A new example of a composite composition flexible phase structure of Ba1+xAx′B1-xO3 (x=0.2124, A′=Ni, B=0.51Ni+0.49Pt)

The structure or Ba(1-x)A(x)'B1-xO3 (x=0.2124, A'=Ni, B=0.51Ni+0.49Pt) has been solved in the superspace formalism with occupational Crenel Functions and sawtooth displacive modulations along the trigonal axis. The structure is described as an incommensurate modulated columnar composite with two subsystems, [(Ni,Pt)O-3] and [Ba], respectively. The superspace group is R (3) over bar m(00 gamma)0s with a = 9.9357(1) Angstrom, c =2.5871(1) Angstrom and q = 0.6062c* (Z=3). The structural modulation has two variable parameters: the modulation wave vector or misfit parameter, which is fixed by the composition, and the mismatch between the sizes of the octahedral and prismatic oxygen polyhedra formed in the structure. The model, although developed within the superspace formalism, is closely related to the previously reported polytype layer picture in direct space. This structure is an example of so-called composition flexible systems. The present structural analysis constitutes a merely demonstrates the efficiency of the four-dimensional superspace formalism for describing such phases. (C) 2000 Published by Elsevier Science Ltd. All rights reserved.