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.

New porous water ice metastable at atmospheric pressure obtained by emptying a hydrogen-filled ice

“…Lattice constant are a =6.3313±0.0002 Å and c =6.1058±0.0002 Å. The other structures considered, namely sT′ ( P 4 2 / mnm ), Ih-C 0 ( Cc )21 and ice I c ( Fd m ), would give diffraction patterns in evident disagreement with the experimental one. Details of the synthesis procedure and of the X-ray measurements are given in the Methods section.…”

“…In fact, the α -quartz structure, with the proposed parameters20, gives rise to atypical, very short, O–O distances. After this, Oganov and colleagues21 used density functional theory-based structure searching to predict several phases of the H 2 –H 2 O mixture, but these searching methods are restricted in the number of atoms; thus, any large clathrate-sized unit cells can easily be missed. In this work we study, by means of Raman spectroscopy, the C 0 phase of the water–hydrogen mixture, produced at ∼400 MPa and recovered at room pressure and liquid nitrogen temperature, still containing a large fraction of molecular hydrogen.…”

Ostwald's rule of stages and metastable transitions in the hydrogen–water system at high pressure

“…At these pressures ice Ih based structures are less stable than C 0 for hydrogen hydrate. 15,35 This means the sample goes through a series of transitions that occur in increasing stability. This cascading through metastable states from an unstable state (ice Ih in this case) to most stable (sII clathrate) is known as Ostwald's Rule of Stages (also known as Law of Steps).…”

“…The energetic order of C À1 and C 0 reverses at 0.5 GPa, in line with literature findings. 15 Around 1.7 GPa, the calculations predict that C À1 would decompose into a linear combination of C 1 and C 2 hydrates. The main results of the calculations -that C 0 is more stable than C À1 at low pressures and C À1 eventually decomposes into higher hydrates -are independent of the choice of exchange-correlation functional, though values for transition pressures do vary.…”

“…1 Recently, a filled ice Ih type structure was proposed to be stable at the same the conditions as the C 0 phase in the H 2 -H 2 O system by Qian et al using an evolutionary structure search. 15 Here we report results from a neutron-diffraction experiments on the D 2 -D 2 O system at low pressures -in the stability region of the sII hydrate. 16 In these we observed the inclusion of deuterium into an ice Isd structure (a stacking disordered structure of ices Ih and Ic), 17,18 and the appearance of the C 0 structure outside its stability range.…”