Rotational and Translational Disorder in High-Pressure Water Ice

Above 2 GPa water ice presents a single sub-lattice of oxygen atoms and all the different phases differ by the behavior of the hydrogen atoms. Behind this apparent simplicity, the sub-lattice of hydrogen atoms undergoes several structural and dynamical transitions: while increasing the pressure leads to the dissociation of the water molecules (VII – X transition), increasing the temperature activates the rotations of the water molecules, which is predicted to form a plastic phase (dynamical orientational disorder of H₂O molecules) and superionic phases (fast diffusion of H atoms in the oxygen sub-lattice). The relations between the superionic domain, the hypothetical plastic phase, the H-bond symmetrization and the melting of the bcc sub-lattice remain poorly constrained. Here, we provide first-principles analysis of the H-bonding regimes encountered in whole stability domain of bcc ices. We report the first ab initio evidence for a plastic phase of water and we propose a coherent phase diagram for bcc water ices compatible with the two groups of melting curves and with the multiple anomalies reported in ice VII around 10 – 20 GPa.