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 + H 2 O 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 ∼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 ∼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 transforming to the C 1 phase. The structure of the new phase is consistent with a water framework similar to α-quartz; the structure could also be related to the tetragonal clathrate phase reported previously for nitrogen and argon guests.