  In the framework of Landau--Ginzburg--Devonshire (LGD) phenomenological
theory, the influence of misfit strains, surface energy, and finite-size
effects on phase diagrams, polar properties, and hysteresis loops has been
calculated for multilayered thin films of the type ferroelectric/virtual
ferroelectric. The influence of elastic deformations that arise at the
interface thin film--substrate owing to a mismatch between the lattice
constants in the film and the substrate on the phase diagrams of multilayered
thin films virtual ferroelectric SrTiO$_{3}$/ferroelectric BaTiO$_{3}$ has been
studied for the first time. In contrast to bulk BaTiO$_{3}$, in which only four
phases (cubic, tetragonal, orthorhombic, and rhombohedral) can exist, it turned
out that six thermodynamically stable BaTiO$_{3}$ phases (paraelectric phase
and tetragonal (FEc), two monoclinic (FEaac and FEac), and two orthorhombic
(FEa and FEaa) ferroelectric phases) can exist in multilayered
SrTiO$_{3}$/BaTiO$_{3}$ films. The main polar properties of hysteresis loops
(shape, coercive field, and spontaneous polarization) in thin multilayered
SrTiO$_{3}$/BaTiO$_{3}$ films are calculated. It is shown that the system
demonstrates a strong dependence of its polar properties on the thickness of
SrTiO$_{3}$ and BaTiO$_{3}$ layers, as well as on the elastic misfit strains,
with SrTiO$_{3}$ playing the role of dielectric layer: the thicker the layer,
the stronger is the depolarization field, which, in its turn, reduces the
spontaneous polarization in the BaTiO$_{3}$ film.
