Molecular character of the metallic high-pressure phase of oxygen

We report Raman measurements of solid oxygen under high static pressures up to 134 GPa through the transition to the metallic phase. Contrary to previous studies, we observed a moderate decrease in intensity of the Raman vibron at the transition, which allows us to characterize unambiguously the metallic phase as a molecular solid. The transition involves a change in lattice of molecular centers and a possible reorientation of the molecules together with a relatively weak change in molecular bonding. We observed a large pressure range of phase coexistence and hysteresis of the transformation.