O8Cluster Structure of the Epsilon Phase of Solid Oxygen

Abstract

Despite many experimental and theoretical studies, the crystal structure of the epsilon phase of solid oxygen has not been determined. We performed powder x-ray diffraction experiments and the Rietveld analyses in this study to show that a new arrangement of the monoclinic space group C2/m could fit the diffraction patterns of the epsilon phase and obtained a structure that consisted of an O8 cluster with 4 molecules. The dependence of the lattice parameters, the molar volume, and the intermolecular distances on the pressure was investigated.

Inelastic x-ray scattering of dense solid oxygen: Evidence for intermolecular bonding

“…Although there is some scatter in the region of the -phase, the overall variation can be largely accounted for by the volume change, providing evidence that the variation in the * transition energy is a consequence of the expected volume reduction under pressure. The decreased variation in the * transition energy Ͼ10 GPa is consistent with the low compressibility of the -phase as determined by x-ray diffraction (3,5). The contrasting behavior of the * and * transition energies indicates the involvement of the 1 g * molecular orbitals on adjacent O 2 molecules, an interaction which evolves as oxygen undergoes phase transitions at high pressure.…”

“…The recent discovery of (O 2 ) 4 molecular clusters in the -phase by x-ray diffraction (3,4) has revived interest in the nature of the intermolecular interactions in dense fluid and solid oxygen, the evolution of these interactions with pressure, and the origin of the stability of the (O 2 ) 4 -based structure of the -phase (13). Using oxygen K-edge inelastic x-ray scattering (IXS) spectroscopy (14), a unique high-pressure probe of local electronic structure and chemical bonding (15)(16)(17), we report an experimental characterization of the electronic structure and bonding changes in condensed phases of oxygen up to 38 GPa that provides direct information on intermolecular interactions on compression.…”

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.…”

X-ray crystallography of simple molecular solids up to megabar pressures: application to solid oxygen and carbon dioxide

“…On one hand, Fujihisa et al performed angle-dispersive X-ray diffraction experiments on powder samplesof pure oxygen [45]. The samples were loaded cryogenically in diamond anvil cells with a conical aperture in order to obtain full Debye-Scherrer rings.…”