Ordering principles for tetrahedral chains in Ga- and Co- substituted YBCO intergrowths

Abstract

A model for superstructure ordering in the «chain» layers of Ga (Co) substituted YBCO intergrowths with general formula (REO 2 ) N Sr 2 MCu 2 O 5 (M=Co, Ga; n=1, 2,...) is proposed. By Ga or Co substitution for Cu, the structure of the chain layer changes: instead of the CuO 4 planar squares, the chains consist of MO 4 tetrahedra (M=Ga, Co) running along the [110] perovskite direction. The existing model for the Ga substituted 123 implies that all the chains are the same. Our new model is based on the results of Electron diffraction and High-resolution electron microscopy investigations. The model reveals the occurrence of two types of chains as a consequence of «opposite» ordering between neighbouring tetrahedra. The comer linked tetrahedra in each chain appear as alternatingly rotated in opposite sense, and a chain itself, as being displaced with respect to the underlying structure in one of two senses; either forth (right) or back (left) along the chain direction. The regular alternation of chains of opposite type doubles the periodicity within a layer and induces the possibility for intrinsic disorder in the chain layer stacking sequence. The planar superstructure and a staggered stacking of the tetrahedral chain layers is found irrespective of the rest of the intergrowth structure. Superstructure ordering in the case of Co substitution is more perfect than for the Ga substitution