  Phase diagram of solid solutions of antiferroelectric and ferroelectric
materials like PbZr$_{1-x}$Ti$_x$O$_3$ (PZT), Rb$_x$(NH$_4$)$_{1-x}$H$_2$PO$%
_4$ (RADP) is calculated at the region $x\leq 0.1$. Antiferroelectric and
ferroelectric order parameters (which correspond respectively to the difference
and sum of two sublattices ions shifts) were introduced for antiferroelectric
host lattice. Small admixture of ferroelectric component was considered as a
random electric field source. The influence of this field on both
aforementioned order parameters was calculated. Self consistent averaging with
calculated random field distribution function made it possible to obtain the
system of equations for order parameters of mixed system. The antiferroelectric
$T_a$ and ferroelectric $T_c$ phase transition temperature concentrational
dependence was expressed through the transition temperatures of the composition
and members. The obtained $T_a(x)$ dependence was shown to fit pretty good the
observed values for PZT and RADP. The application of the proposed theoretical
approach to other types of mixed systems is discussed.
