Influence of Neutron Irradiation on the Characteristics of Phase Transitions in Multifunctional Materials with a Perovskite Structure (A Review)

Exposure to neutron radiation is an efficient way to enter dosed amounts of defects into the crystal and real structures of advanced inorganic materials. Thus-gained experimental data are of importance for both fundamental radiation materials science and for finding ways of usefully modifying the effect of radiation on the properties of practically important multifunctional materials with the perovskite structure. The review considers the specificity of the effect of exposure to neutron radiation on the parameters of crystal and real structures and on the multifunctional (ferroelectric, superconducting, nonlinear optical, magnetic, and other) properties of crystals of the perovskite family that experience various structural phase transitions. The review dwells on the effect of radiation defects on variations of the local and macroscopic properties of crystals, reflected in the anomalies of these properties in phase-transition regions. Ferroelectric perovskite crystals are shown to be particularly susceptible to defects. The main types of radiation-induced structural states are considered, including their formation mechanisms and patterns. Correlation relations between the structural alteration upon radiation exposure and the evolution of phase-transformation temperatures are considered. It is shown that variable irradiation parameters (dose, radiation annealing, impurity content, and concentration of a certain type of initial defects) offer a means to control the kinetics of radiation rearrangement and, ultimately, the radiation resistance of the structure. In addition, taking these factors into account, one can provide conditions for the generation of almost any metastable structural state, close to those formed in intact (non-irradiated) crystals at the pre-transition stage, that would be stable at room temperature. The study of effects of neutron irradiation by precision X-ray structural analysis shows that the structural states that occur under radiation exposure are stable over a wide temperature range, including room temperature, and the stability of metastable states, as a rule, increases with the radiation fluence. These radiation-induced states are actually new structural states, intermediate between low-temperature and high-temperature polymorphs of perovskite crystals. The analysis of literature data implies that the radiation-induced structural rearrangement, amorphization, and decomposition of the initial compound are in many cases competing processes, the activity of which depends on the neutron irradiation fluence and temperature. A decrease in exposure temperature often slows down the decomposition and suppresses the attendant deformation of the crystal, which is essential for the implementation and identification of structural alterations in irradiated crystals.