Proton order-disorder transition of ice investigated by far-infrared spectroscopy under high pressure

In situ high-pressure measurements of the far-infrared absorption spectra in ice VII and VIII covering the region of translational lattice modes are made by utilizing an infrared beam line of synchrotron radiation at the Institute for Molecular Science (Okazaki). Although the lattice dynamics are almost same for both the phases, large spectral changes are observed on the occasion of the proton order-disorder transition. In the proton disordered phase (ice VII), absorption spectra reflecting a density of states of lattice modes are obtained. The proton ordered phase (ice VIII) gives rise to an absorption peak due to the optical phonon at the r point. The anomalous pressure dependence of the phonon energy in ice VIII indicates the occurrence of a lattice instability below 2.7 GPa with decreasing pressure, which is supported by the recent neutron inelastic scattering experiments by Besson et al.