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Observation of an O8 molecular lattice in the ɛ phase of solid oxygen


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 ɛ 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 ɛ-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 ɛ-oxygen. We find that ɛ-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.

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Figure 1: X-ray and Raman measurements of ɛ-oxygen.
Figure 2: The structure of ɛ-oxygen at 17.6 GPa.
Figure 3: Structural evolution of oxygen with pressure.


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We acknowledge discussions with T. Balic-Zunic on the sample twinning. We gratefully acknowledge the assistance of J. Warren and T. Prior in using beamline 9.8 at SRS, Daresbury Laboratory. The work was supported by research grants from the EPSRC, and facilities and other support from Daresbury Laboratory and the CCLRC. S.D. acknowledges the financial support of CEA/DAM Île-de-France and NSERC. Author Contributions P.L. and G.W. prepared the ɛ-oxygen crystals. L.F.L. and M.I.M. performed the X-ray measurements and the structural analysis. G.W. performed the calculations of the modes. S.D. and G.W. performed the Raman measurements. P.L. and M.I.M. wrote most of the paper.

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Correspondence to Malcolm I. McMahon or Paul Loubeyre.

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Supplementary Notes

This file contains the Supplementary Table 1 which is an extended version of Table 1, Supplementary Figures 1–3 and a Supplementary Discussion on the uniqueness of the structure solution and on the observed twinning. (PDF 509 kb)

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Lundegaard, L., Weck, G., McMahon, M. et al. Observation of an O8 molecular lattice in the ɛ phase of solid oxygen. Nature 443, 201–204 (2006).

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