Optical and Thermal Stimulation of Reactions in α‐A12O3 Containing V3+, V4+, Co3+, and Co2+

Optically or thermally stimulated reactions among the species V ,V ,Co , and Co in single crystals of α‐A1 ,O , and the location of the ground state energy levels of these species with respect to the valence or conduction band are studied. A previously unidentified anisotropic absorption band, peaking near 310 nm, is identified as a charge transfer transition of V .Excitation in this band results, with appreciable probability, in the formation of free positive holes and the consequent conversion of V to V and of Co to Co ;this probability is found to increase as the excitation wavelength decreases. At temperatures above 400 oC an equilibrium is readily established among these four ions and the temperature dependence of this equilibrium is studied. It is shown that this equilibrium is arrived at by a mechanism involving the thermal formation of holes at Co and V .