Pressure dependence of hydrogen bonding in a novel H2OH2 clathrate

We have carried out Raman spectroscopy and X-ray diffraction to 60 GPa of the 1:1 H2O:2 clathrate hydrate, which has water molecules in an arrangement similar to pure ices Ic and VII. The pressure dependence of both the νOH frequency and the intermolecular distance dOHO is significantly larger than in pure ice: unexpectedly, both dependencies can be mapped onto those of pure ice by doubling the pressure. Nevertheless, the correlation between νOH and dOHO is the same as in pure ice, making the clathrate a model system for ice at pressures reduced by a factor 2. Thus, the transition to a phase with symmetric hydrogen bonds probably occurs in the range 30–40 GPa, which is significantly lower than that for H2O.