  DC and AC electrical measurements were performed to investigate the
electrical conductivity of alpha-Al2O3:Mg samples with different concentrations
of [Mg]^0 centers .The concentration of [Mg]^0 centers was monitored by the
opticalabsorption peak at 2.56 eV. At low electrical fields, DC measurements
reveal blocking contacts. Steady electroluminescence is emitted at the negative
electrodeindicating that the majority of carriers are holes. Low voltage AC
measurements show that the equivalent circuit for the sample is the bulk
resistance in series with the junction capacitance connected in parallel with a
capacitance, which represents the dielectric constant of the sample. The values
determined for the bulk resistance in both DC and AC experiments are in good
agreement. The electrical conductivity of Al2O3:Mg crystals increases linearly
with the concentration of [Mg]^0 centers, regardless of the amount of other
impurities also present in the crystals, and is four times higher in the
$c_{\perpendicular}$ than in the $c_{\parallel}$ direction. The conductivity is
thermally activated with an activation energy of 0.68 eV, which is independent
of: 1) the [Mg]^0 content, 2) the crystallographic orientation, and 3) the
concentration of other impurities. These results favor the {\it
small-polaron-motion} mechanism.
