New, Lead Free, Perovskites With a Diffuse Phase Transition: NaNbO_{3} ~   Solid Solutions
  Some of (1-x)NaNbO_3-(x)ABO_3 perovskite solid solutions exhibit a dramatic diffusion of the dielectric permittivity epsilon' maximum and relaxor-type behavior when the second component concentration exceeds a threshold value x_0. The concentration phase transition to the relaxor-like phase is abrupt (of the first order kind) that is seen from the step in the dependence of the epsilon'(T) maximum temperature, T_m, on x. The precursor of this transition is a giant (up to 100 K) temperature hysteresis of epsilon'(T). Some relaxor-like properties appear even at x < x_0 in the course of cooling while disappear in the course of heating. The experimental data obtained are qualitatively described within a Landau-type phenomenological approach, assuming the relaxor-type behavior to be local stress-induced. 
I.P. Raevski and S.A. Prosandeev