Atomically resolved structure of step-like uncharged and charged domain walls in polycrystalline BiFeO3

The dynamics of domain walls (DWs) during polarization switching has a great influence on the electromechanical response of ferroelectrics. To understand the influence of DWs on the functional performance of the material, their local structure and properties must be revealed. There are many investigations of the DWs in ferroelectric thin films, while investigations of the DWs in polycrystalline ferroelectrics are fewer. Here, we report on a comparative study of uncharged and charged (“tail-to-tail”) DWs in BiFeO3 ceramics from the point of view of the atomically resolved strain and structure using scanning-transmission electron microscopy. Both types of DWs have a step-like morphology and have a non-Ising behavior. The strain, expressed as the unit-cell distortion, is higher and more concentrated in the case of uncharged walls, which we associate with a narrower transition region of the Fe-displacement vectors. Conversely, in the case of “tail-to-tail” charged DWs, the unit-cell distortion is smaller and more dispersed because of the wider transition region of the Fe-displacement vectors. All the types of step-like DWs reported here, regardless of their charge state, exhibit a comparable amount of Bi-vacancies segregation. Further details about the structural differences are discussed from the point of view of the Fe-displacement evolution through the wall for uncharged/“tail-to-tail” charged DWs. The results are useful as they provide an insight into the local structure and chemistry of charged and uncharged DWs in polycrystalline BiFeO3.