Coexisting morphotropic phase boundary and giant strain gradient in BiFeO3 films.

Large compressive strains may introduce giant tetragonality and morphotropic phase boundaries in perovskite BiFeO₃ films, where the coexisting tetragonal and rhombohedral phases (T like and R like phases) are identified to exhibit large piezoelectric response. Here, we have further achieved mechanical bending deformations in R like BiFeO₃ through its neighboring T like BiFeO₃ phases, where a strain gradient of ∼10⁶/m was identified. Aberration-corrected scanning transmission electron microscopy revealed not only the strain distributions but also the atomic scale Fe polar displacement in the gradient R like BiFeO₃. In spite of the giant strain gradient, the polarization direction in each R like BiFeO₃ unit cell was found mainly along its diagonal direction, suggesting that potential flexoelectric coupling induced polarization in BiFeO₃ is smaller than its spontaneous polarizations, while a large built-in electric field can be obtained via the large strain gradient. Our results indicate that the common phase coexistences in oxide materials could be further manipulated to introduce elastic strain gradients and tune the properties for oxide films. [ABSTRACT FROM AUTHOR]