A phase behavior around the transition between ice VII and a plastic phase of water is investigated by molecular dynamics simulation and the subsequent analysis on the basis of Landau theory. The prior works have predicted that the phase transition between ice VII and plastic ice is a first-order transition on the ground of a weak hysteresis and so on. A rigorous survey in the present report, however, augments their prediction with new evidence that a first-order phase transition line gives way to a second-order one at higher pressures, where a tricritical point joins these phase boundaries together. Critical phenomena are also observed whereby, other than that associated with the hypothetical critical point in the deeply supercooled state, which could influence the physical properties in a wide range of temperatures and pressures. A new critical behavior is affirmed by the result that the scaling law holds at any pressure on the second-order phase transition line for which the critical exponents are estimated. We introduce an appropriate order parameter to obtain the Landau free energy functional and the change in the functional against temperature accounts for the phase behaviors. This also enables an estimate of the coexistence and the spinodal lines at pressures below the tricritical point, all of which compensate those obtained directly by molecular dynamics simulations. These results allow us to anticipate that the critical fluctuations may give us a clue for determining the phase boundary experimentally.

We report various types of pathways emerging in the course of freezing into a high pressure ice VII by large-scale molecular dynamics (MD) simulations at 10 GPa, 425 K. Some trajectories showed an apparently stepwise transition via the unique metastable "phase" in accordance with Ostwald's step rule. The metastable structure was identified as one of the tetrahedrally close-packed structures having the nature of a rotator phase (plastic phase). The unit cell consists of 21 water molecules that has not yet been reported by simulation or by experiments. Structure analysis of numerous trajectories reveals that the nucleation of ice VII easily occurs at the grain boundaries of the metastable embryos, known as epitaxy-mediated transformation in line with Ostwald's step rule. The size of the grain boundaries is thus responsible for the lifetime of the metastable phase. On the other hand, once the embryo of ice VII appears prior to that of the metastable phase in liquid, direct transition to ice VII takes place. We also show that the plastic behavior of the metastable phase is not uniform but is sensitive to the interaction strength of adjacent molecules and the local structure, thus we call "partially plastic ice".

We have investigated structural and energetic characteristics of plastic ice, which was found in a high pressure region such as 10 GPa by molecular dynamics simulation and free energy calculation. It was predicted that plastic ice intervenes between ice VII and liquid water, in which diffusion is suppressed but rotation is allowed. In the present work, the structure in plastic ice is explored from both local and global view points and focus is placed on the local arrangement, the extent of deviation from the ideal lattice position, and the hydrogen-bonded patterns. The roles of the attractive interaction and the repulsive part of Lennard-Jones potential are also examined. It is found that the higher interaction energy in plastic ice induces a large dislocation of water molecules, which eventually conducts a facile rotation. There are a large amount of hydrogen-bonds which do not orient to the tetrahedral directions. These orientational defects give rise to fusion of the two interpenetrating sublattices of ice VII leading to a plastic phase rather than defect-containing ice VII, which results in a unique network structure of the plastic ice.

We propose a spin model compatible with ice VII-plastic ice phase transitions and critical phenomena discovered recently by computer simulations. The Blume-Capel spin-1 Ising model is extended in order to describe the entropic stabilization effect in the plastic ice phase. The model shares the same set of tricritical exponents with simulation, indicating that they are of the same universality class.

A plastic phase predicted to exist between crystalline ice VII and liquid water has been investigated. The present work focuses on the dynamics of the water molecules in the phases at high temperature and pressure, i.e. ice VII, plastic ice, and liquid water. The hydrogen-bond correlation function providing the lifetime of hydrogen bonds is compared to the reorientational correlation function to examine a relation between a rotation of an individual molecule and an energetic relaxation process. The hydrogen-bond correlation function of plastic ice decays in a way similar to liquid but it converges to a finite value as seen in ice VII, reflecting the rotational motion of the water molecule at the fixed location. In addition, the relaxation times of the two correlation functions for plastic ice resemble one another, confirming the fact that only the rotational motion invokes the hydrogen-bond rearrangements in plastic ice.