Detection of hidden oxygen interstitials in neutron-irradiated corundum crystals

An α-Al₂O₃ (corundum) possesses unique optical, electrical and mechanical properties, demonstrates high tolerance to heavy irradiation and, in particular, is in a short list of candidates for optical/diagnostics windows in advanced fission and forthcoming fusion (DEMO) reactors. However, material functionality is strongly affected by structural defects induced by radiation of different types. Optical and magnetic characteristics of Frenkel defects (interstitial-vacancy pairs) as well as the processes of their thermal annealing (radiation damage recovery) have been investigated by means of optical absorption and the EPR methods in α-Al₂O₃ single crystals exposed to fast neutrons with fluence of 6.9 × 10¹⁸ n/cm². For the first time in metal oxides, a single oxygen interstitial, which is not adjacent to any other imperfection, has been detected. The analysis of the EPR spectra/parameters testifies that this interstitial forms with a regular oxygen ion a superoxide ion O₂⁻, stabilized by a trapped hole. The interstitial becomes mobile above 500 K and recombines with a complementary oxygen vacancy that is a part of electronic F⁺ or F centers. The thermal annealing kinetics of the F and F⁺ centers has been theoretically analyzed in terms of interrelated diffusion-controlled recombination reactions of radiation defects. The analysis indicates that both, negatively charged and neutral oxygen interstitials (relevant absorption bands at 5.6 and 6.5 eV, respectively) co-exist in similar concentrations.