Novel Hydrogen Clathrate Hydrate

We report a new hydrogen clathrate hydrate synthesized at 1.2 GPa and 298 K documented by single-crystal x-ray diffraction, Raman spectroscopy, and first-principles calculations. The oxygen sublattice of the new clathrate hydrate matches that of ice II, while hydrogen molecules are in the ring cavities, which results in the trigonal R3c or R (3) over barc space group (proton ordered or disordered, respectively) and the composition of (H2O)(6)H-2. Raman spectroscopy and theoretical calculations reveal a hydrogen disordered nature of the new phase C-1', distinct from the well-known ordered C-1 clathrate, to which this new structure transforms upon compression and/or cooling. This new clathrate phase can be viewed as a realization of a disordered ice II, unobserved before, in contrast to all other ordered ice structures.