Dielectric and Polarization Properties of Na_0.5Bi_0.5TiO₃-BaTiO₃ Solid Solutions with Na and K Niobates

The role of substitution in A- and B-sites of perovskite ABO₃ structure in phase transitions is evaluated, comparing (1-y)(0.939Na_0.5Bi_0.5TiO₃-0.061BaTiO₃)-yK_xNa_1-xNbO₃ solid solutions with x = 0.0, x = 0.5 and x = 1.0. The influence of deviation from a stoichiometric Na/Bi ratio is also studied. The influence of K_0.5Na_0.5NbO₃, NaNbO₃ or KNbO₃ reduces mainly to suppressing of ferroelectric properties, like in other perovskite ferroelectrics where Ti is replaced by Nb, and development of the relaxor state. Besides the frequency-dependent dielectric permittivity, the relaxor state in the studied compositions is characterized also by a temperature dependence of critical electric fields, corresponding to phase transitions between relaxor and ferroelectric states. The influence of Nb in the above mentioned compositions is compared with the one of compositions far from the morphotropic phase boundary by a study of solid solutions (1-y)(0.80Na_0.5Bi_0.5TiO₃-0.20BaTiO₃)-yK_xNa_1-xNbO₃.