Structural modulation and superconducting properties in (Bi, Pb) 2Sr2 (Y, Ca) Cu2Oz

The high-Tc superconductors based on bismuth cuprates (BSCCO) are characterized by an unidimensional structural modulation, quasi-commensurate to the fundamental cell, with a wave vector close to a 1/5a*. The (Bi, Pb)2Sr2(Ca, Y)Cu2Oz system was investigated and the existence of a modulation-free 2212 phase with a wide range of composition (Bi2-xPbxSr2Y1-yCayCu2Oz, 0<=y0.8 and x = (1-y/2)±0.2) was pointed out by electron diffraction. In comparison with Bi2Sr2CaCu2O8 the new phase exibits a larger orthorhombic distortion. The crystal structure of the modulation-free phase was determined by a single crystal X-ray diffraction. The structure differs from the fundamental one of Bi2Sr2CaCu2O8: the unit cell is primitive and a different relative arrangement of the ribbons in adjacent BiO layers was pointed out. The non-modulated Bi2-xPbxSr2Y1-yCayCu2Oz phase sti ll shows superconductivity in the Ca-rich region with Tc decreasing with increasing Y content, supplying a further example of superconductivity not related to structural modulation in BSCCO. The AF-SC transition seems to be driven mostly by the Y/Ca ratio whereas the charge deriving from the Bi/Pb substitution seems to be accomodated mainly in the Bi(Pb)O layers.