High pressure partially ionic phase of water ice

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• Liu, Hanyu
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• Lv, Jian
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• Zhu, Li
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• Wang, Hui
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• Ma, Yanming

Abstract

Water ice dissociates into a superionic solid at high temperature (>2,000K) 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 (~1.5eV) of proton transfer between H₂O molecules. Here we show the pressure-induced formation of a partially ionic phase (monoclinic P2₁ structure) consisting of coupled alternate layers of (OH)^δ- and (H₃O)^δ+ (δ=0.62) in water ice predicted by particle-swarm optimization structural search at zero temperature and pressures of >14Mbar. 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.