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Oxygen is in many ways a unique element: the only known diatomic molecularmagnet and the capability of stabilization of the hitherto unexpected O8cluster structure in its solid form at high pressure. Molecular dissociationsupon compression as one of the fundamental problems were reported for otherdiatomic solids (e.g., H2, I2, Br2, and N2), but it remains elusive for solidoxygen, making oxygen an intractable system. We here report the theoreticalprediction on the dissociation of molecular oxygen into a polymeric spiralchain O4 structure (\theta-O4) by using first-principles calypso method oncrystal structure prediction. The \theta-O4 stabilizes above 2 TPa and has beenobserved as the third high pressure phase of sulfur (S-III). We find that themolecular O8 phase remains extremely stable in a large pressure range of 0.008- 2 TPa, whose breakdown is driven by the pressure-induced instability of atransverse acoustic phonon mode at zone boundary, leading to the ultimateformation of \theta-O4. Remarkably, stabilization of \theta-O4 turns oxygenfrom a superconductor into an insulator with a wide band gap (approximately 5.9eV) originating from the sp3-like hybridized orbitals of oxygen and thelocalization of valence electrons. (This is a pre-print version of thefollowing article: Li Zhu et al, Spiral chain O4 form of dense oxygen, Proc.Natl. Acad. Sci. U.S.A. (2011), doi: 10.1073/pnas.1119375109, which has beenpublished online at http://www.pnas.org/content/early/2011/12/27/1119375109 .)

Spiral Chain O4 Form of Dense Oxygen