Effects of calcium substitution on the superconducting properties of   R(1-x)CaxBa2Cu3Oz (R=Eu, Gd, Er; 0=<x<=0.3)polycrystalline samples
  The influence of Ca substitution on the superconducting properties of polycrystalline R1-xCaxBa2Cu3Oz (R = Eu, Gd, Er; x=0; 0.2; 0.25 and 0.3) samples has been studied by X-ray powder diffraction, ac susceptibility and dc magnetization measurements. The superconducting parameters such as critical temperature, inter- and intra-granular critical current and flux pinning are found to be strongly dependent both on Ca content and type of R element. The best combination of these parameters is found for the system Gd1-xCaxBa2Cu3Oz forming R1+yBa2-yCu3Oz clusters. The level of overdoping and the type of intergrain connection, were found to be influenced by the R element and the Ca concentration. Flux pinning in Gd1-xCaxBa2Cu3Oz is connected with the presence of R1+yBa2-yCu3Oz clusters. 
E. K. Nazarova, K. Nenkov, G. Fuchs, K. -H. Muller