Superconducting and Nonsuperconducting Ca-Free Single Crystals of Pb2Sr2RCu3O8 (R = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Y): Effect of Cation Vacancies and the Metal/Insulator Transition

Ca-free single crystal and polycrystalline samples of Pb2Sr2RCu3O8 have been prepared by a PbO/NaCl flux growth method and a fast solid state reaction, respectively. The electronic character of the single crystals of this series can be roughly categorized into three groups: insulating (no carrier doping in the CuO2 planes) for R = La, Ce, Pr, and Nd, semiconducting for R = Sm, Eu, Gd, and Tb, and poorly metallic for R = Dy, Ho, and Y. Crystals with R = Eu to Ho are found to be superconducting with the exception of Tb. The metallic character in these crystals is likely promoted by a cation vacancy at the rare-earth sites which has been established by crystal structural and chemical analyses. The anomalous behavior of the Tb crystals may originate from a lower dopant level as inferred from the structure determination or possibly from the presence of Tb4+. The fact that metallic behavior is observed in nonstoichiometric polycrystalline samples and not in stoichiometric ones seems consistent with the cation vacancy carrier doping mechanism.