Chemistry of superconducting Cu oxides with the block layers consisting of Tl, Pb and Bi ions

Two interesting topics of layered high-T_c copper oxides, Tl and Pb systems investigated recently in our laboratory are presented to get better understanding of the chemistry and superconductivity of these peculiar compounds. First, a comparative study of double and single Tl layer oxides has been done. The most probable explanation for the origin of holes responsible for a high T_c above 100 K in the double Tl layer oxides is electron transfer mechanism Tl^3-t-Cu^2+2t/n. Evidence for a reduced state in Tl ions is obtained by XPS and XANES measurements. Single Tl layer oxides are shown to be more ionic by XPS and NMR measurements. The block layer of Pb(Ba,Sr)₂YCu₃O₇ lacks one PbO layer from the PbO-Cu-PbO block layer as revealed by high-resolution electron microscopy. In the (Pb,Cu) double layer oxides, an abrupt oxygen removal and uptake accompanied by phase transition was observed to occur in a reversible fashion. Pb(Ba,Sr)₂YCu₃O₇ became a 60 K superconductor by partial substitution of Y with Ca.