DISSOCIATION OF O-2(2-) DEFECTS INTO PARAMAGNETIC O- IN WIDE BAND-GAP INSULATORS - A MAGNETIC-SUSCEPTIBILITY STUDY OF MAGNESIUM-OXIDE

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.