  The paper contains an application of lattice model to ferroelectric solid
solutions. Short-range parts of interatomic potentials are taken into account
by means the lattice structure introduction. Long-range parts are considered in
effective field approximation. The equations for the mean values of the
components contributions into polarization are derived. The general equation
for the Curie temperature dependence on solution composition is obtained.
Theoretical calculation of Curie temperature of the system
Pb$_{1-x}$Sn$_x$TiO$_3$ fulfilled.
