SYNCHROTRON X-RAY DIFFRACTION STUDY OF SOLID O₂: THE CRYSTALLINE STRUCTURE OF THE e-PHASE Serge Desgreniers^a and Keith E. Brister^b, ^aInstitut de physique Ottawa- Carleton, Ottawa, Canada K1N 6N5, ^bCHESS, Cornell University, Ithaca NY, USA.

We present recent angle-dispersive X-ray diffraction results of dense solid oxygen samples obtained at high pressure in diamond anvil cells, using synchrotron radiation and image plate detection. At room temperature, solid oxygen is known to undergo a pressure-induced phase transition from [[partialdiff]]-O₂ to e-O₂ at around 10 GPa, the latter being stable to 96 GPa [1]. Although the structure of [[partialdiff]]-O₂ has been well characterized (Fmmm, Z=4), the proposal for the structure of e-O₂ had yet to be confirmed and refined. Angle-dispersive X-ray diffraction patterns of polycrystalline e-O₂ samples are well described by the A/2m monoclinic space group, as suggested earlier by Johnson et al. [2]. The following lattice parameters are measured for e-O₂ at 12.5+/-0.1 GPa and 297+/-3 K: a=3.68 Å, b=5.62 Å, c=7.94 Å and ß=116.6deg.. Rietveld refinements, performed on diffraction patterns generated from data acquired on image plates, provide atomic positions in the lattice as well as insights on the crystal growth habit on the different phases occuring at high pressure. Furthermore, we comment on the existence of other solid oxygen phases at room temperature at around 10 GPa, intermediate to [[partialdiff]]-O₂ and e-O₂. Finally, from angle-dispersive and energy-dispersive diffraction measurements, we calculate an equation of state for e-O₂ to high pressures.

[1] Akahama, Y, Kawamura, H., Häusermann, D., Hanfland, M., and Shimomura, O. Phys. Rev. Lett. 74, 4690 (1995).

[2] Johnson, S., Nicol, M. and Schiferl, D., J. Appl. Cryst. 26, 236 (1993).