A new, lead free, family of perovskites with a diffuse phase transition: NaNbO3-based solid solutions

(1−x)NaNbO₃-(x)ABO₃ perovskite solid solutions belonging to group II according to the Krainik classification [Izv. Akad. Nauk SSSR, Ser. Phys. 28 (1964) 643] exhibit a dramatic diffusion of the dielectric permittivity ε′ maximum and relaxor-type behavior when the second component concentration exceeds a threshold value x₀. The concentration phase transition to this relaxor-like phase is abrupt (of the first order kind) that is seen from the step in the dependence of the ε′(T) maximum temperature, T_m, on x. Some relaxor-like properties appear even at x<x₀ in the course of cooling while disappear during the course of heating. Due to this fact and because of coupling of the antiferroelectric (AFE) and ferroelectric (FE) order parameters a giant (up to 100 K) temperature hysteresis of ε′(T) arises at AFE-AFE first order phase transition. The T_m values of all the known NaNbO₃-ABO₃ relaxor-type compositions are well below the room temperature and the dielectric permittivity maximal values, ε′_m, are much lower than in the case of Pb-containing relaxors. However both T_m and ε′_m values can be increased substantially by Li or K-doping leading to the formation of NaNbO₃-ABO₃-LiNbO₃ (KNbO₃) solid solutions.