J. Phys. I France 3 (1993) 1219-1234
Ordering principles for tetrahedral chains in Ga- and Co- substituted YBCO intergrowthsO. Milat1, 2, T. Krekels1, G. Van Tendeloo1 and S. Amelinckx1
1 EMAT University of Antwerp (RUCA), Groenenborgerlaan 171, B-2020 Antwerpen, Belgium
2 Institute of Physics, University of Zagreb, Bijenicka 46, POB 304, 4100 Zagreb, Croatia
(Received 26 October 1992, accepted in final form 18 January 1993)
A model for superstructure ordering in the " chain " layers of Ga (Co) substituted YBCO intergrowths with general formula (REO ) NSr MCu O (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 planar squares, the chains consist of MO tetrahedra (M = Ga, Co) running along the  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" R ordering between neighbouring tetrahedra. The corner 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. Superstmctuie ordering in the case of Co substitution is more perfect than for the Ga substitution.
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