科研之友
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摘要
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.
