Temperature-induced topological differentiation of the two high-density amorphous ices

Physique des Milieux Denses, Institut de Minéralogie et Physique des Milieux Condensés CNRS-UMR 7590, Université Pierre et Marie Curie - F-75252 Paris, France

Received: 17 January 2006
Accepted: 7 March 2006

Abstract

The high-density (HDA) and the very high-density (VHDA) amorphous forms of ice are studied through computer simulations. The structural transformations involve, at different stages of compression, two different subgroups of second-shell neighbors. The interplay between these subgroups is found to be crucial in the mechanism of high-pressure differentiation between HDA and VHDA: in HDA they are more separated and the first subgroup is locked in the anti-tetrahedral, ice-VII-like positions all along the downstroke to ambient pressure. In VHDA the annealing under pressure triggers instead the merging of the two subgroups through a counter-intuitive outward displacement of one subgroup of neighbors, resulting then in a local decrease of the coordination number, a more random-close-packed arrangement and a different equation of state.