A centrosymmetric stress cannot induce a polar response in centric materials,piezoelectricity is, for example, possible only in non-centrosymmetricstructures. An exception is meta-materials with shape asymmetry, which may bepolarized by stress even when the material is centric. In this case themechanism is flexoelectricity, which relates polarization to a strain gradient.The flexoelectric response scales inversely with size, thus a large effect isexpected in nanoscale materials. Recent experiments in polycrystalline,centrosymmetric perovskites [e.g., (Ba,Sr)TiO3] have indicated values offlexoelectric coefficients that are orders of magnitude higher thantheoretically predicted, promising practical applications based on bulkmaterials. We show that materials with unexpectedly large flexoelectricresponse exhibit breaking of the macroscopic centric symmetry throughinhomogeneity induced by the high temperature processing. The emergingelectro-mechanical coupling is significant and may help to resolve thecontroversy surrounding the large apparent flexoelectric coefficients in thisclass of materials.
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