  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.
