Radiation-induced states in crystals with ionic-covalent bonds

Based on a review of data in the literature and on the results of a systematic investigation of the effect of various kinds of radiation on a number of crystals with ionic-covalent bonds (ICB crystals), the author describes mechanisms and regularities associated with the creation of radiation-induced structural states. A classification of these states is given, along with their characteristics and models of their behavior. The author proposes a criterion, tested on specially selected compounds, for high-temperature structural instability leading to the realization of a structural state, which in terms of the symmetry and parameters of the unit cell is close to the high-temperature modification of the unirradiated crystal. As a basis for choosing this special class of compounds thermodynamic parameters are adopted which determine the character of their thermal phase transitions. It is shown that, apart from amorphization and decomposition of the original compound, this is the only possible overall change in the structure of ICB crystals which extends throughout their entire volume. The author concludes that for the majority of ICB crystals in which such changes in crystal structure are observed, a decisive role is played by isolated point defects caused by radiation. She also investigates the influence of impurities, growth conditions and conditions of irradiation on the radiation hardness of a structure. In conclusion, she points out factors which determine the structural states of an irradiated ICB crystal, and analyzes the causes of discrepancies in the experimental data of various authors regarding the eifect of radiation on the structure of the same material.