A superspace approach to the composition flexible composite Ba1+x(CuxIr1-x)O-3 (x=0.2708)

The structure of Ba1+xCuxIr1-xO3, x = 0.2708 has been solved using the supe rspace 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). Th e structure can be described as a modulated columnar composite containing t wo 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 f unction is defined by its center (x) over cap (4) along the fourth dimensio n, its width (Delta) and the maximum amplitude of the displacive modulation (delta). The paper describes how, as a first approximation, the columns (C uxIr1-x)O-3 can be simply described by a single free parameter, based on th e height difference of the prisms and octahedra that make up the transition metal chains. As a result, this superspace approach requires only a very s mall number of variables to be refined, compared to the conventional supers tructure description. In addition, this paper presents a structural compari son of this structure with other members of this family and outlines how th e structure determination is completely general and can be applied to other modulated composite structures of this family. The structural analysis pre sented in this paper is another demonstration of the efficiency of the 4-di mensional superspace formalism for describing the structures of so-called ' composition flexible' systems. (C) 2000 Elsevier Science Ltd. All rights re served.