Structure and a bond-valence-sum study of the 1-2-3 superconductors (CaxLa1-x)(Ba1.75La0.25+x)Cu3Oy and YBa2Cu3Oy -: art. no. 174510

We present a study of the crystal structure of the 1-2-3 superconductor (CaxLa1-x) (Ba1.75-xLa0.25+x)Cu3Oy (CLBLCO). Because of the presence of bath La and Ba ions in the "Ba" layer, the local symmetry of YBa2Cu3Oy (YBCO) is lost. For example, one can no longer assume that an "apical" oxygen atom always lies strictly on the line joining Cu I and Cu II 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 bend-valence-sum method to study the mean oxidation states of the Cu I and Cu II 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 Cu II plane, it definitely does not yield the concentration of mobile holes, nor the concentration of superconducting charge carriers.