In-plane polarization states and their instabilities in polydomain epitaxial ferroelectric thin films

A Landau–Ginsburg–Devonshire-type thermodynamic theory is used to describe epitaxial ferroelectric films with dense laminar domain structures, where spontaneous polarizations in both domains are oriented in the film plane. For PbTiO3 films, it is found that, at positive misfit strains, the domain patterning transforms the orthorhombic aa phase, which is stable in single-domain films, into a pseudotetragonal a1/a2/a1/a2 polydomain state. In BaTiO3 films, the latter is further replaced at low temperatures by a polydomain aa1/aa2/aa1/aa2 state. With decreasing misfit strain, both a1/a2/a1/a2 and aa1/aa2/aa1/aa2 patterns become unstable with respect to the appearance of polarization component orthogonal to film surfaces. This results in the formation of polydomain states in BaTiO3 films, which do not exist in bulk crystals. The stability ranges of various states in the misfit strain-temperature phase diagram are determined.