Structural transitions in hydrogen hydrate at high pressures.

Gas hydrates are crystalline hydrogen-bonded water frameworks encaging gas atoms or molecules at high gas densities. Hydrates of natural gas occurs on the sea floor on Earth and hydrates of astrophysically relevant gases are candidate components for the interiors of the icy bodies of the outer Solar system. Due to its high weight percent of hydrogen, H2-hydrate is also a promising material for solid hydrogen storage, and pressure is a key parameter to explore this direction as it promotes novel stoichiometries with increased storage capacities as well as increased temperature ranges of stability. We propose to explore the stability and the structures formed by the H2-hydrate in the 50 GPa-150 GPa pressure range by x-ray diffraction in diamond anvil cell. Based on published calculations and by analogy with the high-pressure behaviour of CH4 hydrate, we expect that this system forms a new high-pressure structure above 50 GPa which will be stable in the Mbar range.