An illustration of the structural model of filled ice Ic hydrogen hydrate, HH-C2 (modified from that shown by Vos et al. 16). Water molecules are drawn by red and blue spheres and hydrogen molecules are drawn by light blue spheres.
Experimental paths. Eleven paths were performed for five samples. Different colors indicate different samples used. The paths indicated by dotted lines only show changes in the pressure-temperature conditions, i.e., XRD measurements were not made along the dotted lines.
Changes in XRD patterns with pressures from 6.5 to 46.6 GPa at 300 K. The 220, 311, 400, and 331 diffraction lines split above 20 GPa and 25 GPa, respectively. Above 45 GPa, the split reduced. Red arrows show the splitting diffraction lines of the tetragonal phase and blue arrows show those of the HP-phase.
Peak fitting of the split 220 and 311 peaks at 34.7 GPa and 300 K (left) and at 24.9 GPa and 300 K (right). The split peaks are indexed as a tetragonal structure.
(Left) Changes in XRD patterns with pressure from 24.5 to 55.5 GPa at 100 K. (Right) Changes in XRD patterns with increasing temperature from 84 to 300 K. Red arrows show the splitting diffraction lines of the tetragonal phase and blue arrows show those of the HP-phase.
Summary of phase changes. Reaction diagram incorporating all experimental data from the 11 paths of five samples. The solid blue rhombuses, green squares, and pink circles indicate the cubic, tetragonal, and high-pressure phases, respectively. The open pink circles indicate the high-pressure phase reported previously.21 The open green square at 0 K indicates the calculated tetragonal phase.28
Variation of lattice parameters with pressure change at 300 K (top) and 100 K (bottom).
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