  The mean-field Landau-type theory is used to analyze the polarization
properties of epitaxial ferroelectric thin films grown on dissimilar cubic
substrates, which induce biaxial compressive stress in the film plane. The
intrinsic effect of the film surfaces on the spontaneous polarization is taken
into account via the concept of the extrapolation length. The theory
simultaneously allows for the influence of the misfit strain imposed on the
film lattice by a thick substrate. Numerical calculations are performed for
PbTiO3 and BaTiO3 films under an assumption of the polarization reduction in
surface layers. The film mean polarization is calculated as a function of film
thickness, temperature, and misfit strain. It is shown that the negative
intrinsic size effect is reduced in epitaxial films due to the in-plane
compression of the film lattice. At room temperature, strong reduction of the
mean polarization may take place only in ultrathin films (thickness ~ 1 nm).
Theoretical predictions are compared with the available experimental data on
polarization properties of BaTiO3 films grown on SrRuO3 coated SrTiO3.
