Ca substituted tetragonal 123 superconductor with T_c above 85 K synthesized in 1-15 atm oxygen overpressure

Abstract

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 [Ca_xY_(1-x)Ba₂Cu₃O_y], 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 T_c 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 T_c 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.