Structure and properties of oxyanion derivatives of 123 cuprate superconductors: effect of substitution of carbonate and nitrate ions in the Cu(1) position of YBaSrCu₃O_7−δ

Abstract

When the Cu(1) site in orthorhombic YBaSrCu₃O_7−δ (T_c=80 K), is partly substituted by the carbonate ion (upto 50%), the crystal structure becomes tetragonal and the electron diffraction pattern shows evidence for 2a×2c superstructure; the material however is not superconducting. The same is true when Y is partly replaced by Ca as in YCaBa₂Sr₂Cu₅(CO₃)O_y. When the CO₃ group is partly replaced by NO₃ group as in the YCaBa₂Sr₂Cu₅(CO₃)_1−x(NO₃)_xO_y, the structure remains the same but superconductivity is retained. IR spectroscopic studies show that both CO₃ and NO₃ coordinate strongly and are not present as free ions in these oxyanion cuprate derivatives. Cu K-EXAFS studies on the carbonate and carbonato-nitrate derivatives confirm the presence of the oxyanions in the place of CuO₄ units in the Cu---O chains.