The Structure and a Bond-Valence-Sum Study of the 1-2-3 Superconductors CLBLCO and YBCO

摘要

We present a study of the crystal structure of the 1-2-3 superconductor[Ca(x)La(1-x)][Ba$(1.75-x)La(0.25+x)]Cu(3)O(y) (``CLBLCO''). Because of thepresence of both La and Ba ions in the ``Ba'' layer, the local symmetry ofYBa(2)Cu(3)O(y) (``YBCO'') is lost. One can no longer assume that an ``apical''oxygen atom always lies strictly on the line joining CuI and CuII ions, and aPauling bond-valence calculation is very useful as an adjunct to the Rietveldrefinement of the positions of the La and Ba ions. When a Rietveld analysis isperformed allowing for the possibility that the apical oxygen atoms may beslightly displaced along the a or b direction, the bond valence sums for bothLa and Ba are close to the natural oxidation states of these ions. We have also used the bond-valence-sum method to study the mean oxidationstates of the CuI and CuII ions in both CLBLCO and YBCO. Our results for YBCOdiffer from some previously published results. Our main conclusion is that,while the method may be useful in finding the charge concentration on the CuIIplane, it definitely does not yield the concentration of mobile holes, nor theconcentration of superconducting charge carriers.