  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.
