  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.
