The chemical control of superconductivity in (Y1-yCay)(Sr2-xBax)Cu2.7Co0.3O7±δ

Although the Sr solubility limit in YBa2Cu3O7 corresponds to the approximate composition Y(BaSr)Cu3O7, partial replacement of Cu at the ''chain'' sites can stabilise the Sr-containing phases. In this paper we consider Co-stabilised materials and describe the superconducting characteristics of the series Y(Sr2-xBax)Cu2.7Co0.3O7±δ for 0≤x≤2, for which Tc increases from 0K (x=0) to 34K (x=2) for samples annealed in oxygen. Significant increases in critical temperature have been achieved by annealing at 500 C in oxygen at 350 bar (e.g. Tc = 45K for x=2), and by partially replacing Y3+ ions by Ca2+ ions (Tc=84K for (Y0.7Ca0.3)Ba2Cu2.7Co0.3O7±δ). (orig.)