ИСТИНА

Though Pb-based perovskite materials are widely used in different branches of industry, environmental problems caused by toxic elements stimulated intensive studies of lead-free oxide materials in order to replace toxic Pb-based ones. Ferroelectrics with perovskite structure on the base of BaTiO3 (BT), (K0.5Na0.5)NbO3 (KNN) and relaxor (Na0.5Bi0.5)TiO3 (NBT) are being among the most popular objects studied. They are promising for development of new lead-free capacitor, piezoelectric, electrocaloric and other materials. We studied influence of cation modification of compositions on structure, microstructure, and functional properties of compositions close to the Morphotropic Phase Boundary in the KNN-BT and NBT-BT systems additionally modified with donor (La3+), and acceptor (Li+, Ag+, K+, Mn3+, Ni3+, and Fe3+) cations and with overstoichiometric additives ZnO, SiO2, CuO. Ceramic samples were prepared using the two-step solid-state reaction and sol-gel methods. Their properties were characterized and studied by the X-ray Diffraction, Scanning Electron Microscopy, Second Harmonic Generation, Dielectric Spectroscopy, and Piezorespone Force Microscopy methods. The observed unit cell parameters and size of grains changed in modified compositions correlating with ionic radii of substituting cations. Ferroelectric phase transitions were observed in the range of ~ 400 – 700 K. Phase transitions in NBT-based ceramics revealed typical relaxor behavior related to the presence of polar nanoregions in a nonpolar matrix. At temperatures > 700 K effects of dielectric relaxation caused by formation of oxygen vacancies due to deficiency in the A-sites and/or presence of cations with mixed valency in the B-sites of perovskite lattice were observed in some acceptor doped compositions. Local PFM hysteresis loops observed for KNN- and NBT-based samples indicated to ferroelectric polarization switching at nanoscale. In KNN-based samples high effective d33 piezoelectric coefficient values correlated with dielectric permittivity at the room temperature were observed. The results obtained confirmed prospects of new lead-free materials development on the base of the KNN- and NBT-perovskites.