High-resolution imaging of the ferroelectric perovskite Ba₂Bi₄Ti₅O₁₈

The ferroelectric material Ba₂Bi₄Ti₅O₁₈ has been examined by high-resolution electron microscopy at 200 and 500 kV. With the directly interpretable resolution limit in each case extending to better than 2.5 Å, it was demonstrated that both the A-cation and B-cation positions could be resolved and, furthermore, that analysis of structural disorder at this level of resolution allowed an explanation of some observed lattice defects. In particular, unit shifts of a perovskite cell along a bismuthate sheet were identified as occurring in the manner previously postulated, and a Burgers vector circuit analysis around a low-angle domain boundary permitted the formulation of a model for the atomic configuration at the boundary. The implications of these observations for the study of perovskites generally are briefly discussed.