EFFECTS OF CE ON THE CRYSTAL-STRUCTURE AND SUPERCONDUCTIVITY IN TLSR2CACU2O7-DELTA

The partial substitution of Sr or Ca in TlSr2CaCu2O7-delta (T1 1:2:1:2 ) with the rare-earth element Ce has the advantage to stabilize the te tragonal phase with space group P4/mmm and shows a prominent deviation from Vegard's law for the changes in the a, c and v parameters. The t ransitions ranging from non-superconducting metal to superconductor to insulator are displayed in both the Tl(Sr2-xCex)CaCu2O7 (0 less than or equal to x less than or equal to 0.6) and TlSr2(Ca1-yCey)Cu2O7-delt a (0 less than or equal to y less than or equal to 0.7) series. As exp ected from the lattice-parameter data, the T-c value and the supercond ucting volume fraction increase with increasing Ce concentration up to x = y = 0.35 (T-c = 62 K), then decrease. In the insulating region, t he metal-insulator (M-I) transition appears near the composition of x = y = 0.6. The resistivity of the sample with x = 0.6 or y = 0.7 does follow the three-dimensional Mott variable-range-hopping (VRH) depende nce in the temperature below similar to 100 K.