ROLE OF CA IN ENHANCING THE SUPERCONDUCTIVITY OF YBA2CU3O7-Y

It is known that Ca2+ substitution at the Y3+ site reduces the superconducting transition temperature (T(c)) of YBa2Cu3O7-y (Y-123) when y congruent-to 0 [Phys. Rev. B 49, 6385 (1994)]. On the other hand, in the y congruent-to 1 sample, which is nonsuperconducting, 20 at.% Ca at the Y site makes the sample superconducting [J. Solid State Chem. 78, 192 (1989)]. We have prepared oxygen-deficient samples of the series Y1-xCaxBa2Cu3O7-y by annealing them in flowing Ar atmosphere at 400-degrees-C. We find that substitution of divalent Ca at the trivalent Y site enhances the superconducting critical temperature T(c) of the oxygen-deficient Y-123. From x-ray-diffraction studies, it is observed that the cause of such a contrasting behavior of Ca substitution is the existence of divalent Ca in two different coordination numbers in the discussed system. In the oxygen-deficient Y-123, Ca exists in eightfold coordination while in the fully oxygenated Y-123, it is sixfold coordinated. It is argued that in these compounds the coordination number of Ca is dependent on the oxygen content and the concentration of doped Ca in the material.