A model for quantitative description of the relaxer ferroelectrics pro
perties in the random field theory framework is proposed. An order par
ameter, phase transition temperature, critical concentration of random
field sources were calculated with the help of the fields distributio
n function allowing for nonlinear and spatial correlation effects. Ess
ential differences were obtained in comparison with linear approach, n
amely the phase transition order change, appearance of maximum in tran
sition temperature T-c concentrational dependence. The comparison betw
een calculated and experimental data has been made for the model ferro
electric relaxer PLZT x/65/35. Fairly good agreement between calculate
d and measured T-c(x) and critical concentration of lanthanum have bee
n obtained.