Novel Hydrogen Hydrate Structures under Pressure

Gas hydrates are systems of prime importance. In particular, hydrogen hydrates are potential materials of icy satellites and comets, and may be used for hydrogen storage. We explore the H2O-H-2 system at pressures in the range 0-100 GPa with ab initio variable-composition evolutionary simulations. According to our calculation and previous experiments, the H2O-H-2 system undergoes a series of transformations with pressure, and adopts the known open-network clathrate structures (sII, C-0), dense filled ice'' structures (C-1, C-2) and two novel hydrate phases. One of these is based on the hexagonal ice framework and has the same H2O : H-2 ratio (2 : 1) as the C-0 phase at low pressures and similar enthalpy (we name this phase Ih-C-0). The other newly predicted hydrate phase has a 1 : 2 H2O : H-2 ratio and structure based on cubic ice. This phase (which we name C-3) is predicted to be thermodynamically stable above 38 GPa when including van der Waals interactions and zero-point vibrational energy, and explains previously mysterious experimental X-ray diffraction and Raman measurements. This is the hydrogen-richest hydrate and this phase has a remarkable gravimetric density (18 wt.%) of easily extractable hydrogen.