A modified thermodynamic potential based on the eight-order Landau-Devonshire polynomial is derived for ferroelectric thin films grown on dissimilar substrates that induce anisotropic tensile/compressive strains and the shear strain in the film plane. The effect of the shear strain on the ferroelectric phase transition occurring in considered films is analyzed theoretically. It is shown that the application of the shear strain suppresses the formation of ferroelectric c phase and raises the temperature of aa* phase formation. The directional dependence of in-plane dielectric permittivity of a ferroelectric film is also calculated and compared with the dependence observed in (Ba0.60Sr0.40)TiO3 films deposited on NdGaO3. (c) 2008 American Institute of Physics.
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