HRTEM study of cation-deficient perovskite-related AnBn−δO3n (n⩾4δ) microphases in the Ba5Nb4O15–BaTiO3 system

The occurrence of coherent intergrowths of cation-deficient perovskites in the Ba5Nb4O15–BaTiO3 system has been examined by high-resolution transmission electron microscopy and selected area electron diffraction. Because of their structural similarity, the simple members Ba5Nb4O15 (n=5) and Ba6TiNb4O18 (n=6) form coherent intergrowths—noted 5P61—by the juxtaposition along the c-axis of P perovskite-like blocks n=5 and one perovskite-like block n=6, with P=1, 2 and 3. More generally, the ability to form intergrowths in the hexagonal perovskite systems is discussed considering the structural characteristics of the simple members. Examples taken from various systems show that the formation of such intergrowths is highly dependent on the size of the A cation present in simple members.