Structure and properties of oxyanion derivatives of 123 cuprate superconductors: Effect of substitution of carbonate and nitrate ions in the Cu(1) position of YBaSrCu3O7−δ

When the Cu(1) site in orthorhombic YBaSrCu3O7−δ (Tc = 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)Oy. When the CO3 group is partly replaced by NO3 group as in the YCaBa2Sr2Cu5(CO3)1−x(NO3)xOy, 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.