ИСТИНА

Lead-free perovskite oxide materials are among the most intensively studied in order to replace widely used toxic lead containing ones last ten years. We studied influence effects of cation substitutions in A- and B-sites on stoichiometry, structure parameters, microstructure, dielectric, ferroelectric, and piezoelectric properties of ceramics based on sodium-bismuth titanate (Na0.5Bi0.5)TiO3 (NBT) and sodium-potassium niobate (K0.5Na0.5)NbO3 (KNN) perovskite ceramics. Ceramic samples in the systems (Na0.5Bi0.5)TiO3 - BaTiO3 (NBT-BT) and (K0.5Na0.5)NbO3 – BaTiO3 (KNN-BT) with compositions close to the Morphotropic PhaseBoundaries (MPB) were prepared by the two-step solid-state reaction method at hightemperatures up to 1500 K. The samples were additionally modified by Li+, Ag+, K+, Mn3+, Ni3+, and Fe3+ cations. Over stoihiometric additives KCl, LiF, Bi2O3, CuO and ZnO were additionally used to improve sintering of ceramics. The samples were characterized using complex of physico-chemical methods using the X-ray Diffraction, Scanning Electron Microscopy (SEM), Second Harmonic Generation (SHG), Dielectric Spectroscopy, and Piezorespone Force Microscopy (PFM) methods. Changes in the unit cell volume of the KNN- and NBT-based ceramics were observed correlating with radii of A- and B-cation substitutions. In the dielectric permittivity versus temperature curves anomalies related to ferroelectric phase transitions marked by steps at ~ 450 K (NBT) and by peaks at ~ 550 K (NBT) and at ~ 700 K (KNN) were revealed of the compositions studied. Phase transitions near 450 K revealed typical relaxor-type behavior attributed to the presence of polar nanoregions in a nonpolar matrix. Non monotonous changes in the spontaneous polarization value were checked in modified ceramics using the SHG method. Local PFM hysteresis loops for the KNN- and NBT-based ceramics were observed indicating ferroelectric polarization switching at nanoscale. At the room temperature, non monotonous changes of the dielectric parameters εrt and tanδrt and maximum effective d33 values were observed in modified BNT- and KNN-based ceramics, thus confirming their prospects for new lead-free materials development. In addition, a decrease in the magnitude of the coercive voltage in the modified samples, which is important for measurements of piezoand electrocaloric properties of processing samples was observe