Magnetic Properties of the Oxygen Molecule in Solid Oxygen-Argon Mixtures

Recent neutron diffraction and crystallographic studies of the magnetically ordered phase (≤24°K) of solid oxygen have aroused new interest in magnetic exchange interactions between oxygen molecules. The interpretation of the magnetic ground state in the pure oxygen system is difficult because multiple solid‐phase transitions and the narrow range of temperatures over which each phase is stable prevent a determination of the magnitude of the exchange interaction. We report here on magnetic susceptibility studies of O₂‐A solid mixtures which exist from 0°–70°K in a single hcp phase for O₂ concentrations in argon ranging from 20%–50% and in a single fcc phase from 0%–20%. The measurements can be quantitatively interpreted using a statistical molecular field theory of disordered solid‐solutions. We find ${\text{θ̄/c≅60}}^{\circ }\mathrm{K}$ , where $\mathrm{\theta ̄}$ is the ``statistical'' Curie‐Weiss temperature and c is the concentration of O₂ in A. This is comparable to the paramagnetic Curie‐Weiss temperature θ=50°–60°K observed in pure liquid and γ solid phase O₂. The relatively large temperature range over which the data and theory agree contrasted to the narrow temperature range available for the liquid γ solid phases reenforces the interpretation that strong exchange interactions do occur between O₂ molecules.