Structural instability in ice VIII under pressure

Below 125 K, tetragonal ice VIII remains in a metastable form at ambient pressure. This form has long been considered to be identical to ice VIII, although unexplained discrepancies exist between their vibrational spectra. Neutron diffraction studies reveal an isostructural phase transformation below 2 GPa related to a weakening of the bonds between the two D2O sublattices of ice VIII. In this new phase, ice VIII', the tetragonal distortion is 20% larger than in ice VIII, which accounts for the differences between the vibrational frequencies of the two phases.