  Flexoelectricity is a type of ubiquitous and prominent electromechanical
coupling, pertaining to the response of electrical polarization to mechanical
strain gradients while not restricted to the symmetry of materials. However,
large elastic deformation in most solids is usually difficult to achieve and
the strain gradient at minuscule is challenging to control. Here we exploit the
exotic structural inhomogeneity of grain boundary to achieve a huge strain
gradient (~ 1.2 nm-1) within 3 ~ 4 unit-cells, and thus obtain atomic-scale
flexoelectric polarization up to ~ 38 {\mu}C/cm2 at a 24 LaAlO3 grain boundary.
The nanoscale flexoelectricity also modifies the electrical activity of grain
boundaries. Moreover, we prove that it is a general and feasible way to form
large strain gradients at atomic scale by altering the misorientation angles of
grain boundaries in different dielectric materials. Thus, engineering of grain
boundaries provides an effective pathway to achieve tunable flexoelectricity
and broadens the electromechanical functionalities of non-piezoelectric
materials.
