Thermal stability of radiation-induced optical centers in non-stoichiometric spinel crystals

The successive anneals of the neutron-irradiated non-stoichiometric spinel crystals MgO . 2.2Al2O3 leads to incremental change of optical spectra that demonstrates two main bands whose intensity and spectral position depends on annealing temperature. While temperature increases from 450 to 750 K, one of the bands shifts from photon energy of 4.2-5.1 eV. Another one shifts in the opposite direction from 6.4-5.6 eV. This effect can be attributed to coagulation of radiation induced defects near cation vacancies and change of the energy levels and transitions in F+- and F-centers in neutron-irradiated crystals. The final position of these two bands 5.1 and 5.6 eV corresponds to transitions in F+- and F-centers in non-stoichiometric spinel, respectively. The investigation of optical centers induced at subsequent UV-illumination of neutron-irradiated crystals annealed to temperature of 750 K, and comparison with as-grown crystals, shows the existence of residual concentration mainly of the antisite defects and partially of the anionic vacancies in the neutron irradiated and annealed crystals. The existence of two temperature stages where optical centers at antisite defects are effectively destroyed may indicate the presence of spatially correlated and isolated antisite defects in irradiated spinel