Spiral chain O 4 form of dense oxygen

Abstract

Oxygen is in many ways a unique element: It is the only known diatomic molecular magnet, and it exhibits an unusual O₈ cluster in its high-pressure solid phase. Pressure-induced molecular dissociation as one of the fundamental problems in physical sciences has been reported from theoretical or experimental studies of diatomic solids H₂, N₂, F₂, Cl₂, Br₂, and I₂ but remains elusive for molecular oxygen. We report here the prediction of the dissociation of molecular oxygen into a polymeric spiral chain O₄ structure (space group I4₁/acd, θ-O₄) above 1.92-TPa pressure using the particle-swarm search method. The θ-O₄ phase has a similar structure as the high-pressure phase III of sulfur. The molecular bonding in the insulating ε-O₈ phase or the isostructural superconducting ζ-O₈ phase remains remarkably stable over a large pressure range of 0.008–1.92 TPa. The pressure-induced softening of a transverse acoustic phonon mode at the zone boundary V point of O₈ phase might be the ultimate driving force for the formation of θ-O₄. Stabilization of θ-O₄ turns oxygen from a superconductor into an insulator by opening a wide band gap (approximately 5.9 eV) that originates from the sp³-like hybridized orbitals of oxygen and the localization of valence electrons.