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

The detailing of the intermolecular interactions in dense solid oxygen is essential for an understanding of the rich polymorphism and remarkable properties of this element at high pressure. Synchrotron inelastic x-ray scattering measurements of oxygen K-edge excitations to 38 GPa reveal changes in electronic structure and bonding on compression of the molecular solid. The measurements show that O(2) molecules interact predominantly through the half-filled 1pi(g)* orbital <10 GPa. Enhanced intermolecular interactions develop because of increasing overlap of the 1pi(g)* orbital in the low-pressure phases, leading to electron delocalization and ultimately intermolecular bonding between O(2) molecules at the transition to the epsilon-phase. The epsilon-phase, which consists of (O(2))(4) clusters, displays the bonding characteristics of a closed-shell system. Increasing interactions between (O(2))(4) clusters develop upon compression of the epsilon-phase, and provide a potential mechanism for intercluster bonding in still higher-pressure phases.