title: Novel hydrogen hydrate structures under pressure.
abstract: 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-H₂ 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-H₂ system undergoes a series of transformations with pressure, and adopts the known open-network clathrate structures (sII, C₀), dense "filled ice" structures (C₁, C₂) and two novel hydrate phases. One of these is based on the hexagonal ice framework and has the same H₂O:H₂ ratio (2:1) as the C₀ phase at low pressures and similar enthalpy (we name this phase Ih-C₀). The other newly predicted hydrate phase has a 1:2 H₂O:H₂ ratio and structure based on cubic ice. This phase (which we name C₃) 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.
authors.affiliation: Department of Geosciences, Stony Brook University, Stony Brook, New York 11794-2100, USA.
authors.forename: Guang-Rui
authors.initials: GR
authors.lastname: Qian
authors.affiliation: Department of Geosciences, Stony Brook University, Stony Brook, New York 11794-2100, USA.
authors.forename: Andriy O
authors.initials: AO
authors.lastname: Lyakhov
authors.affiliation: Department of Geosciences, Stony Brook University, Stony Brook, New York 11794-2100, USA.
authors.forename: Qiang
authors.initials: Q
authors.lastname: Zhu
authors.affiliation: 1] Department of Geosciences, Center for Materials by Design, and Institute for Advanced Computational Science, State University of New York, Stony Brook, NY 11794-2100 [2] Moscow Institute of Physics and Technology, 9 Institutskiy lane, Dolgoprudny city, Moscow Region 141700, Russia [3] School of Materials Science, Northwestern Polytechnical University, Xi'an 710072, China.
authors.forename: Artem R
authors.initials: AR
authors.lastname: Oganov
authors.affiliation: 1] Department of Geosciences, Stony Brook University, Stony Brook, New York 11794-2100, USA [2] School of Physics and MOE Key Laboratory of Weak-Light Nonlinear Photonics, Nankai University, Tianjin 300071, China.
authors.forename: Xiao
authors.initials: X
authors.lastname: Dong