STUDY OF THE FORMATION OF RADIATION-STIMULATED IMPURITY DEFECTS IN CaF₂CRYSTALS ACTIVATED BY TRIVALENT RARE-EARTH IONS

Ionizing radiation gives rise to impurity defects in activated crystals due to the transition of impurity ions from the trivalent to divalent state. An approach is proposed for studying the influence of the energy position of R³⁺ ions in the energy-band structure of СaF₂ crystals on the degree of stability of rare-earth ions in the divalent state as a result of the transition of 4fⁿ electronic states of R³⁺ → R²⁺ ions under ionizing irradiation. The processes of direct and reverse photochromism occurring on impurity defects, which are related, respectively, to the coloring of activated crystals under γ irradiation and their bleaching under UV irradiation, have been studied. A mechanism of photochromic transformation taking into account the participation of radiation-induced color centers (CCs) in this process is proposed. The valence transition R³⁺ → R²⁺ is considered in terms of photooxidation reaction. The possibilities and conditions of the ion transformation reaction in dependence of the type of ionizing radiation acting on R³⁺-containing crystals are analyzed based on calculations of the change in the Gibbs energy.