Plastic crystal phases of simple water models

摘要

We report the appearance of two plastic crystal phases of water at high pressure and temperatureusing computer simulations. In one of them the oxygen atoms form a body centered cubic structure(bcc) and in the other they form a face centered cubic structure (fcc). In both cases the watermolecules were able to rotate almost freely. We have found that the bcc plastic crystal transformedinto a fcc plastic crystal via a Martensitic phase transition when heated at constant pressure. We haveperformed the characterization and localization in the phase diagram of these plastic crystal phasesfor the SPC/E, TIP4P, and TIP4P/2005 water potential models. For TIP4P/2005 model free energycalculations were carried out for the bcc plastic crystal and fcc plastic crystal using a new method(which is a slight variation of the Einstein crystal method) proposed for these types of solid. Theinitial coexistence points for the SPC/E and TIP4P models were obtained using Hamiltonian Gibbs-Duhem integration. For all of these models these two plastic crystal phases appear in the highpressure and temperature region of the phase diagram. It would be of interest to study if such plasticcrystal phases do indeed exist for real water. This would shed some light on the question of whetherthese models can describe satisfactorily the high pressure part of the phase diagram of water, and ifnot, where and why they fail.