Theoretical estimation of longitudinal flexoelectric response in polar perovskite oxides

Abstract

The quest for pure flexoelectricity has brought about various theoretical and experimental approaches to eliminate piezoelectric contributions from the flexoelectric measurements. Herein, we propose a first-principles approach to measure flexoelectricity. We compute the corresponding piezoelectric contribution and subtract it from the total local polarization rather than devising the system having the inversion symmetry to acquire the pure flexoelectric contribution. This approach is applied to experimentally accessible strain gradients in the order of ∼107 m−1 with sufficient structural relaxation and the precise Born effective charge tensors, which play a dominant role in probing the correct strain gradient and polarization responses. We validate the proposed approach by calculating the longitudinal flexoelectric coefficient μ3333 (−0.513±0.004 nC/m) for SrTiO3 (Pm-3m), which is comparatively closer (irrespective of their signs) to its experimental value of 0.2 nC/m with a smaller uncertainty than previous works. Its application is extended to other ABO3 perovskites in cubic (Pm-3m) phases and perovskite BaTiO3 in its tetragonal (P4mm) phase. The analysis of obtained results enables us to comment on the possible origin of flexoelectricity and its additive interaction with piezoelectricity in the local polarization response.