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 96 GPa.[1]
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,[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]
