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

Abstract

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 D2,3 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 D2,3 defects is suggested.