The dc conductivity and ionic transference number of MgO single crystals doped with Al, Fe, and Sc were measured as a function of temperature (1330' to 1600°C) and oxygen pressure (lo-' to MPa). Magnesium oxide is a mixed conductor with its ionic component directly proportional to the trivalent solute concentration. The activation energy for ionic conduction equals 202 kJ/mol and is independent of the specific trivalent solute used. This behavior results from a defect structure for a single trivalent solute for which the charge balance condition is 2[vM/] =[fM;], where [vD(/] is the Mg vacancy concentration and [I,,'] the trivalent solute concentration. At lower temperatures and high Sc concentrations, effects attributed to Sc ionvacancy association are observed. Ionic conduction results from Mg vacancy motion with a diffusion coefficient of V,,"given by 1 D~M~ =(0.38?0.15) exp [ -2202;k~/mo1Deduced values of the Mg-ion diffusion coefficient lead to the conclusion that the scatter in the published DM, data largely results from differences in impurity content.

The electronic conductivity of MgO was determined by measuring the dc conductivity and ionic transference number as a function of temperature (1200° to 1600°C), oxygen pressure (10−1 to 10−13 MPa), and trivalent solute concentration (65 to 1500 ppm) using samples doped with Al, Fe, or Sc so that only one dopant controlled the defect structure. The electronic conductivity showed ap‐ to n‐type transition with decreasing oxygen pressure, consistent with a defect structure in which the concentrations of the electronic species are negligible with respect to those of the ionic defects, so that MgO is a fully compensated semiconductor. From the temperature dependence of the electronic conductivity minima, a thermal band gap of 650±50 kj/mol (6.8±0.5 eV/electron) and electron and hole mobilities of 24 and 7 cm2/V·s were determined at 1400°C. From these results, the defect‐reaction equilibrium constants were determined.

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