CHARACTERIZATION OF PB-DOPED AND CA-DOPED BI2+XSR2-XCUO6+DELTA

A phase diagram study has been made to determine the range of compositions of single phase, Pb-doped R solid solutions in the Bi-Sr-Pb-Cu-O system. These have the formula Bi(2+z-z)PbzSr(2-x)CuO6+delta, with 0.15 < x < 0.40 and 0 < z < 0.55 at 750-degrees-C. With increasing z, the unit cell symmetry changes from tetragonal to orthorhombic; cell parameters are given as a function of x and z. From the compositional extent of the solid solutions, in terms of x and z, it is possible to infer that Pb enters predominantly "Bi" sites but also "Sr" sites, in the crystal structure of the R phase, in contrast to Ca which appears to enter exclusively the "Sr" sites. The change from tetragonal to orthorhombic symmetry may be interpreted as indicating the onset of Pb occupation of the "Bi" sites.