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Gas hydrates are systems of prime importance. In particular, hydrogenhydrates are potential materials of icy satellites and comets, and may be usedfor hydrogen storage. We explore the H2O-H2 system at pressures in the range0-100 GPa with ab initio variable-composition evolutionary simulations.According to our calculation and previous experiments, the H2O-H2 systemundergoes a series of transformations with pressure, and adopts the knownopen-network clathrate structures (sII, C0), dense "filled ice" structures (C1,C2) and two novel hydrate phases. One of these is based on the hexagonal iceframework and has the same H2O:H2 ratio (2:1) as the C0 phase at low pressuresand similar enthalpy (we name this phase Ih-C0). The other newly predictedhydrate phase has a 1:2 H2O:H2 ratio and structure based on cubic ice. Thisphase (which we name C3) is predicted to be thermodynamically stable above 38GPa when including van der Waals interactions and zero-point vibrationalenergy, and explains previously mysterious experimental X-ray diffraction andRaman measurements. This is the hydrogen-richest hydrate and this phase has aremarkable gravimetric density (18 wt.%) of easily extractable hydrogen.

Predicted Novel Hydrogen Hydrate Structures under Pressure from First Principles