  Oxygen is in many ways a unique element: the only known diatomic molecular
magnet and the capability of stabilization of the hitherto unexpected O8
cluster structure in its solid form at high pressure. Molecular dissociations
upon compression as one of the fundamental problems were reported for other
diatomic solids (e.g., H2, I2, Br2, and N2), but it remains elusive for solid
oxygen, making oxygen an intractable system. We here report the theoretical
prediction on the dissociation of molecular oxygen into a polymeric spiral
chain O4 structure (\theta-O4) by using first-principles calypso method on
crystal structure prediction. The \theta-O4 stabilizes above 2 TPa and has been
observed as the third high pressure phase of sulfur (S-III). We find that the
molecular 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 a
transverse acoustic phonon mode at zone boundary, leading to the ultimate
formation of \theta-O4. Remarkably, stabilization of \theta-O4 turns oxygen
from a superconductor into an insulator with a wide band gap (approximately 5.9
eV) originating from the sp3-like hybridized orbitals of oxygen and the
localization of valence electrons. (This is a pre-print version of the
following 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 been
published online at http://www.pnas.org/content/early/2011/12/27/1119375109 .)
