Role of Ca in enhancing the superconductivity of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$

It is known that ${\mathrm{Ca}}^{2+}$ substitution at the ${\mathrm{Y}}^{3+}$ site reduces the superconducting transition temperature (${\mathit{T}}_{\mathit{c}}$) of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ (Y-123) when y≃0 [Phys. Rev. B 49, 6385 (1994)]. On the other hand, in the y≃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 ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ca}}_{\mathit{x}}$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$ by annealing them in flowing Ar atmosphere at 400 °C. We find that substitution of divalent Ca at the trivalent Y site enhances the superconducting critical temperature ${\mathit{T}}_{\mathit{c}}$ 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.