Octaoxygen

Preparation and properties

As the pressure of oxygen at room temperature is increased above 10 gigapascals (1,500,000 psi), it undergoes a dramatic phase transition to a different 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 consists of O
₄ molecules in a crystal lattice. However, in 2006, it was shown by X-ray crystallography that this stable phase is in fact O
₈. No one predicted the structure theoretically: a rhomboid O
₈ cluster consisting of four O
₂ molecules.

In this phase, it exhibits a dark-red color, very strong infrared absorption, and a magnetic collapse. It is also stable over a very large pressure domain and has been the subject of numerous X-ray diffraction, spectroscopic and theoretical studies. 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 11 GPa, the intra-cluster bond length of the O
₈ cluster is 0.234 nm, and the inter-cluster distance is 0.266 nm, both longer than the 0.120 nm bond-length in the oxygen molecule O
₂.

The formation mechanism of the O
₈ 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.