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 intr
oduced 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.
