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 β and ε phases. The occurrence of a first‐order phase transition, positioned at 5.5–6.5 GPa, between the monoclinic α ( C 2/ m ) and the orthorhombic δ ( 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 $^{3}\Sigma_{g}^{-} \rightarrow{}^{1}\Delta_{g}$ , 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. Copyright © 2003 John Wiley & Sons, Ltd.