MAGNETIC PROPERTIES OF 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 O 2-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 θ̄/c≅60°K, where θ̄ 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. © 1969 The American Institute of Physics."