High pressure antiferromagnetic phases of solid oxygen.

We report on the antiferromagnetic character of solid oxygen in the high-pressure, low-temperature region enclosed between the b and e phases. The occurrence of a first-order phase transition, positioned at 5.5-6.5 GPa, between the monoclinic a (C2/m) and the orthorhombic d (Fmmm) phases was shown by means of an accurate x-ray diffraction analysis. The two crystal structures were assessed to be antiferromagnetically ordered, owing to a narrow peak observed in the IR spectrum and assigned to a vibron component. The absorption of this mode is allowed owing to the coupling of the molecular spins, which produces a doubling of the primitive cell. The vibrational coupling parameter was quantitatively related to the exchange integral J involved in the Heisenberg interaction. Finally, absorption measurements at different pressures of the electronic transition 3 sigma g- -> 1 delta g, , lying in the near-IR spectral range, allowed us, by means of a simple model, to obtain directly the quantity J as a function of the nearest-neighbour distance.