High pressure phase transitions and hydrogen-bond symmetry in ice polymorphs

Abstract

A recently developed ground state variational wave function theory of hydrogen-bonded crystals is employed to investigate the high pressure behavior of several ice polymorphs. The theory accurately accounts for short and long range proton correlations and quantum mechanical tunneling, all of which are important in the high pressure regime. Attention is focused on pressure induced order–disorder phase transitions and hydrogen-bond symmetrization for the ices VII, VIII, and Ic. We find a strongly first order transition (driven by proton tunneling) at approximately 330 kbar from the antiferroelectrically ordered ice VIII phase to a highly ionized form of proton disordered ice VII. Despite the large degree of ionization, significant short range proton order remains at the transition as indicated by the bimodality of the proton charge density along the bond. The crossover to a unimodal situation, i.e., hydrogen-bond symmetrization, is predicted to occur at 450 kbar. The effects of deuterium isotope substitution on these phenomena are also studied. Calculations for the lower density cubic form of ice indicate that it may be possible to observe symmetrical hydrogen bonds in this material at a significantly lower pressure than those required for the dense ices.