Effect of external stress on ferroelectricity in epitaxial thin films

A nonlinear thermodynamic theory is used to describe the influence of an external mechanical loading on the ferroelectric, dielectric, and piezoelectric properties of epitaxial thin films grown on dissimilar cubic substrates. The calculations are performed for single-domain perovskite films in the approximation of a homogeneous loading of the film upper surface. The ``misfit strain-stress'' and ``stress-temperature'' phase diagrams are developed for epitaxial PbTiO₃ and BaTiO₃ films. It is shown that the loading may lead to drastic changes of the film polarization state. The most remarkable theoretical prediction is the stress-induced ferroelectric to paraelectric phase transition, which may take place at room temperature in films grown on ``compressive'' substrates that provide large negative misfit strains in the epitaxial system. The small-signal dielectric and piezoelectric constants of single-domain PbTiO₃ and BaTiO₃ films are also calculated and found to be very sensitive to the external stress under certain misfit strain-temperature conditions. The theory thus predicts that the mechanical loading of ferroelectric films can be employed for the fine tuning of their physical properties. The results of calculations may be also useful for the interpretation of experimental data obtained via scanning force microscopy and the indentation of ferroelectric films.