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

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 tie 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.