Defect Centers in Gamma-Irradiated Single-Crystal Al2O3

The γ-irradiation-induced optical absorption spectra of annealed Al₂O₃ single crystals were analyzed. The samples were beat-treated in O₂ or vacuum from 1600° to 1800°C. Four absorption bands are reported with average peak positions at 3.4,4.8,5.5, and 6.6 eV for samples annealed in O₂ at ≥ 1750°C. Vacuum anneals up to 1800°C and O₂ anneals at < 1700°C failed to generate observable bands in the energy range studied. There is also evidence that one or more bands exist at >6.6 eV. The radiation-induced change in the absorption spectra is reported and the defect centers responsible for the measured optical absorption bands are discussed with respect to the aliovalent impurities in the host lattice. Special attention is given to the role of the Fe³⁺ impurity ion.