Effect of ca Substitution on Superconducting and Structural Properties of Gd1- XPrXBa2Cu3O7-δ

The Gd_1-X-ZPr_XCa_ZBa₂Cu₃O_7-δ (GdPrCa-123) high- T_c cuprate superconductor samples were prepared by standard solid state reaction and characterized by XRD technique. The electrical resistivity of the samples with x =0 showed reduction in T_c. with increasing in Ca content. For these samples, T_c(z) curve is nonlinear and a plateau appeared at z≈0.05-0.15. A detailed study of the structure of the system by X-ray diffraction showed that with increasing Ca concentration a-lattice parameter increased and the oxygen content increased slightly up to z=0.05, and then oxygen content stayed constant in the plateau. Moreover, it is observed that in-plane Cu(2)-O(2) bond distance increased with increase in the Ca content. The same bond distances in Pr-Ca substituted samples decreases slowly with increasing Ca doping, but it increases with increasing Ca concentration after an optimal value. Besides, the electrical resistivity of Pr-Ca-doped samples shows increases in T_c with Ca doping up to an optimal value where it begins to decrease. We suggest that, besides affecting the number of holes and overdoping, the Ca substitution give rise to interesting crystallographic changes, i.e. CuO₂ planes become flatter with successive Ca doping which it causes suppression of superconductivity. These effects result in nonlinear decrement in T_c(z).