Ferroelastic switching in ferroelectric/multiferroic oxides plays a crucial role in determining their dielectric, piezoelectric, and magnetoelectric properties. In thin films of these materials, however, substrate clamping is generally thought to limit the electric-field- or mechanical-force-driven responses to the local scale. Here, we report mechanical-force-induced large-area, non-local, collective ferroelastic domain switching in PbTiO 3 epitaxial thin films by tuning the misfit-strain to be near a phase boundary wherein c/a and a 1 / a 2 nanodomains coexist. Phenomenological models suggest that the collective, c - a - c - a ferroelastic switching arises from the small potential barrier between the degenerate domain structures, and the large anisotropy of a and c domains, which collectively generates much larger response and large-area domain propagation. Large-area, non-local response under small stimuli, unlike traditional local response to external field, provides an opportunity of unique response to local stimuli, which has potential for use in high-sensitivity pressure sensors and switches.

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One effective way to extend the functional degree of freedom for semiconductors is to introduce structural defects (e.g., surface or interface) and chemical defects as they commonly exist and modify the properties of the entity. Here, the optoelectronic properties of bismuth ferrite thin films and their tuning by the interface between the film and the strontium titanate substrate are reported. The defects that have been demonstrated, especially oxygen vacancies, are of paramount importance in the photoelectric properties of a film‐substrate system. A detailed analysis of the oxygen vacancy levels supports the role of the interface in carrier transport. These results provide a new strategy to design the optoelectronic device via usual defect doping and interface coupling.

Phase morphology and corresponding piezoelectricity in ferroelectric solid solutions were studied by using a phenomenological theory with the consideration of phase coexistence. Results have shown that phases with similar energy potentials can coexist, thus induce interfacial stresses which lead to the formation of adaptive monoclinic phases. A new tetragonal-like monoclinic to rhombohedral-like monoclinic phase transition was predicted in a shear stress state. Enhanced piezoelectricity can be achieved by manipulating the stress state close to a critical stress field. Phase coexistence is universal in ferroelectric solid solutions and may provide a way to optimize ultra-fine structures and proper stress states to achieve ultrahigh piezoelectricity.