  The Landau theory of phase transitions of Ba0.8Sr0.2TiO3 thin film under
external electric field applied in the planar geometry is developed. The
interfacial van-der-Waals field Ez=1.1x10^8 V/m oriented normal to the
film-substrate interface was introduced in to the model calculation to explain
experimentally observed behavior of the polarization as a function of planar
electric field. The Ez - misfit strain phase diagram of the film is constructed
and discussed.
