by the orientational ordering of the guest molecules. The pressure and temperature conditions at the split of the vibration modes agreed well with those of the compressibility change. The results indicate the following: (i) the orientational ordering of the guest methane molecules from an orientationally disordered state occurred at high pressures and low temperatures; and (ii) this guest ordering led to anisotropic contraction in the host framework. Such guest orientational ordering and subsequent anisotropic contraction of the host framework were similar to that reported previously for filled ice Ic hydrogen hydrate. Since phases with different guest-ordering manners were regarded as different phases, existing regions of the guest disordered-phase and the guest ordered-phase were roughly estimated by the X-ray study. In addition, above the pressure of the guest-ordered phase, another high-pressure phase developed in the low-temperature region. The deuterated-water host samples were also examined, and the influence of isotopic effects on guest ordering and phase transformation was observed.

Orientational Ordering, Locking-in, and Distortion of CH4 Molecules in Methane Hydrate III under High Pressure

“…While our low-pressure data are in excellent agreement with previous authors, 14,19,21 we observed a different behaviour above 15-20 GPa. Specifically, both peaks progressively broaden with pressure (see Figure 1, right panel); however, we detected no splitting of either the ν 1 or ν 3 modes in the range 15-20 GPa or beyond; details of this frequency range are plotted in Figure 3 and compared with our simulation data.…”

“…8 Therefore, several experimental studies in the last years focused on the high-pressure behavior of methane hydrates. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] As pressure increases beyond the kbar range, typically above 1-2 GPa, most ice clathrates undergo profound structural changes: the cages shrink and reorganize into structures bearing some ressemblance to ice phases, known as filled ices, 9 where the guest molecules occupy interstices in the ice lattices. 10 In filled ices, three different structures have been observed so far, and more recently a new "chiral hydrate" was established.…”

“…MH-III was found to be stable up to 86 GPa at least, 15 though a possible transition to an unresolved high-pressure structure was reported to occur around 40 GPa. 14,15,19,21 A splitting of the symmetric (ν 1 ) and antisymmetric (ν 3 ) CH stretching mode peaks above 15 − 20 GPa has also been observed, which some authors attributed to the CH 4 orientational ordering 14,19,21 and others ascribed to the distortion of the methane molecules at high pressure. 22 Finally, the appearance of a possible Fermi resonance between the overtone of the D 2 O bending mode and the OD stretching mode was observed 22 at 15 GPa, while symmetrization of the hydrogen bond network was predicted to occur above 60 GPa from ab-initio molecular simulations.…”

Water and methane stay together at extreme pressures

“…The evidence for these discoveries is based on Raman spectroscopy carried out in a diamond anvil cell and supported by high-level computational calculations. Schaack et al (8) also show that their proposed structure for MH-IV is consistent with previously unresolved X-ray diffraction data reported by Tanaka et al (12). The mechanism of the MH-III to MH-IV phase transition is quite subtle and requires the reorganization of only a few hydrogen bonds, which explains why the H 2 O:CH 4 ratio can remain constant at 2:1 during the phase transition.…”

Observation of methane filled hexagonal ice stable up to 150 GPa

“…Recent studies also suggested that under high-pressure conditions methane and water could form stoichiometric mixed molecular crystals beyond the known MH-III phase: The existence of a new high-pressure phase in methane hydrate at pressures beyond 40 to 50 GPa was first hinted by Hirai and coworkers (13–16), who observed a slow transition from MH-III into a different, yet unsolved structure during synchrotron X-ray diffraction experiments. The high-pressure structure was observed to survive up to 86 GPa, but the diffraction patterns did not allow the authors to identify it in detail.…”