A study of the production and annealing of defect cluster centers in MgO single crystals has been undertaken. A comparison of the annealing of single negative-ion vacancies in neutron-irradiated, electron-irradiated, and Mg-additively colored samples is made. The results indicate that isolated negative-ion vacancies are not mobile below 900°C, and the annealing of these defects at lower temperatures in irradiated crystals is due to interstitial migration. The broad absorption bands at 352, 573, and 975 nm and the zero-phonon lines observed in neutron-irradiated crystals, previously proposed as due to $F$-aggregate centers, are not observed in electron-irradiated and additively colored samples even after annealing. Therefore, there is some uncertainity whether these lines and/or bands are due to $F$-aggregate centers.

• Received 15 November 1968

DOI:https://doi.org/10.1103/PhysRev.182.960