Spiral chain O4 form of dense oxygen

Oxygen is in many ways a unique element: It is the only known diatomic molecular magnet, and it exhibits an unusual O-8 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 H-2, N-2, F-2, Cl-2, Br-2, and I-2 but remains elusive for molecular oxygen. We report here the prediction of the dissociation of molecular oxygen into a polymeric spiral chain O-4 structure (space group I4(1)/acd, theta-O-4) above 1.92-TPa pressure using the particle-swarm search method. The theta-O-4 phase has a similar structure as the high-pressure phase III of sulfur. The molecular bonding in the insulating epsilon-O-8 phase or the isostructural superconducting zeta-O-8 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 O-8 phase might be the ultimate driving force for the formation of theta-O-4. Stabilization of theta-O-4 turns oxygen from a superconductor into an insulator by opening a wide band gap (approximately 5.9 eV) that originates from the sp(3)-like hybridized orbitals of oxygen and the localization of valence electrons.