Electron microscopy of ferroelectric bismuth oxides containing perovskite layers

Ferroelectric bismuth oxides of the general formula (Bi$_{2}$O$_{2}$)$^{2+}$(A$_{n-1}$B$_{n}$O$_{3n+1}$)$^{2-}$ (A = Ba, etc., B = Ti or other transition metal) have been examined by high-resolution lattice imaging electron microscopy. The lattice images show dark bands at the positions of the Bi$_{2}$O$_{2}$ layers, with $n$ - 1 lines between them due to the layers of the perovskite A cations or, in favourable circumstances, the fully resolved 0.4 nm square perovskite grid. Dislocations and domain boundaries have been imaged for the first time in ferroelectric crystals. The structure of the dislocations and domain walls is discussed in the light of the microstructural evidence.