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

Abstract

We investigate the effects of high pressure on the reorientational and vibrational dynamics of methane molecules embedded in MH-III hydrate -the stable form of methane for pressures above 2 GPa at room temperature -by combining high-pressure Raman spectroscopy with ab-initio simulations including nuclear quantum effects. We observe a clear evolution of the system from a gas-filled ice structure, where methane molecules occupy the channels of the ice skeleton and rotate almost freely, to a CH 4 :D 2 O compound where methane rotations are hindered, and methane and water dynamics are tightly coupled. The gradual orientational ordering of the guest molecules results in a complete locking-in at approximately 20GPa. This happens along with a progesssive distortion of the guest molecules. Finally, as pressure increases beyond 20 GPa the system enters a strong mode coupling regime where methane guests and water hosts dynamics are intimately paired.

Observation of methane filled hexagonal ice stable up to 150 GPa

“…The 2 D 2 O stretching modes shifted to smaller frequencies and lost intensity swiftly between 35 and 45 GPa, as previously reported in the literature for methane hydrate (15, 16)> and in strong analogy to what is commonly observed in pure ice (19) and salt-doped ices (20). In our previous work (11), we described in detail the mode mixing occurring between the stretching mode of D 2 O and the rocking mode of CH 4 in this pressure range. A peak at 950 cm−1 appeared upon pressure increase at ∼100 GPa.…”

“…Measurements of the same sample up to 45 GPa were previously performed by our group, using the same experimental apparatus but different diamonds, and are reported in ref. 11. Details of sample preparation and Raman spectra are shown in SI Appendix .…”

“…They showed peaks appearing in the diffraction patterns beyond 40 GPa, which are not compatible with the MH-III structure. Moreover, MH-III presents nonnegligible angular frustration for the water and methane molecules (11), which increases upon compression and might weaken the whole structure. All of the previous results indicate that a different MH structure is formed, although experiments did not allow identifying it uniquely.…”

“…The phase transition from MH-III to MH-IV can also be detected by looking into the behavior of the vibration modes as a function of the increasing pressure. In the MH-III structure, the methane molecules undergo an increasing distortion from tetrahedral symmetry upon compression (11); at P≃40 GPa, a HCH^ angle diminishes while another widens by 3○ with respect to 109.47○. The departure from normalTd symmetry implies the loss of degeneracy of methane stretching and bending modes.…”

“…As pressure increases in MH-III, the O–C distances shorten and methane vibrations become more and more entangled with those of the neighboring water molecules. Above ∼20 GPa, the enclathrated methane appreciably deviates from the tetrahedral symmetry and shows orientational ordering (11, 12)>.…”