8 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)Sr2(Ca,R)Cu2O7 (R = Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm). T(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 (T(c), decreasing for larger ions separating the two CuO2 superconducting sheets) is similar to the behavior of RBa2Cu4O8 (1:2:4) and R2Ba4Cu7O(y) (2:4:7), but not RBa2Cu3O7 (1:2:3). As prepared in air the samples were not superconductors; superconductivity was induced by annealing in oxygen under 100 bars of oxygen pressure at 940-degrees-C followed by fast cooling from 940 to 100-degrees-C (50-degrees-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.