CATION ORDERING IN Y, TB-(123)-TYPE STRUCTURES

An ordering of atoms in a cation sublattice of a (123)-type crystal do ped by Tb has been observed by means of detailed single-crystal x-ray diffraction studies. On the basis of structural and chemical compositi on data analysis for a series of single crystals the cation ordering i s predicted to be a general feature of (123)-type crystals. Two main f actors responsible for superstructural lattice formation have been rev ealed. The first one is related to the presence of cations of differen t radii in Y sites. As a result, the large cation gains a particular p osition instead of a random distribution. The second factor is connect ed with the presence of atoms in the Cu(1) site which are able to ensu re the octahedral oxygen coordination and hence to cause the crossing of Cu-O chains and the formation of closed loops in the basal plane.