Polarization Instability in Polydomain Ferroelectric Epitaxial Thin Films and the Formation of Heterophase Structures

A thermodynamic theory is developed for dense laminar domain structures in epitaxial ferrolectric films. It is found that, at some critical misfit strain between the film and substrate, the 90 degrees c/a/c/a domain structure becomes unstable with respect to the appearance of the polarization component parallel to domain walls, which results in the formation of a heterophase structure. For PbTiO_3 and BaTiO_3 films, the stability ranges of polydomain and heterophase states are determined using misfit strain - temperature diagrams. Dielectric anomalies accompanying misfit-strain-driven structural transformations are described.