STRUCTURE AND SUPERCONDUCTING PROPERTIES OF TL1-YPBYY1-XCAXSR2CU2O7 - DEPENDENCE OF TC ON HOLE CONCENTRATION

Structure and superconducting properties of Tl1-yPb(y)Y1-xCa(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-yPb(y)Y1-xCa(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 approximately 0.25, which is close to the hole concentration at maximum T(c) in other cuprate superconductors containing two CuO2 layers.