Dielectric Properties of Ice VII. Ice VIII: A New Phase of Ice

Abstract

The dielectric properties of ice have been studied up to 107 kbar using opposed circular anvils. Ice VII undergoes orientational polarization, and consequently its molecules are orientationally disordered. The volume of activation for the relaxation of the orientational polarization is ∼3 cm³ mole⁻¹, which suggests that the reorientation occurs by the migration of Bjerrum defects, as it appears to do in ice I, III, V, and VI. The energy and entropy of activation at 23 kbar are about 11.6 kcal mole⁻¹ and 9.2 cal deg⁻¹·mole⁻¹, which are similar to those for other phases of ice. At about 0°C, ice VII undergoes a transformation to a phase, here designated ice VIII, whose molecules are orientationally immobile at audio frequencies. This phase had previously been suggested by an analysis of the boundary line between ice VI and VII. The transformation occurs with a hysteresis of about 5.5°C at constant pressure, and the transformation temperature is independent of pressure to within about 0.5° up to 50 kbar. Some measurements previously identified with ice VII must now be identified with ice VIII or perhaps with another phase occurring at much lower temperatures. The results illustrate the usefulness of dielectric measurements in an opposed‐anvil apparatus for the study of phase changes in nonconductors at high pressures.