Synthesis and structural analysis of the superconducting lead cuprates, (Pb, Cu) (Sr, R)₂ (R′, Ce)₂Cu₂O_z (R, R′: rare-earth element)

The structural analysis of a superconducting (Pb/Cu)-1222 phase, (Pb_0.5Cu_0.5)(Sr_0.875Eu_0.125)₂(Eu_0.75Ce_0.25)₂Cu₂ O_z (z≈9), is performed by Rietveld analysis using X-ray powder diffraction data. Pb, Cu and O atoms constituting the (Pb/Cu)O monolayer in this crystal are found to be displaced from their ideal crystallographic sites as previously reported for the (Pb/Cu)-1212 compound. In addition, (Pb, Cu)-1222 compounds with nominal compositions of (Pb, Cu)(Sr, R)₂(R', Ce)₂Cu₂O_z (R, R': rare-earth element) are synthesized and characterized. Nearly single-phase samples are successfully obtained for the compounds with R = R′ = Pr, Sm, Eu, Gd, Dy, Y or Ho, and for those with a pair of rare-earth elements, (R, R′) = (La, Eu), (Nd, Eu), (Nd, Dy), (Nd, Y), (Nd, Er), (Sm, Eu) or (Eu, Gd). Superconductivity is observed for the samples with R = R′ = Sm, Eu or Gd and also for those with (R, R′) = (La, Eu), (Nd, Eu), (Sm, Eu) or (Eu, Gd).