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' crystal that displays unusual magnetic order. At very high pressures, solid oxygen changes from an insulating to a metallic state; at very low temperatures, it even transforms to a superconducting state. Structural investigations of solid oxygen began in the 1920s and at present, six distinct crystallographic phases are established unambiguously. Of these, the epsilon phase of solid oxygen is particularly intriguing: it exhibits a dark-red colour, very strong infrared absorption, and a magnetic collapse. It is also stable over a very large pressure domain and has been the subject of numerous X-ray diffraction, spectroscopic and theoretical studies. But although epsilon-oxygen has been shown to have a monoclinic C2/m symmetry and its infrared absorption behaviour attributed to the association of oxygen molecules into larger units, 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 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.

Oxygen‐Rich Lithium Oxide Phases Formed at High Pressure for Potential Lithium–Air Battery Electrode

“…As presented in Figure 4 , the best fit of the XRD pattern is from the combination of phases LiO 2 (P4/mbm), LiO 4 (Ibam), and Li 2 O 3 (Im‐3m) along with high pressure ε‐O 8 phase (C2/m)22 and pristine Li 2 O 2 phase (P6 3 /mmc). From the fitting volumetric percentage, the amount of LiO 2 is quite small (3.27%), but the amount of Li 2 O 2 has increased dramatically.…”

High-pressure studies with x-rays using diamond anvil cells

“…19). The striking structural complexity is found to exist in many elements, including the widespread existence of incommensurate structures at HP [347][348][349][350].…”

“…Enhanced intermolecular interactions develop because of increasing overlap of the 1πg* orbital in the low-pressure phases, leading to electron delocalization and ultimately intermolecular bonding between O2 molecules at the transition to the ε-phase. The ε-phase, consisting of (O2)4 clusters [347,348], displays the bonding characteristics of a closedshell system. HP studies also provide detailed knowledge of atomic-scale structural changes across phase transitions, which is essential for understanding the process and mechanism of the transitions.…”

First-principles equation of state and electronic properties of warm dense oxygen

“…X-ray diffraction and optical experiments reveal that oxygen condenses to a molecular solid with a rich phase diagram made up of at least ten different structural phases [1][2][3][4][5][6] . Static compression experiments on the solid have been performed up to 1.3 Mbar and 650 K 1 .…”

“…Eventually, intermediate mass stars evolve into white dwarfs, which have most of their hydrogen and helium depleted, leaving a remnant composed mostly of carbon and oxygen. The core density of a white dwarf 16 is likely higher than 10 5 g/cm 3 . For comparison, we also plotted the interior profile of the current-day Sun 14 as well as the profile of a 25 M ⊙ star at the end of its helium burning time 15 .…”