Octaoxygen

Octaoxygen
Part of the crystal structure of ε-oxygen
Names
Other names
  • ε-Oxygen
  • epsilon oxygen
  • Red oxygen
Identifiers
Properties
O8
Molar mass 127.992 g·mol−1
Appearance Deep red
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Octaoxygen, also known as ε-oxygen or red oxygen, is an allotrope of oxygen consisting of eight oxygen atoms. This allotrope forms at high pressure and remains stable across a wide pressure range at room temperature (between 10 and 96 GPa). If it is further compressed past 96 GPa, then it transitions into a metallic, superconducting state.

The material is dark red and, unlike other allotropes of oxygen, has no magnetic character, due to its unpaired electrons being forced to overlap.

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 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 was unclear. A crystal lattice of tetraoxygen O4 units was suggested in 1999, based on infrared spectroscopy. However, in 2006, two independent groups showed by X-ray crystallography that the structure was a rhomboid O8 ((O2)4) cluster consisting of four O2 molecules.

(O2)4, which exhibits strong bonds in the diatomic units but with much weaker π*–π* bonds between them

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 11 GPa, the intra-cluster bond length of the O8 cluster is 2.34 Å, and the inter-cluster distance is 2.66 Å, both longer than the 1.2 Å bond-length in the oxygen molecule (O2). It is stable over a very large pressure domain, from 10 GPa to 96 GPa. Above 96 GPa, 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]

References

  1. 1 2 3 Fujihisa, Hiroshi; Akahama, Yuichi; Ohishi, Yasuo. "O8 Cluster in the ε Phase of Solid Oxygen" (PDF). www.spring8.or.jp. SPring 8. Retrieved 31 March 2026.
  2. 1 2 Akahama, Yuichi; Kawamura, Haruki; Häusermann, Daniel; Hanfland, Michael; Shimomura, Osamu (5 June 1995). "New High-Pressure Structural Transition of Oxygen at 96 GPa Associated with Metallization in a Molecular Solid". Physical Review Letters. 74 (23): 4690–4693. Bibcode:1995PhRvL..74.4690A. doi:10.1103/PhysRevLett.74.4690. PMID 10058574.
  3. 1 2 Nicol, Malcolm; Hirsch, K.R.; Holzapfel, Wilfried B. (December 1979). "Oxygen phase equilibria near 298 K". Chemical Physics Letters. 68 (1): 49–52. Bibcode:1979CPL....68...49N. doi:10.1016/0009-2614(79)80066-4.
  4. ↑ Gorelli, Federico A.; Ulivi, Lorenzo; Santoro, Mario; Bini, Roberto (November 1999). "The ε Phase of Solid Oxygen: Evidence of an O4 Molecule Lattice". Physical Review Letters. 83 (20): 4093–4096. Bibcode:1999PhRvL..83.4093G. doi:10.1103/PhysRevLett.83.4093.
  5. ↑ Gorelli, Federico A.; Ulivi, Lorenzo; Santoro, Mario; Bini, Roberto (20 February 2001). "Spectroscopic study of the ε phase of solid oxygen". Physical Review B. 63 (10). doi:10.1103/PhysRevB.63.104110.
  6. ↑ Fujihisa, Hiroshi; Akahama, Yuichi; Kawamura, Haruki; Ohishi, Yasuo; Shimomura, Osamu; Yamawaki, Hiroshi; Sakashita, Mami; Gotoh, Yoshito; Takeya, Satoshi; Honda, Kazumasa (25 August 2006). "O8 Cluster Structure of the Epsilon Phase of Solid Oxygen". Physical Review Letters. 97 (8) 085503. doi:10.1103/PhysRevLett.97.085503. PMID 17026315.
  7. ↑ Lundegaard, Lars F.; Weck, Gunnar; McMahon, Malcolm I.; Desgreniers, Serge; Loubeyre, Paul (2006-09-14). "Observation of an O8 molecular lattice in the phase of solid oxygen". Nature. 443 (7108): 201–204. Bibcode:2006Natur.443..201L. doi:10.1038/nature05174. PMID 16971946. S2CID 4384225.
  8. ↑ Steudel, Ralf; Wong, Ming Wah (2007). "Dark-Red O8 Molecules in Solid Oxygen: Rhomboid Clusters, Not S8-Like Rings". Angewandte Chemie International Edition. 46 (11): 1768–1771. doi:10.1002/anie.200604410. PMID 17450606.Open access icon
  9. ↑ Freiman, Yu A.; Jodl, H. J. (2004). "Solid Oxygen". Physics Reports. 401 (1–4): 1–228. Bibcode:2004PhR...401....1F. doi:10.1016/j.physrep.2004.06.002.
  10. ↑ Ochoa-Calle, A. J.; Zicovich-Wilson, C. M.; Hernández-Lamoneda, R.; Ramírez-Solís, A. (10 March 2015). "Understanding the ε and ζ High-Pressure Solid Phases of Oxygen. Systematic Periodic Density Functional Theory Studies Using Localized Atomic Basis". Journal of Chemical Theory and Computation. 11 (3): 1195–1205. doi:10.1021/acs.jctc.5b00017. PMID 26579768.
  11. ↑ Ball, Phillip (16 November 2001). "New form of oxygen found". Nature News. doi:10.1038/news011122-3.