EPR-active dimer centers with S=1 in α-Al2O3 single crystals irradiated by fast neutrons

Corundum (α-Al₂O₃) possesses a number of unique properties, including high tolerance to harsh radiation environment. Material functionality is strongly affected by radiation-induced structural defects, single interstitial-vacancy Frenkel pairs and their aggregates. Three novel paramagnetic two-electron defects with total spin S = 1 have been revealed in fast neutron irradiated α-Al₂O₃ single crystals by means of the EPR method. Analysis of the EPR characteristics of these defects allows to conclude that the main novel D₁ defect (initial concentration after irradiation about 2.8 × 10¹⁶ cm⁻³) is oriented along one of three equivalent b crystal axes, that connect two adjacent anion sites of oxygen triangles on the base corundum plane. The D₁ defect is ascribed to the F₂²⁺ center (two adjacent single-charged oxygen vacancies), while the D₂ and D₃ have the same dimer structure with an additional defect in their vicinity. The suggested defect structure is supported by pulse annealing of the EPR signal of relevant defects: defect concentration dependence contains a rising stage at 500–630 K, while complete thermal destruction due to the recombination with complementary oxygen interstitials occurs by 720–750 K. Such behavior qualitatively coincides with earlier studied thermal annealing of the F₂²⁺ center optical absorption in neutron-irradiated corundum.