Phase Behavior of H2 + H2O at High Pressures and Low Temperatures

Whereas several clathrate-like structures are known to exist from mixtures of H-2 + H2O under pressure, the combined high-pressure and low-temperature region of the phase diagram remains largely unexplored. Here we report a combined Raman spectroscopy and synchrotron X-ray diffraction study on the low-temperature region of the phase diagram. Below similar to 120 K, the H-2 vibron originating from the clathrate 2 (C-2) phase splits into two distinct components, yet X-ray diffraction measurements reveal no structural change between room temperature and 11 K. We suggest that the two vibrons of the C-2 phase at low temperature originate from vibrational transitions of hydrogen molecules in the ground and first excited rotational energy levels. At similar to 1 GPa we observe the clathrate 1 (C-1) phase to persist to the lowest temperature measured (80 K). Upon decompression from the C-2 phase we observed the appearance of cubic ice (I-c), which converted to a new phase before trans forming to the C-1 phase. The structure of the new phase is consistent with a water framework similar to a-quartz; the structure could also be related to the tetragonal clathrate phase reported previously for nitrogen and argon guests.