Computer Simulations on Phase Transitions in Ice

Abstract

Ice has a very rich phase diagram (Petrenko and Whitworth 1999) as shown in Figure 1⇓. Up to now, sixteen crystalline phases have been identified experimentally. All crystalline phases except ice X consist of water molecules connected to four adjacent ones by hydrogen bonds. Broadly speaking, the crystalline phases of ice can be described as follows: the low-pressure forms ( P < ~1 GPa) consist of a unique hydrogen-bond network (1HBN). The high-pressure forms (~1 GPa < P < ~80 GPa) are made up of two interpenetrating networks (2HBN); they are often referred to as self-clathrates. Ice Ih and XI are prototypical forms of low-pressure ice with low density, and ice VII and VIII are those of high-pressure ices with high density. Figure 2⇓ shows atomic structures of ice XI and VIII, hydrogen-ordered phases. Between these prototypical phases, there are many complex phases. Figure 1. Phase diagram of ice. Tetrahedra denote water molecules connected to each other by hydrogen bonds represented by solid lines. In ice VI, VII, VIII, X, and XV, two interpenetrating lattices are distinguished by different colors of tetrahedra. Ice Ih, Ic, III, IV, V, VI, VII, and XII are hydrogen-disordered. Other phases are hydrogen-ordered. Ice X is a hydrogen-bond-symmetrized form. Figure 2. Atomic structures of (a) ice XI and (b) ice VIII. Small white and large gray spheres denote hydrogen and oxygen atoms, respectively. Ice XI is a hydrogen-ordered form of ice Ih, the most usual form of ice. Ice Ih is hexagonal ( P 63/ mmc ) and paraelectric. Ice XI is orthorhombic ( Cmc 21) and ferroelectric.For ice VIII, light and dark gray spheres represent two sublattices interpenetrating to each other. Ice VIII is a hydrogen-ordered form of ice VII. Ice VII is cubic ( Pn 3 m ) and …