Phenomenological analysis of phase transitions in epitaxial perovskite ferroelectric thin films

A phenomenological Landau–Ginzburg–Devonshire theory is employed to investigate the effect of biaxial misfit strains and in-plane shear strain on thermodynamic equilibrium states and dielectric properties of single domain KNbO3, BaTiO3, and PbTiO3 thin films. For KNbO3 thin films grown on cubic substrates, the constructed isotropic misfit strain–temperature phase diagram is in qualitative agreement with the result from first-principle calculations. For BaTiO3 and PbTiO3, misfit strain–temperature phase diagrams at constant in-plane shear strains are constructed. In-plane shear strain is shown to restrain formation of a phase (polarization along [1 0 0]). The anisotropic misfit strain–temperature and misfit strain–misfit strain phase diagrams are also developed for KNbO3 thin films grown on orthorhombic substrates. The anisotropic biaxial misfit strains lead to appearance of orthorhombic phases (a1, a2) and monoclinic phases (a1c, a2c).