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).

Abstract

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 \ifmmode^\circ\else\textdegree\fi{}C followed by fast cooling from 940 to 100 \ifmmode^\circ\else\textdegree\fi{}C (50 \ifmmode^\circ\else\textdegree\fi{}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.