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

We investigate the effects of high pressure on the reorientational and vibrational dynamics of methanemolecules embedded in methanehydrate III—the stable form of methanefor 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 methanemolecules occupy the channels of the ice skeleton and rotate almost freely, to a CH4:D2O compound where methanerotations are hindered, and methaneand water dynamics are tightly coupled. The gradual orientational ordering of the guest molecules results in a complete locking-in at approximately 20 GPa. This happens along with a progressive distortion of the guest molecules. Finally, as pressure increases beyond 20 GPa, the system enters a strong mode couplingregime where methaneguests and water hosts dynamics are intimately paired.