On the new phase (Bi,Pb)₃Sr₂Ca₂CuO_y in the Bi-Pb-Sr-Ca-Cu-O system

A new Pb-rich compound (Bi,Pb)₃Sr₂Ca₂CuO_y (3221) has been identified and synthesized. Electron diffraction and X-ray diffraction data revealed that the 3221 phase has unit cell parameters of a=0.9919 nm and c=0.3471 nm with a hexagonal symmetry, which is in contrast to the commonly found tetragonal or orthorhombic symmetry in the Bi-Pb-Sr-Ca-Cu-O systems. The ordered superlattice structure consisting of three unit cells was observed along the (h0-h0) direction. The 3221 compound shows no superconducting transition to liquid helium temperature. By using a combination of the volumetric measurement technique and iodometric titration it was found that 100% Pb were tetravalent while 10% Cu were trivalent when the samples were treated in air. For the sample treated in nitrogen no Cu³⁺ were detected, while Pb remained in the tetravalent state.