A superspace approach to the composition flexible composite Ba1+x(CuxIr1-x)O3 (x=0.2708)

The structure of Ba1+xCuxIr1-xO3, x = 0.2708 has been solved using the superspace formalism. The superspace group is R (3) over barm(00 gamma )0s with a=10.1490(8) Angstrom, c=2.7164(3) Angstrom and q=0.6354(2)c(1)* (Z=3). The structure can be described as a modulated columnar composite containing two subsystems [(Cu, Ir)O-3] and [Ba]. A sawtooth function was used to model the z-component of the atomic modulation functions. Each atomic sawtooth function is defined by its center (x) over cap (4) along the fourth dimension, its width (Delta) and the maximum amplitude of the displacive modulation (delta). The paper describes how, as a first approximation, the columns (CuxIr1-x)O-3 can be simply described by a single free parameter, based on the height difference of the prisms and octahedra that make up the transition metal chains. As a result, this superspace approach requires only a very small number of variables to be refined, compared to the conventional superstructure description. In addition, this paper presents a structural comparison of this structure with other members of this family and outlines how the structure determination is completely general and can be applied to other modulated composite structures of this family. The structural analysis presented in this paper is another demonstration of the efficiency of the 4-dimensional superspace formalism for describing the structures of so-called 'composition flexible' systems. (C) 2000 Elsevier Science Ltd. All rights reserved.