Labile Ferroelastic Nanodomains in Bilayered Ferroelectric Thin Films

Heterostructured thin films differing either in their structure, composition, or in both have shown novel magnetic, superconducting, ferroelectric, or electromechanical responses. In the case of ferroelectrics, multilayers or superlattices have displayed enhanced polarization, high dielectric permittivity and in some instances, entirely new structural phases. These observations have been accounted for on the basis of electric-field-induced coupling, epitaxial strain, and specific polar interactions between the interfacial layers. As most ferroelectrics have a strong non-negligible ferroelastic self-strain associated with their phase transformation, an aspect that should be equally fascinating is that of the elastic interactions between such multilayers. The interactions lead to the formation of ferroelastic domains, arranged in the form of periodalternating lamellae in order to relax the excess elastic energy. When these lamellae are of the same phase but of different crystallographic orientations, they are generally referred to as ‘‘twins’’. The ferroelastic-domain-wall (extrinsic) contribution to the dielectric, piezoelectric, and elastic properties of ferroelectrics is indeed quite significant in the case of bulk ceramics and single-crystals, and this can be several times larger than the intrinsic lattice piezoresponse. In the case of ferroelectric thin films, the issue of movement of ferroelastic domains is not without debate. Earlier experimental studies proposed that the ferroelastic ‘‘herringbone’’ pattern in thin films demonstrated very limited (if any) ability to move under external stress or electric field. This has been contradicted in more recent experiments on polycrystalline films. Nevertheless, gross quantitative enhancement has been reported only under special conditions, such as films patterned into