Dissociation of O2−2 defects into paramagnetic O− in wide band-gap insulators: A magnetic susceptibility study of magnesium oxide

Abstract

The magnetic susceptibility of single-crystal MgO has been measured in the temperature range 300–1000 K, using a Faraday balance. The high-purity crystal (<100 ppm transition metals), grown from the melt in a H2O-containing atmosphere, was found to be paramagnetic due to the presence of defects on the O2− sublattice. The defects derive from OH− introduced into the MgO matrix by the dissolution of traces of H2O during crystal growth. The OH− converts into O2−2 and H2. Each O2−2 represents two coupled, spin-paired O− states. The observed strongly temperature-dependent paramagnetism can be described by three contributions that overlay the intrinsic diamagnetism of MgO and arise from (i) the low level of transition-metal impurities, (ii) O− generated by O2−2 dissociation, and (iii) O− states trapped by quenching from high temperatures from previous experiments.