We report here a consolidated study of structural and superconducting properties of Ca substituted R1-xCaxBa2Cu3O7-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) Tc(ρ=0) as a function of x and δ 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.
This article was originally published as: Sedky, A, Gupta, A, Awana, VPS & Narlikar, AV, Structural and superconducting properties of R1 – xCaxBa2Cu3O7 – delta with 0.50 >= x >= 0.00, Physical Review B, 1998, 58(18), 12495-12502. Copyright 1998 American Physical Society. The original journal can be found here.