Electrical Conductivity in Magnesium-Doped Al_2 O_3 Crystal at Moderate   Temperatures
  AC and DC electrical measurements between 273 and 800 K were used to characterize the electrical conductivity of Al_2 O_3:Mg single crystals containing [Mg]^{0} center. At low fields contacts are blocking. At high fields, electrical current flows steadily through the sample and the I-V characteristic corresponds to a directly biased barrier whit a series resistance (bulk resistance). AC measurements yield values for the junction capacitance as well as for the sample resistance, and provide perfectly reproducible conductivity values. The conductivity varies linearly whit the [Mg]^{0} concentration and a thermal activation energy of 0.68 eV was obtained, which agrees very well with the activation energy previously reported for motion of free holes. 
M. Tard\'io, R. Ram\'irez, R. Gonz\'alez, Y. Chen and M.R. Kokta