Appearance of ferroelectricity in thin films of incipient ferroelectric

The consideration of a size-induced ferroelectric-paraelectric phase transition for conventional and incipient ferroelectric thin films with perovskite structure was carried out in the phenomenological theory framework of Ginsburg -Landau -Devonshire. The more general form of surface free energy expansion that includes intrinsic surface stress tensor, the surface piezoelectric effect and electrostriction as well as quadratic and quartic powers of surface polarization has been considered. The analytical expressions for the thickness dependence of the transition temperature was derived both for the conventional and incipient ferroelectrics. It was shown that although there is no ferroelectricity in the bulk incipient ferroelectrics it appears in thin film for the negative extrapolation length that is realized, e.g., at positive surface stress coefficients and negative or zero misfit strain. In our consideration we came to the conclusion about thickness-induced ferroelectricity in incipient ferroelectrics KTaO3 at room temperature for films that were sufficiently thin. The similar surface effects can strongly influence the phase transitions in the thin films of another incipient ferroelectric, SrTiO3.