EFFECTS OF SIMULTANEOUS PB BI AND Y/CA SUBSTITUTION ON STRUCTURAL ANDSUPERCONDUCTING PROPERTIES OF THE 2212-BSCCO PHASE/

By substituting Pb for Bi and Y for Ca in Bi2Sr2CaCu2O8, it is possibl e to prepare a Bi2-xPbxSr2Y1-yCayCu2Oz phase with a 2212-type structur e that does not show structural modulation in a wide region of the pha se diagram. In comparison with Bi2Sr2CaCu2O8 the non modulated phase e xibits a larger orthorhombic distorsion and a different arrangements o f the adjacent BiO layers. The vanishing of the structural modulation, which occurs in a similar way for any Ca/Y ratio by increasing the Pb content, involves changes in the modulation that are discussed in ter m of decrease of the oxygen content and adjustement of the structural mismatch which seem to play a role in different regions of the phase d iagram. The non modulated Bi2-xPbxSr2Y1-yCayCu2Oz phase still shows su perconductivity in the Ca-rich region with T(c) decreasing with increa sing the Y content, supplying a further example of superconductivity n ot related to structural modulation in BSCCO. The AF-SC transition in this system seems to be drived mostly by the Y/Ca ratio whereas the ch arge deriving from the Pb-->Bi substitution seems to be accomodated ma inly in the Bi(Pb)O layers through a partial oxydation of Pb2+ to Pb4. The existance of a similar mechanism has been confirmed by nitrogen annealing experiments showing a stabilizing effect of lead on the T(c) .