Fully oxygenated single phase (La1-x- zCaxPrz)(Ba1.5Sr0.5)Cu3Oy compounds with tetragonal triple-perovskite structure were prepared by a solid state reaction method. In the Pr series, The Tc of (La1- zPrz)(Ba1.5Sr0.5)Cu3Oy quickly dropped from 54 K for /z=0 and the samples became non-superconducting for /z>0.30. However, in the Ca series, the Tc of (La1- xCax)(Ba1.5Sr0.5)Cu3Oy increased from 54 K for /x=0 to 73 K for /x=0.20. With the amount of Pr ion kept constant, the Tc of (La1-x- zCaxPrz)(Ba1.5Sr0.5)Cu3Oy was also increased by increasing the amount of Ca ion in each series. Assuming 20% of Pr ions to be present in the tetravalent state, we calculated the hole concentration in the CuO2 layer, psh, for all the compounds, which was then correlated with Tc. By introducing Ca ions into La sites, Tc and the hole concentration in the CuO2 layer were both also increased. On the other hand, introducing Pr4+ ions into the La sites by fully oxygenating the samples leads to a decreased hole concentration in the CuO2 layer. The hole doping and filling effects caused by the Ca2+ and Pr4+ ions, respectively, were demonstrated. At constant psh, Tc is decreased with increasing amount of Pr ions, indicating a further deterioration of superconductivity by the Pr ions in the La sites.
Physica C: Superconductivity and its Applications 364-365, pp.575-581