Fig. 15: Phase diagram of ice showing the two high pressure polymorphs ice VII and ice VIII, and their boundary measured using Raman scattering by Pruzan et al. (oxygen atoms in green and hydrogen atoms in red). Hollow squares: H2O, filled squares: D2O. Ice VIII has a proton ordered tetragonal structure which transforms into a proton disordered cubic structure, ice VII, upon heating. We now have some evidence that the hydrogen bonds (indicated by the blue dashed lines) transform to give «symmetric ice X» around 165 GPa.