About complexity of the 2.16-eV absorption band in MgO crystals irradiated with swift Xe ions

The precise study of the accumulation and subsequent thermal annealing of the defects responsible for the complex absorption band around 2.16 eV, being under discussion in the literature for a long time, has been performed in highly pure MgO single crystals exposed to 0.23-GeV ¹³²Xe ions with a fluence of Φ = 5 × 10¹¹ − 3.3 × 10¹⁴ ions/cm². Three Gaussian components with the maxima at 2.16, 2.02 and 2.40 eV have been considered as a measure of so-called D₁, D₂ and D₃ defects. Similar to the F and F⁺ centers, the concentration of these defects increases at high fluences without saturation marks, thus confirming their radiation-induced nature (involvement of novel Frenkel defects). The accumulation of D₁ and the first stage of D-type defect annealing up to 700 K occurs similarly to single F-type centers, while the following increasing stage for D_2,3 starts above 700 K; their concentration reaches the maximum at 900 K (practically plateau is seen in this temperature region for D₁) and complete annealing of the D-type defects proceeds at 1100 K. Behavior of the D₁ defects (accumulation rate, annealing kinetic at high temperatures) clearly differs from that for other D-type defects (especially D₂). In our opinion, the D₁ defects are high-order aggregates of anion vacancies (and not the anion vacancy dimers), while the involvement of additional structural defects into the D_2,3 defects is suggested.