SUPERCONDUCTIVITY DUE TO CHARGE-TRANSFER TO THE CUO2 SHEET FROM THE (TL,PB)O LAYER IN TL0.5PB0.5SR2-XNDXCUO5-DELTA

The superconductivity of Tl0.5Pb0.5Sr2-xNdxCuO5-delta was observed for x greater than or equal to 0.3, although the calculated Cu valence as suming Tl3+ and Pb4+ is slightly above or below 2.0. The binding energ y of the T1 and Pb 4f core levels measured by XPS shifted to higher en ergy than those of Tl3+ and Pb4+ in the reference materials, showing t hat the Tl and Pb valences are lower than 3 + and 4 +, respectively. T hese observations strongly suggest that holes in the CuO2 sheet are cr eated by charge transfer from the (Tl,Pb)O layer, similar to the doubl e-layered Tl-Ba cuprates. In contrast, the temperature variation of el ectrical conductivity of Tl0.5Pb0.5Sr2CuO5-delta changed from metallic to semiconducting with increasing oxygen deficiency, delta, and no su perconductivity was observed through any control of delta. Although XP S measurement also suggested that Tl and Pb valences lowered with incr easing oxygen deficiency, delta, the reason why the system did not sho w superconductivity can be understood by the depletion of oxygen from the CuO2 plane during deoxygenation.