On the polarization dynamics in the presence of flexoelectricity and morphotropic phase boundary in ferroelectrics

Abstract

It is shown that anomalous piezoelectric properties of epitaxial nanostructures arise on the morphotropic phase boundary (MPB) due to the strong flexoelectric effect on dislocation walls. The MPB (typical of many materials) exhibits a coexistence of various phases and partition of these phases to minimum sizes. This minimum size l_с (nanoscale) is found using the dislocation theory; it coincides with the distance between individual dislocations in dislocation walls, which is much larger than the Burgers vector b, regardless of the type of crystalline material. The flexoelectric coefficients f are estimated taking into account dimensional relations and experimental data on the rotations of ferroelectric nanodomains in multiferroics. These estimates coincide with classical values. The critical value l_с ~ 10b specifies the measured dependence on the dielectric susceptibility χ_e, f ~ χ_e^1/2. The quantity χ_e depends on the frequency of the ac electric field applied to a sample and on the dislocation density. The Ba_0.6Sr_0.4TiO₃/Ni_0.8Zn_0.2Fe₂O₄ ceramic composite shows typical frequency dispersion of χ_e in a wide frequency range. The frequency dependence of flexoelecric coefficients is shown to reproduce the frequency dependence of permittivity at high frequencies.