Structural and superconducting properties of R<SUB>1-x</SUB>Ca<SUB>x</SUB>Ba<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-&#948;</SUB> with 0.50&#62;~x&#62;~0.00

We report here a consolidated study of structural and superconducting properties of Ca substituted R1-xCaxBa2Cu3O7-&#948; system, with R=Er, Y, Sm, and Nd, and for 0.50&#62;~x&#62;~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) &#961;(T) is found to show an upward curvature for Er/Y samples, which is absent for the Sm/Nd samples, and (4) Tc(&#961;=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