Octaoxygen, also known as ε-oxygen or red oxygen, is an allotrope of oxygen consisting of eight oxygen atoms. This allotrope forms above 600 K (327 °C; 620 °F) at pressures greater than 17 GPa.[1]
Ball-and-stick model of O8 |
Part of the crystal structure of ε-oxygen
|
As the pressure of oxygen at room temperature is increased through 10 gigapascals (1,500,000 psi), it undergoes a dramatic phase transition to a different allotrope. Its volume decreases significantly,[2] and it changes color from sky-blue to deep red.[3] 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.[4] However, in 2006, it was shown by X-ray crystallography that this stable phase is in fact O
8.[5][6] No one predicted the structure theoretically:[1] a rhomboid O
8 cluster[7] consisting of four O
2 molecules.
In this phase, it exhibits a dark-red color, very strong infrared absorption, and a magnetic collapse.[8] It is also stable over a very large pressure domain[citation needed] 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.[1]
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.[1]
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.[9]
Researchers think that this structure may greatly influence the structural investigation of elements.[1]