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

Abstract

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 the presence of both La and Ba ions in the ``Ba'' layer, the local symmetry of YBa(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 a Pauling bond-valence calculation is very useful as an adjunct to the Rietveld refinement of the positions of the La and Ba ions. When a Rietveld analysis is performed allowing for the possibility that the apical oxygen atoms may be slightly displaced along the a or b direction, the bond valence sums for both La 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 oxidation states of the CuI and CuII ions in both CLBLCO and YBCO. Our results for YBCO differ from some previously published results. Our main conclusion is that, while the method may be useful in finding the charge concentration on the CuII plane, it definitely does not yield the concentration of mobile holes, nor the concentration of superconducting charge carriers.

LaTeX, 32 pages, 5 postscript figures

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