Optical transitions in magnesium aluminate spinel crystals of different compositions exposed to irradiation

Optical absorption of magnesium aluminate spinel single crystals of different compositions MgO · nAl2O3 (n=1.0 and 2.5) were investigated after irradiation with neutrons, X-rays and the UV-light to elucidate existence of specific absorption bands related to lattice defects. The radiation induced absorption envelope is shifted to higher energy and is larger in intensity in non-stoichiometric spinel in comparison with stoichiometric one at the same irradiation conditions. Deconvolution of difference optical spectra of irradiated crystals shows the shift of the F-type centers from 4.75 and 5.3 eV in stoichiometric crystals to 5.08 and 5.63 eV in MgO · 2.5Al2O3 for F+- and F-centers, respectively. The absorption bands at 3.1 and 3.78 eV are present in both stoichiometric and non-stoichiometric crystals and are identified as V-type centers. By changing the irradiation time and using isochronal annealing it was revealed that additional bands appear in optical absorption spectra at 4.15 eV for MgO · 1.0Al2O3 crystals and at 4.46 eV for MgO · 2.5Al2O3 crystals. From the shift of the band energy position and the high intensity of this band in non-stoichiometric spinel crystals, it could be identified with electronic centers related to lattice defects