The Role of Oxide Surface Structure and Polarity in Flexoelectricity

Interest in flexoelectricity has grown significantly over the past decade owing to its universality and importance at the nanoscale. Unlike many other oxide properties, a material’s macroscopic flexoelectric response is predicted to be highly surface sensitive. This surface sensitivity has been demonstrated with first principles calculations on (100) SrTiO₃ bulk terminations [PRB 90, 201112(R) (2014)]. However, experimentally observed oxide surfaces often possess structural and chemical differences from the corresponding bulk material, the effects of which remain unclear in the context of flexoelectricity. Using density functional calculations, we compare the flexoelectric response of a number of low-energy (100) SrTiO₃ reconstructions to assess the impact of surface stoichiometry and periodicity on flexoelectricity. Additionally, we address the role of polar surfaces with calculations on (100) and (111) MgO surfaces. Our calculations on experimentally observed oxide surface structures corroborate previous work highlighting the importance of surfaces in flexoelectricity and have important implications for flexoelectric measurements and thin film growth.