Nonpoled ferroelectric ceramics are thought to be nonpolar because of randomly oriented grains and the formation of ferroelectric domains in the grains. Here, we discover the surfaces ($\sim \mathrm{several}\mu \mathrm{m}$ thick) of ferroelectric ceramics are spontaneously polarized. Because the orientations of ferroelectric polarization of the opposite surfaces are antiparallel, ferroelectric ceramics are nonpolar as a whole. However, the ceramics exhibit a strong flexoelectriclike electromechanical response from the piezoelectric response of the polarized surfaces if they are asymmetrically strained (such as bending). Our results reveal a major mechanism to resolve one important but largely unresolved issue: the experimentally measured flexoelectric effect is typically orders of magnitude larger than the theoretically predicted value in ferroelectrics.

• Received 12 December 2017
• Revised 3 May 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.057602