Direct and indirect studies of the electrocaloric effect were carried out in poled and depoled Na_0.5Bi_0.5TiO₃. For this purpose, polarization and electrocaloric effect temperature change measurements were made at different electric field pulses as a function of temperature. The applicability of the widely used indirect electrocaloric effect determination method, using the Maxwell relation, was critically analyzed with respect to the reliable direct measurements. Quantitative differences were observed between the results obtained by both approaches in the case of the poled Na_0.5Bi_0.5TiO₃ sample. These differences can be explained by the temperature-dependent concentration of domains oriented in the direction of the applied electric field. Whereas in depoled Na_0.5Bi_0.5TiO₃, which is characterized by the electric field dependence of polar nanoregions embedded in a nonpolar matrix, the Maxwell relation is not applicable at all, as it is indicated by the obtained results. Possible mechanisms which could be responsible for the electrocaloric effect in the relaxor state were considered. The results of this study are used to evaluate the numerous results obtained and published by other authors, using the Maxwell relation to indirectly determine the electrocaloric effect. The reason for the negative values of the electrocaloric effect, obtained in such a way and widely discussed in the literature in the case of Na_0.5Bi_0.5TiO₃, has been explained in this study.

Studies of nature of polarization in ternary solid solutions of Na_1/2Bi_1/2TiO₃-SrTiO₃-PbTiO₃ were done, using measurements of dielectric properties and polarization hysteresis loops. Various stages of relaxor behavior depending on concentration of PbTiO₃ were established and compared with other known ferroelectric relaxors.

All sequential steps of transfer from relaxor to normal ferroelectric phase transition are found in Na_1/2Bi_1/2TiO₃-SrTiO₃-PbTiO₃ if concentration of PbTiO₃ increases. The relaxor properties in Na_1/2Bi_1/2TiO₃-SrTiO₃-PbTiO₃ solid solutions are described by the power law and Vogel-Fulcher relationship. Concentration dependence of the relevant parameters is discussed. Stability of relaxor and ferroelectric phases is discussed by analyzing behaviour of critical electric fields.

The long and short range orders in 0.4Na_1/2Bi_1/2TiO₃-(0.6-x)SrTiO₃-xPbTiO₃ solid solutions were studied by x-ray diffraction and Raman spectroscopy. X-ray diffraction patterns for these compositions in the relaxor state are different for bulk and powdered samples. It was found that, in spite of different tetragonality depending on PbTiO₃ content, the Raman spectra suggest similarity of local structure for all studied compositions. Both methods indicate a disappearance of tetragonal phase upon increasing temperature.

Phase coexistence region is studied by x-ray diffraction for Na_1/2Bi_1/2TiO₃-BaTiO₃ solid solutions in the tetragonal phase side from the morphotropic phase boundary. The first order ferroelectric phase transition, determined from a jump in the temperature dependence of dielectric permittivity, is located inside the coexistence region of cubic and tetragonal phases and is below the temperature, where tetragonality disappears. At low BaTiO₃ concentrations phase transition into ferroelectric state at cooling is slowly approached in time and is smeared over large temperature range. Rietveld method, applied for more precise evaluation of phase content, reveals large local deformations inside the ferroelectric phase.