Structural and superconducting properties of R_1-xCa_xBa₂Cu₃O_7-δ with 0.50>~x>~0.00

Abstract

We report here a consolidated study of structural and superconducting properties of Ca substituted R_1-xCa_xBa₂Cu₃O_7-δ system, with R=Er, Y, Sm, and Nd, and for 0.50>~x>~0.00. Interestingly, with increasing x, Er/Y samples show a behavior significantly different from Sm/Nd. In particular (1) orthorhombic distortion of R:123 lattice is found to be little affected for Er/Y samples with increasing x in comparison to Sm/Nd samples, (2) although Ca substitution leads to oxygen depletion for all the four sets of samples, the average Cu valence is, in general, found to remain invariant with x, (3) ρ(T) is found to show an upward curvature for Er/Y samples, which is absent for the Sm/Nd samples, and (4) T_c(ρ=0) as a function of x and d shows a much steeper decrease in Er/Y based samples, than in Sm/Nd based ones. Our results unequivocally point to a different structural order or disorder in Er/Y samples as compared to Sm/Nd based samples. We suggest that different site preferences of oxygen vacancies, predominantly created in CuO₂ planes (CuO chains) of Er and Y (Sm and Nd) based samples, might be responsible for the observed difference in the behavior. This contention is supported by a host of other considerations and experimental observations.