Study on modulation structure and superconductivity of Bi-Sr-Cu-O system doped with Ca

A series of samples with the nominal composition of Bi2Sr2-xCaxCuO6+δ (x=0.1, 0.3, 0.5, 0.7, 0.9) were prepared by sol-gel method. By means of XRD, TEM and standard four-probe technique, it was investigated that the effects of Ca doping on lattice types and parameters of Bi-2201 phase, the incommensurate modulation structure and the onset temperature of superconducting critical transition (TC, onset). It was found that Bi-2201 phase with orthorhombic symmetry can be maintained with the introduction of Ca2+ in the whole doping range. With increasing x, lattice parameter c was decreasing gradually. When x>0.3, the b* component of the modulation wave vector was decreasing while the c* component was increasing with increasing x. Experimental R-T curves show that TC,onset was the highest when x=0.3, and then has a declining trend with increasing x, which was in agreement with the variation of the b* component of the modulation wave vector and can be interpreted by the crystal misfit model.