New metallic ice phase under high pressure†

Crystal materials can exhibit novel properties under high pressure, which are completely different from properties under ambient conditions. Water ice has an exceptionally rich phase diagram with at least 20 known crystalline ice phases from experiments, where the high-pressure ice X and ice XVIII behave as an ionic state and a superionic state, respectively. Thus, the ice structures stabilized under high pressure are very likely to possess other novel properties. Herein, we constructed a sequence of hypothetical high-pressure ices whose structures were generated according to the topological frameworks of common metal oxides. Based on density functional theory calculations, the pressure-induced phase transition sequence is in order that the known Ag₂O-Pnm structure (ice X) transformed into a previously undiscovered TiO₂_brookite-Pbca structure at a pressure of 300 GPa, followed by a transition to a new NaO₂-Pa3 structure at a pressure of 2120 GPa. Hitherto unreported NaO₂-Pa3 ice with a cubic structure is in the ionic state, where the oxygen atoms in NaO₂-Pa3 have a face-centered cubic (fcc) sublattice, and the coordination number of H atoms increases to 3. These two structures are dynamically stable according to phonon spectrum analysis and remain stable at temperature of 100 K based on ab initio molecular dynamics (AIMD) simulations. More importantly, the NaO₂-Pa3 ice exhibits novel metallic properties with a closing band gap above a pressure of 2600 GPa, owing to the electron orbital coupling of oxygen atoms in close proximity induced by pressure.