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.

Intrinsic Conductivity in Magnesium–Oxygen Battery Discharge Products: MgO and MgO2

“…This phenomenon, along with anomalies in magnetic susceptibility and charge distribution data, were argued by Batllo and co-workers to arise from the disassociation of holes bound to doubly negative Mg vacancies. These hole–vacancy complexes are created by the incorporation of trace H 2 O during crystal growth and are thus of extrinsic origin . The holes were postulated to be localized on the oxygen sublattice, which would make them very similar to the small hole polarons identified in the present study.…”

“…Our prediction that doubly negative Mg vacancies, V Mg 2− , and holes, p + , comprise the dominant defects in MgO is consistent with experiments. [33][34][35]88 For example, magnetic susceptibility measurements indicate the presence of paramagnetic species in MgO, 34 a feature which could be explained by a nonzero concentration of p + . Also, abrupt changes in the magnetic properties, charge distribution, and conductivity in MgO with respect to temperature were attributed to holes localized on the oxygen sublattice that are formed upon dissociation of a so-called "peroxy defect".…”

“…21, [31][32][33] These differing values suggest the transport mechanism in MgO varies as a function of temperature. Although speculation regarding the different mechanisms has been offered, [32][33][34][35] little direct evidence exists.…”

“…Unusually, the conductivity data for MgO exhibits three distinct W values of approximately 2.4, 1, and 0.2 eV. ,− These differing values suggest the transport mechanism in MgO varies as a function of temperature. Although speculation regarding the different mechanisms has been offered, − little direct evidence exists.…”

Intrinsic Conductivity in Magnesium-Oxygen Battery Discharge Products: MgO and MgO2

“…21, [31][32][33] These differing values suggest the transport mechanism in MgO varies as a function of temperature. Although speculation regarding the different mechanisms has been offered, [32][33][34][35] little direct evidence exists.…”