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

Abstract

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 diffraction^5,6,7, spectroscopic^8,9,10,11 and theoretical studies^12,13,14. But although ɛ-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 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.