Of the simple diatomic molecules, oxygen is the only one to carry a magnetic moment. This makes solid oxygen particularly interesting: it is considered a 'spin-controlled' crystal¹ that displays unusual magnetic order². At very high pressures, solid oxygen changes from an insulating to a metallic state³; at very low temperatures, it even transforms to a superconducting state⁴. Structural investigations of solid oxygen began in the 1920s and at present, six distinct crystallographic phases are established unambiguously¹. Of these, the phase of solid oxygen is particularly intriguing: it exhibits a dark-red colour, very strong infrared absorption, and a magnetic collapse¹. It is also stable over a very large pressure domain and has been the subject of numerous X-ray diffraction^5,6,7, spectroscopic^8,9,10,11 and theoretical studies^12,13,14. But although -oxygen has been shown to have a monoclinic C2/m symmetry^5,6,7,15 and its infrared absorption behaviour attributed to the association of oxygen molecules into larger units^9,14, its exact structure remains unknown. Here we use single-crystal X-ray diffraction data collected between 13 and 18 GPa to determine the structure of -oxygen. We find that -oxygen is characterized by the association of four O₂ molecules into a rhombohedral molecular unit, held together by what are probably weak chemical bonds. This structure is consistent with existing spectroscopic data, and further validated by the observation of a newly predicted Raman stretching mode.

1. SUPA, School of Physics and the Centre for Science at Extreme Conditions, The University of Edinburgh, Edinburgh, EH9 3JZ, UK
   
2. Département de Physique Théorique et Appliquée, Commissariat à l'Énergie Atomique, 91680 Bruyères-le-Châtel, France
   
3. Department of Physics, University of Ottawa, Ontario K1N 6N5, Canada

Correspondence to: Malcolm I. McMahon¹Paul Loubeyre² Correspondence and requests for materials should be addressed to M.I.M. (Email: mim@ph.ed.ac.uk) or P.L. (Email: paul.loubeyre@cea.fr).