Eight rare-earth-substituted Pb-based 1:2:1:2 superconducting compounds: (Pb,Cu)Sr2(Ca,R)Cu2O7(R=Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm)

Rare-earth substitution has given a series of Pb-based 1:2:1:2 superconducting compounds (Pb,Cu)${\mathrm{Sr}}_2$(Ca,R)${\mathrm{Cu}}_2$ ${\mathrm{O}}_7$ (R=Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm). ${\mathit{T}}_{\mathit{c}}$’s vary inversely with rare-earth ionic size from 72 K (Tm) to 60 K (Gd) and then remain constant for larger ions up to Nd. This behavior (${\mathit{T}}_{\mathit{c}}$ decreasing for larger ions separating the two ${\mathrm{CuO}}_2$ superconducting sheets) is similar to the behavior of ${\mathrm{RBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$ (1:2:4) and ${\mathit{R}}_2$ ${\mathrm{Ba}}_4$ ${\mathrm{Cu}}_7$ ${\mathrm{O}}_{\mathit{y}}$ (2:4:7), but not R${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ (1:2:3). As prepared in air the samples were not superconductors; superconductivity was induced by annealing in oxygen under 100 bar of oxygen pressure at 940 °C followed by fast cooling from 940 to 100 °C (50 °C/min). Lattice constants of the compounds correlate with the ionic radii of the rare-earth elements, as in the 1:2:3, 1:2:4, and 2:4:7 compounds.