New Phases of Water Ice Predicted at Megabar Pressures

TL;DR

This study predicts two new high-pressure water ice phases with distinct structures and electronic properties, including a metallic phase, at pressures of 7.6 and 15.5 Mbar using density functional calculations.

Contribution

It introduces two novel crystal structures of water ice at extreme pressures, expanding understanding of ice's phase diagram under megabar conditions.

Findings

Predicted new ice phases Pbca and Cmcm at 7.6 and 15.5 Mbar.

Cmcm phase is metallic, unlike other insulating ice phases.

Structural details of hydrogen atom positions in high-pressure ice.

Abstract

Based on density functional calculations we predict water ice to attain two new crystal structures with Pbca and Cmcm symmetry at 7.6 and 15.5 Mbar, respectively. The known high pressure ice phases VII, VIII, X, and Pbcm as well as the Pbca phase are all insulating and composed of two interpenetrating hydrogen bonded networks, but the Cmcm structure is metallic and consists of corrugated sheets of H and O atoms. The H atoms are squeezed into octahedral positions between next-nearest O atoms while they occupy tetrahedral positions between nearest O atoms in the ice X, Pbcm, and Pbca phases.