Thermal annealing and transformation of dimer F centers in neutron-irradiated Al2O3 single crystals

The precise study of the thermal annealing of the F₂-type dimer defects, being under discussion in the literature for a long time and responsible for the number of absorption bands below 4.5 eV, has been performed in corundum single crystals irradiated by fast neutrons with a fluence of 6.9 × 10¹⁸ n/cm². The Gaussian components of the radiation-induced optical absorption with the maxima at 4.08, 3.45 and 2.75 eV have been considered as a measure of the F₂, $F_2^{+}$and $F_2^{2+}$centers, respectively. In contrast to the F and F⁺ centers, the concentration of which continuously decreases at the sample heating up to 1100 K, the concentration of dimer defects with different charge states passes the increasing stages above 500 K starting from the $F_2^{2+}$centers. The tentative mechanisms of such rise of the $F_2^{2+}$centers as well as of the subsequent transformation/rise of dimer centers, $F_2^{2+}\to F_2^{+}\to F_2$at 650-800 K are considered. The possible sources of carriers needed for the recharging of dimer centers are also analysed on the basis of thermally stimulated luminescence measurements up to ~850 K.