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

Whereas several clathrate-like structures are known to exist from mixtures of H2 + 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 ∼120 K, the H2 vibron originating from the clathrate 2 (C2) 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 C2 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 (C1) phase to persist to the lowest temperature measured (80 K). Upon decompression from the C2 phase we observed the appearance of cubic ice (Ic), which converted to a new phase before transforming to the C1 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.