Orientational ordering, locking-in, and distortion of CH4 molecules in Methane Hydrate III under high pressure

• Published in: The Journal of Physical Chemistry C
• Main Authors: Schaack, Sofiane, Ranieri, Umbertoluca, Depondt, Philippe, Gaal, Richard, Kuhs, Werner F., Falenty, Andrzej, Gillet, Philippe, Finocchi, Fabio, Bove, Livia E.
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Chemical Society 2018
• Subjects: Electronic; optical and magnetic material; energy (all); Physical and Theoretical Chemistry; surface; coatings and films; Methane hydrate
• Online Access: http://hdl.handle.net/11573/1195897; https://doi.org/10.1021/acs.jpcc.8b02783

We investigate the effects of high pressure on the reorientational and vibrational dynamics of methane molecules embedded in methane hydrate III - 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 CH4:D2O 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 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 coupling regime where methane guests and water hosts dynamics are intimately paired.