High-temperature superconductivity in YBa2Cu3O7 and related perovskite oxides: Some aspects of solid state chemistry

Thermogravimetry and Cu K-absorption edge measurements show that the superconducting YBa2Cu3O7 sample (annealed in O2) contains more oxygen than the nonsuperconducting sample annealed in N2, the former has a less distorted structure with the actual composition not far from the ideal one. Oxides of the formula Y1−xBa2Cu3O7 (x = 0.05, 0.10 and 0.15) annealed in oxygen also possess the perovskite structure and show high-Tc superconductivity. The superconduccting transition temperature seems to generally increase with the nominal Cu 3+ Cu 2+ ratio in the superconducting copper oxides. Control of the Cu 3+ Cu 2+ ratio (and hence the density of states at the metal-nonmetal transition in the non-superconducting state) through the oxygen stoichiometry or an appropriate cation substitution seems to provide a basis for synthesizing high-Tc copper oxides.