High-density structures and phase transition in an ionic model of H2O ice

A self-consistent ionic model of water (protons and O2-ions) has been used to explore the low-temperature, high-pressure properties of ice. Interionic interactions were calculated with the electron-gas model, and the self-consistent changes in the electronic structure of the oxide ions were modeled with Watson spheres. A phase transition is predicted at about 330 GPa from the observed, low-pressure, symmetric hydrogen-bonded cuprite structure to a fully ordered antifluorite structure. The transition pressure is higher than the 100-GPa estimate obtained from a molecular-dynamics study [Phys. Rev. Lett. 60, 2284 (1988)] that employed more approximate potentials.