Harnessing atomic-scale order at grain boundaries for giant flexoelectricity

Flexoelectricity is a ubiquitous electromechanical coupling mechanism that produces a polarization response to strain gradients and requires no material symmetry constraints. Here, we investigated flexoelectricity in the La_0.24Sr_0.76Al_0.62Ta_0.38O₃ (LSAT) grain boundaries using atomic-resolution scanning transmission electron microscopy, including high-angle annular dark-field (HAADF) imaging, energy-dispersive x-ray spectroscopy (EDX), and electron energy-loss spectroscopy (EELS). Our results reveal that tantalum (Ta) segregates in the grain boundaries, forming unique chemically ordered structures. EELS uncovers pronounced distortions of the aluminum (Al)/Ta─oxygen (O) octahedra in the grain boundaries. We exploited the pronounced structural inhomogeneity of the 36.8° grain boundary to achieve a large strain gradient (∼2.0 per nanometer) within two to three unit cells, resulting in an atomic-scale flexoelectric displacement of up to ∼114.8 picometers. Quantitative analysis indicates that the flexoelectric displacement correlates with local nonstoichiometry induced by Ta segregation. We further demonstrated that the segregation-enhanced strain gradient exists generally in both symmetric and asymmetric grain boundaries. Our atomic-scale findings provide insight into tunable giant flexoelectricity in electroceramic grain boundaries.