Superspace description of the hexagonal perovskites in the system Ba5Nb4O15-BaTiO3 as modulated layered structures

A general structural model of B-site deficient hexagonal layer perovskites AB(1-x)X(3) having no tilting of anion octahedra in consecutive layers is presented. They are described as modulated layered structures using a (3 + 1)-dimensional superspace approach. The presence or not of a given atom along the stacking direction is described in superspace through step-like (crenel) occupational modulation functions. Being essentially composition independent, the model is applicable to any compounds related to this structural type. The component of the primary modulation vector q = gammac* is connected to the chemical composition by the relation gamma = (1 + x)/3. The model is confirmed and the efficiency of the superspace approach is demonstrated by the structural analysis of some of the hexagonal perovskite found in the system Ba5Nb4O15-BaTiO3, where long-period intergrowth compounds have been recently identified. (C) 2003 Elsevier Science (USA) All rights reserved.