SUPERCONDUCTIVITY AT 105 K IN TL0.5PB0.5CASR2-XLAXCU2O7

Substitution of La3+ for Sr2+ in the lattice Tl0.5Pb0.5Sr2CaCu2O7 has been used to reduce the hole carrier concentration on the CuO2 planes resulting in T(c) passing through a maximum at 105 K. This behaviour is similar to the previously reported effect of substitution of Y3+ for Ca2+ in the same parent material. However, in the La3+-substituted series: (a) superconductivity is maintained to higher substitution levels; (b) the c-axis lattice parameter initially contracts at only 30% of the rate; and (c) the a- b-planes lattice parameter expands at 140% of the rate; compared to the Y3+-substituted series. These effects are consequences of the differing sites of substitution-La3+ on the Sr-O plane bounding the CuO2 planes, and hence influencing the bridging oxygen atom; Y3+ on the bare Ca2+ site between the CuO2 planes. The superconducting phase diagrams of T(c) versus substitution level are similar to the phase diagram for the CuO2 planes established for La2-xSrxCuO4.