Structure and superconducting properties of Tl1−yPbyY1−xCaxSr2Cu2O7

Structure and superconducting properties of Tl1−y Pb y Y1−x Ca x Sr2Cu2O7 (y=0.0, 0.1, 0.25 and 0.5) , derived from the parent insulator TlYSr2Cu2O7, have been investigated for different values of x. XANES studies show Pb to be in the 4+ state while Tl is in the 3+ state, suggesting thereby that in this series, (x−y) approximately corresponds to the hole concentration. The in-plane CuO distance decreases with increase in x for all values of y. The apical CuO distance as well as the Tl(Pb)O(2) distance show anomalies at x=0.5 in the series with y=0.25; at this composition, T c also reaches a maximum. In general, the composition (value of x) at which the T c of Tl1−y Pb y Y1−x Ca x Sr2Cu2O7 reaches a maximum depends on the Pb content and the maximum T c itself increases with increase in y, reaching a value of 105 K at y=0.5. More interestingly, a maximum T c occurs at an (x−y) value of ∼0.25, which is close to the hole concentration at maximum T c in other cuprate superconductors containing two CuO2 layers.