TEM study of cation-deficient-perovskite related AnBn-1O3ncompounds:: the twin-shift option

Among hexagonal perovskites, the B-cation deficient perovskite-related compounds A(n)B(n-1)O(3n) present two kinds of structural types depending on the stacking sequences of their AO(3) layers. All these structures derive from the perovskite, which is periodically disturbed by planar defects. The structures with (hhc... c)-type sequences, present successive perovskite blocks shifted from one another by a 1/3<01 (1) over bar0>(H) vector, while (hc...c)-type sequences show twin plane boundaries. The non-stoichiometry of these compounds is in all cases closely associated with the distribution of these planar defects: shift planes or twin boundaries. In the hexagonal perovskites of the A(n)B(n-1)O(3n) series belonging to the Ba5Nb4O15-BaTiO3 system, the twin type structures is favored by high values of it (number of octahedra layers within a perovskite blocks) and t (Goldschmidt tolerance factor). The stability of the "shift" type structure surely comes from the preservation of a bee cationic sub-lattice. However, this structural type results in the occurrence of a long-range order of vacant octahedra layers. The stability of the "twin" type structure is related to the drastic decrease of the periodicity of vacancies along the c-axis (every n/2 octahedra layers) but needs a very expanded three dimensional BO6 octahedra sub-lattice (high value of t) and a specific order of the B-cations in the vicinity of the twin planes. (C) 2003 Elsevier Inc. All rights reserved.