Infrared investigation on ice VIII and the phase diagram of dense ices

The phase diagram of the dense ices VII, VIII (molecular phases), and X (nonmolecular phase) was investigated over a P-T region of 12-298 K and 2-100 GPa by infrared absorption measurements. The ice VII-VIII and VIII-X phase boundaries determined in this study agree with those provided by previous Raman studies. The ice VII-X and VIII-X transitions can be characterized by the completion of rotational to distortional mode conversion (the disappearance of the ν_R rotational peak), and the ice VII-X boundary was determined accordingly. Triple points among the three phases are located at 100 K/62 GPa and 100 K/75 GPa for H₂O and D₂O ice, respectively. Substitution of hydrogen with deuterium pushes the boundary between the molecular (VII, VIII) and nonmolecular (X) ice phases toward higher pressures by 12–14 GPa, indicating clearly isotope effect on the phase transition due to dynamical translational disordering. Careful observation of infrared spectra near the triple point reveals that translational proton transfer arising from thermal activation and mutisite disordering becomes significant over a pressure span of about 10 GPa prior to the determined ice VII-X boundary. The frequency of the bending mode shows an anomalous softening below 8 GPa in both ice VII and ice VIII as a result of structural relaxation.