Lowering symmetry of filled ice Ic hydrogen hydrate under low temperatures and high pressures

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  • Filled ice Ic 水素ハイドレートの低温高圧下における相変化
  • Filled ice Ic スイソ ハイドレート ノ テイオン コウアツカ ニ オケル ソウヘンカ

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Abstract

Low-temperature and high-pressure experiments were performed with filled ice Ic structure of hydrogen hydrate at the unexplored conditions of 5 to 55 GPa and 30 to 300 K using diamond anvil cells and a heliumrefrigeration cryostat. In-situ X-ray diffractometry revealed that the cubic filled ice Ic structure transformed to a tetragonal structure at low temperatures and high pressures and the axis ratio of the tetragonal phase changed depending on pressure and temperature. These results were consistent with theoretical predictions determined by first principle calculations. In addition, further change in the lattice parameters was observed from about 45 to 50 GPa, which suggested the existence of a transformation to another high-pressure phase above 50 GPa throughout the temperature region examined. The reason for transforming to the tetragonal structure was inferred; it might be induced due to changes in the vibrational or rotational modes of the hydrogen molecules under low temperature and high pressure.

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