Water under the extreme conditions of planetary interiors: Symmetric hydrogen bonding in the superionic phase

The predicted superionic phase of water is investigated via ab initio molecular dynamics at densities of 2.0-3.0 g/cc (34-115 GPa) along the 2000K isotherm. We find that extremely rapid (superionic) diffusion of protons occurs in a fluid phase at pressures between 34 and 58 GPa. A transition to a stable body-centered cubic (bcc) 0 lattice with superionic proton conductivity is observed between 70 and 75 GPa, a much higher pressure than suggested in prior work. We find that all molecular species at pressures greater than 75 GPa axe too short lived to be classified as bound states. Above 95 GPa, a transient network phase is found characterized by symmetric O-H hydrogen bonding with nearly 50% covalent character.