Letter

Nature 443, 201-204 (14 September 2006) | doi:10.1038/nature05174; Received 2 May 2006; Accepted 16 August 2006

Observation of an O8 molecular lattice in the alt epsilon phase of solid oxygen

Lars F. Lundegaard1, Gunnar Weck2, Malcolm I. McMahon1, Serge Desgreniers3 and Paul Loubeyre2

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 alt epsilon 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 diffraction5, 6, 7, spectroscopic8, 9, 10, 11 and theoretical studies12, 13, 14. But although alt epsilon-oxygen has been shown to have a monoclinic C2/m symmetry5, 6, 7, 15 and its infrared absorption behaviour attributed to the association of oxygen molecules into larger units9, 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 alt epsilon-oxygen. We find that alt epsilon-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).

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