Antiferromagnetic order in the delta phase of solid oxygen.

Infrared spectroscopy has been applied to the study of solid oxygen in the pressure-temperature region enclosed among the ?, ?, and ? phases which is now unambiguously assigned to the ? phase. The narrow peak, detected in the fundamental mode region, is assigned to a vibron component allowed because of the long-range antiferromagnetic order of the molecular spins which, as in the ? phase, produces a doubling of the primitive cell. The integrated infrared absorption evolution with temperature confirms this model. The molecu- lar vibrational coupling is quantitatively interpreted as due to the Heisenberg interaction via the exchange integral J.