Observation of an O8 molecular lattice in the ε 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' crystal(1) that displays unusual magnetic order(2). At very high pressures, solid oxygen changes from an insulating to a metallic state(3); at very low temperatures, it even transforms to a superconducting state(4). Structural investigations of solid oxygen began in the 1920s and at present, six distinct crystallographic phases are established unambiguously(1). Of these, the 1 phase of solid oxygen is particularly intriguing: it exhibits a dark-red colour, very strong infrared absorption, and a magnetic collapse(1). It is also stable over a very large pressure domain and has been the subject of numerous X-ray diffraction(5-7), spectroscopic(8-11) and theoretical studies(12-14). But although epsilon-oxygen has been shown to have a monoclinic C2/m symmetry(5-7,15) 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 epsilon-oxygen. We find that epsilon-oxygen is characterized by the association of four O-2 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.