The phase diagram of water at high pressures as obtained by computer simulations of the TIP4P/2005 model: the appearance of a plastic crystal phase

摘要

In this work the high pressure region of the phase diagram of water has beenstudied by computer simulation by using the TIP4P/2005 model of water. Freeenergy calculations were performed for ices VII and VIII and for the fluidphase to determine the melting curve of these ices. In addition moleculardynamics simulations were performed at high temperatures (440K) observing thespontaneous freezing of the liquid into a solid phase at pressures of about80000 bar. The analysis of the structure obtained lead to the conclusion that aplastic crystal phase was formed. In the plastic crystal phase the oxygen atomswere arranged forming a body center cubic structure, as in ice VII, but thewater molecules were able to rotate almost freely. Free energy calculationswere performed for this new phase, and it was found that for TIP4P/2005 thisplastic crystal phase is thermodynamically stable with respect to ices VII andVIII for temperatures higher than about 400K, although the precise valuedepends on the pressure. By using Gibbs Duhem simulations, all coexistencelines were determined, and the phase diagram of the TIP4P/2005 model wasobtained, including ices VIII and VII and the new plastic crystal phase. TheTIP4P/2005 model is able to describe qualitatively the phase diagram of water.It would be of interest to study if such a plastic crystal phase does indeedexist for real water. The nearly spherical shape of water makes possible theformation of a plastic crystal phase at high temperatures. The formation of aplastic crystal phase at high temperatures (with a bcc arrangements of oxygenatoms) is fast from a kinetic point of view occurring in about 2ns. This is incontrast to the nucleation of ice Ih which requires simulations of the order ofhundreds of ns.