CA SUBSTITUTED TETRAGONAL 123 SUPERCONDUCTOR WITH TC ABOVE 85-K SYNTHESIZED IN 1-15 ATM OXYGEN OVERPRESSURE

We report studies on Ca substitution in the Y-Ba-Cu-O system at elevated oxygen pressure. We find that elevated P(O2) increases the solubility range of Ca, and that Ca stabilizes the 123 phase over 247 or 124 in which the oxygen content is less variable. In the 123 phase [CaxY(1-x)Ba2Cu3Oy], the orthorhombic distortion decreases systematically as Ca increases, and a, b and c/3 become equal at x≈0.25, as measured by X-ray diffraction. Then the structure remains tetragonal (CaT-123) throughout the extended range of Ca solubility beyond x≈0.25. The synthesis of CaT-123 has been achieved in oxygen overpressure as small as 1 bar. Doping by Ca is compensated by a corresponding reduction in oxygen content, and Tc remains consistently high in CaT-123 (80-86 K), comparable to orthorhombic 123, and in striking contrast to the well-known oxygen deficient tetragonal 123. This work establishes that the controlling factor in determining Tc in this class of superconductors is the optimum oxidation (or formal average copper valency), and not any specific oxygen content, ordering into chains, or orthorhombic distortion. © 1990.