Effects of Ca Substitution in High-Temperature Superconducting

Rare-earth (RE) element in the high-temperature superconductor can be partly or totally replaced by the chemically similar elements. But Tc tends to become slightly lower with decrease of the RE ionic radius. One of the possible reasons is that small ionic radius makes 123 structure unstable. We here take up YbBa2Cu3O7-δ superconductor with the second smallest rare-earth ion Yb 3+ in the Lanthanide series, and investigate substitutional effects by Ca 2+ ion of larger ionic radius than Yb 3+ . Ceramic samples of Yb1-xCaxBa2Cu3O7-δ were obtained by the standard solid state reaction method with x ＝ 0%, 5%, 10%, 15%. The electrical resistance was measured by the fourprobe method, and surface and crystal structure of samples were analyzed by electron scanning microscope and by X-ray diffraction. Although Yb pure sample is apt to melt, melting is prevented by substituting Yb ion with larger Ca ion. As a result of structural stabilization, Yb-Ca sample with small amount of Ca has indicated higher Tc than Yb-pure pristine.