Revealing the structure of the new C3 phase: the denser phase of hydrogen-hydrate

Gas hydrates are crystalline compounds formed by polyhedral water cages of different size filled with gas molecules in a non-stochiometric ratio. Under high pressure conditions most of gas hydrates transform gas-filled ices, constituted by a water-ice skeleton isostructural to one of the low pressure ice phases filled with gas molecules in its channels. The gas molecules prevent the empty ice structure to collapse exerting a strong hydrophobic repulsion on the water molecules, and are in such a way confined at remarkable high densities in the ice skeleton. In a recent HP powder diffraction experiment on the ID 15 beamline we have observed the formation of the DFT predicted C3 phase of the hydrogen-hydrate. This filled ice is based on ice Ic frame and has an exceptionally high H2O: 2H2 ratio. Here we propose a new experiment on the Id28 beamtime to fully determine C3 structure, including H2 positions, by single-crystal X-ray diffraction. This study is part of a PhD thesis project.