Intergrowths and defect structures in Bi₉Ti₃Fe₅O₂₇ revealed by high‐resolution electron microscopy

A high‐resolution 500 kV study of the long‐period (n = 8) perovskite Bi₉Ti₃Fe₅O₂₇ has revealed a variety of order/disorder effects, in marked contrast to low‐n materials, as well as the occurrence of a new two‐dimensional superlattice structure corresponding to a 3 × d₁₁₀ ordering along the bismuthate layers. Intergrowths of the 2‐D SL phase with the ‘usual’ perovskite/bismuthate phase, as well as sharp interfaces between these two phases, were observed and the common occurrence of steps along these interfaces suggested ledge migration as one possible transformation mechanism. Coherent intergrowths of structural elements rather similar to tetragonal‐tungsten‐bronze (TTB)‐type were also found and these sometimes appeared to be relics of the super‐lattice phase. The chemical implications of the sevarious observations are briefly discussed. 1983 Blackwell Science Ltd