Octaoxygen, also known as ε-oxygen or red oxygen, is an allotrope of oxygen consisting of eight oxygen atoms. This allotrope forms at room temperature at pressures between 10 and 96GPa.

Preparation and properties

As the pressure of oxygen at room temperature is increased above 10 gigapascals (1,500,000psi), it undergoes a dramatic phase transition to the 𝛆-phase allotrope. Its volume decreases significantly, and it changes color from sky-blue to deep red. This ε-phase was discovered in 1979, but the structure has been unclear. Based on infrared spectroscopy, researchers assumed in 1999 that this phase consisted of O4 molecules in a crystal lattice. However, in 2006, it was shown by X-ray crystallography that this stable phase is a rhomboid O8 ((O2)4) cluster consisting of four O2 molecules.

In this phase, it exhibits a dark-red color, very strong infrared absorption, and a magnetic collapse. It has been shown to have a monoclinic C2/m symmetry, and its infrared absorption behaviour was attributed to the association of oxygen molecules into larger units. At 11GPa, the intra-cluster bond length of the O8 cluster is 2.34Å, and the inter-cluster distance is 2.66Å, both longer than the 1.2Å bond-length in the oxygen molecule (O2). It is stable over a very large pressure domain, from 10GPa to 96GPa. Above 96GPa, ζ-oxygen forms which is metallic and superconductive; ε-oxygen is non-conductive.

The formation mechanism of the O8 cluster found in the work is not clear yet, and the researchers think that the charge transfer between oxygen molecules or the magnetic moment of oxygen molecules has a significant role in the formation.

Potential applications

Liquid oxygen is already used as an oxidant in rockets, and it has been speculated that octaoxygen could make an even better oxidant, because of its higher energy density.[need quotation to verify]