NEUTRON STRUCTURAL STUDIES OF SUPERCONDUCTING BI2SR2CUO-SIMILAR-TO-6 AND BI2-XPBXSR1.8LA0.2CUO6+DELTA(X = 0.0 AND 0.2)

We have investigated the influence of incorporation of La- and Pb-ions on the structural features of Bi-2201 system by Rietveld refinement o f the neutron diffraction patterns of samples with three different com positions, viz. (A) Bi2Sr2CuO approximately 6 with T(c)R=0=9.5 K; (B) Bi2Sr1.8La0.2CuO6+delta with T(c)R=0=16.5 K; and (C) Bi1.8Pb0.2Sr1.8La 0.2 CuO6+delta with T(c)R=0=21 K. The large fraction of vacancies obse rved at Bi-, Sr-, Cu- and O(2)-sites in the sample A get filled up on substitution of La in the sample B. However, these vacancies reappear to a smaller extent for the sample C. The O(3)-ions belonging to the B i-double layer are found to be disordered for all the three samples, t hough the nature of the disorder is highly sensitive to the type of su bstitution. A correlation exists between bond valence of the Cu-sites and the measured T(c)-value of the three samples.