Investigation on incommensurate modulation structure and Tc in Ca-doped Bi-2201 system

A series of samples with the nominal composition of Bi2Sr2−xCaxCuO6+δ (x = 0.2, 0.4, 0.6, 0.8, 1.0) ceramics are synthesized by sol-gel growth. The microstructure and the incommensurate modulation structure (IMS) were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD) methods. The superconductivity and the onset temperature of superconducting transition (Tc,onset) were analysed. In the whole Ca-doping range, an IMS in b∗-c∗ planes can be identified and the values of Tc,onset are all exceed 80 K. With an increase of x, the b-component of modulation vector q∗, the basic structural parameters b and c of Bi-2201 phase have a trend of fall first then rise, whereas the c-component of modulation vector, the angle θ between q∗ and b∗ vectors and the values of Tc,onset are reversely varied. It can be verified that Ca concentration x = 0.6 is an extreme value, at which the distortion of the CuO2 layers makes the concentration of mobile carriers and the value of Tc,onset optimizing.