Dielectric properties of ice VII and VIII and the phase boundary between ice VI and VII

Abstract

The dielectric properties of H2O ice VII have been measured in a piston-cylinder cell. The limiting low-frequency permittivity implies a value of the orientational correlation parameter g that increases from 1.41 at 0°C to 1.54 at 70°C. This is about a half the corresponding value for ice I and suggests that the molecular axes are on the average more antiparallel to one another than required by the ice rules, and are consequently not fully disordered. The VI-VII and VI-VIII phase boundaries for both H2O and D2O have been measured. The entropy of the transition VII-VIII is 0.66 cal K−1 · mole−1 for H2O and 0.64 cal K−1 · mol−1 for D2O, which are 0.15 and 0.17 cal K−1 · mole−1 less than expected if ice VIII were fully ordered and ice VII were fully disordered within the limits of the ice rules. Both dielectric and phase boundary measurements therefore indicate incomplete disorder in ice VII.