Total and partial ionic and electronic c-axis dc conductivities were measured for singlecrystal A1203.8 ppm Mg as a function of oxygen pressure and temperature. The sample is unsaturated at T & 1360'C but is saturated in equilibrium with a spinel precipitate phase at lower temperatures.The results are analyzed to give ionic mobility, thermodynamic parameters of oxidation reduction of A1~03, and the solubility of MgO as a function of temperature. Oxidized samples are grayish purple due to the presence of a broad absorption band with maximum at 480 nm. Discoloration can occur by reduction (at high temperature) or by precipitation (at T & 1000 C); both proceed from the surface, involving migration of oxygen out of or of Al into the crystal. An explanation for the rapid diffusion regulating precipitation belo~1000'C is proposed. Recoloration at low temperature proceeds without a front and possibly involves oxygen diffusing into the sample along dislocations or subgrain boundaries. Hole-release studies at low temperature indicate that the hole binding energy in the defect MgAb E, &0.68 eV. The combined results of the present work suggest that free holes move as large, rather than small, polarons and that E, =0.68 eV.

Partial ionic and electronic dc conductivities and compressional creep rate were measured for hot-pressed polycrystalline AlrOs made from Al-isopropoxide (A1203(II)). The undoped material was found to contain 1.5X 1OI8 cm-J fixed valency acceptors (Mg). Properties of undoped material and material doped with Fe or Ti were investigated as a function of grain size, dopant concentration, oxygen pressure, and temperature. No fast ionic conduction along grain boundaries is found in either acceptor-or donor-dominated material. Absolute values of self-diffusion coefficients calculated from conductivity and creep indicate that both effects are limited by migration of Al, involving VAl"' in donor-, Al, ' . ' in acceptor-dominated material.In creep, oxygen is transported along grain boundaries in a neutral form (Oix). The po2 dependence of u, and oh are as expected on the basis of a defect model. That of creep is weaker for reasons that are not entirely clear. An ionic conductivity with low activation energy, observed a t low temperature, is attributed to the presence of AI-silicate second phase.

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