Crystal structure of a modulated composite structure with two subsystems:: Ba1.1064CoO3

The structure of Ba1.1064CoO3 has been solved in the (3 + 1)-dimensional formalism. The structure is described as a modulated chain composite with two subsystems, [Coos] and [Ba], respectively. The superspace group is R-3m (00 gamma )0s with a = 9.8842(20) Angstrom, c = 2.4785(12) Angstrom, and q = 0.5532(4) c* (Z = 3). A saw-tooth function was used to model both the occupational and displacive modulations. Each atomic saw-tooth function is defined by its center (x) over cap (4), along the fourth dimension, its width (A), and the maximum amplitude of the displacive modulation (delta). The paper describes how, as a first approximation, the columns (CoO3) can be mainly described by a single free parameter, based on the height difference of the trigonal prisms and octahedra that constitute the transition metal chains. As a result, this superspace formalism requires only a small number of variables to be refined, compared to the conventional superstructure description. (C) 2001 Academic Press.