PHASE-TRANSITIONS IN KTAO3LI+, NB-5+, NA+ AND THEIR INVESTIGATIONS BY RADIOSPECTROSCOPY METHODS

This review contains up-to-date information about experimental and theoretical investigations of phase transitions and specific features of properties of mixed crystals involving the incipient ferroelectric KTaO3 with the off-center Li+, Nb5+, Na+ impurities. Main attention is paid to the results of the study of the mixed systems by radiospectroscopy methods (ESR and NMR). Theoretical criteria and experimental evidence of ferroelectric and ferroelastic phase transitions as well as of dipole and quadrupolar glass states induced by the off-center impurities are discussed. It is shown that at high enough concentrations of the off-center impurities phases with long-range order appear, but at low concentrations dipole glass states occur. At intermediate concentrations of the off-center impurities new phases appear in which the coexistence of long-range properties and those of dipole glasses is possible. These phases could be named ferroelectric or ferroelastic glasses to stress their similarity with the ferroglass phases which were discovered earlier in mixed magnetic systems. The usefulness of radiospectroscopy methods for the study of phase transitions induced by off-center impurities in mixed systems, which can be considered as model disordered systems, is underlined. In conclusion, some questions on the physics of phase transitions in disordered systems which should be solved in the future are discussed.