Effect of high pressure on the Raman spectra of ice VIII and evidence for ice X

Raman spectra of H2O and D2O ice VIII have been measured up to 50 GPa at 100 K. The changes in line positions and intensities of ice VIII and the appearance of a new band above 40 GPa point to the formation of symmetric hydrogen bonds in ice X. A lattice dynamical analysis of the frequency shifts up to 36 GPa suggest a second order phase transition to symmetric hydrogen bonds near 47.5 GPa. However, the observed changes in the lattice modes indicate that this phase transition is already formed by a first order transition at 42 GPa.