High pressure partially ionic phase of water ice

Water ice dissociates into a superionic solid at high temperature (> 2,000 K) and pressure, where oxygen forms the lattice, but hydrogen diffuses completely. At low temperature, however, the dissociation into an ionic ice of hydronium (H3O)(+) hydroxide (OH)(-) is not expected because of the extremely high energy cost (similar to 1.5 eV) of proton transfer between H2O molecules. Here we show the pressure-induced formation of a partially ionic phase (monoclinic P2(1) structure) consisting of coupled alternate layers of (OH)(delta-) and (H3O)(delta+) (delta = 0.62) in water ice predicted by particle-swarm optimization structural search at zero temperature and pressures of > 14 M Mbar. The occurrence of this ionic phase follows the break-up of the typical O-H covalently bonded tetrahedrons in the hydrogen symmetric atomic phases and is originated from the volume reduction favourable for a denser structure packing.