A NEW PARAMETRIZATION FOR INVESTIGATING RELATIONSHIPS BETWEEN CHEMICAL-COMPOSITION AND CRYSTAL-STRUCTURE IN LAYERED ABO3 CERAMICS

The relationship between the layer and framework pictures of layered ABO3 ceramics is examined, and expressed quantitatively. This permits a calculation of the volumes of cationic coordination polyhedra in terms of AO3 layer separations and distances between oxygen ions within the layers. Thus the observed stacking sequences (combinations of cubic- and hexagonal-type stacking) can be quantitatively related to the sizes of the cations, and hence chemical composition. Several ABO3 systems are examined, with Ba2+ and Sr2+ as A ions. In the (Ba,Sr)RuO3 system, the occurrence of 9L, 4L and perovskite phases is rationalized in terms of <V(A)>/<V(B)> polyhedral volume ratios, and the solubility of different B ions in the 9L BaRuO3 structure is discussed. Considerations of ionic size are generally adequate to understand the structures obtained, except in compounds with the hexagonal barium titanate structure, where metal-metal bonding occurs. The influence of temperature and pressure on stacking sequence is considered, by examining BaMnO3-x and Ba1-ySryMnO3-x compositions. A rationalization in terms of <V(A)>/<V(B)> ratios is again found to be appropriate. Whereas the layer picture breaks down in some ABO3 systems, the framework picture, with its quantitative expression in terms of cationic polyhedral volumes, is of general validity.