High-temperature superconductivity in YBa/sub 2/Cu/sub 3/O/sub 7/ and related perovskite oxides: Some aspects of solid state chemistry

Thermogravimetry and Cu K-absorption edge measurements show that the superconducting YBa/sub 2/Cu/sub 3/O/sub 7/ sample (annealed in O/sub 2/) contains more oxygen than the nonsuperconducting sample annealed in N/sub 2/, the former has a less distorted structure with the actual composition not far from the ideal one. Oxides of the formula Y/sub 1-x/Ba/sub 2/Cu/sub 3/O/sub 7/ (x = 0.05, 0.10 and 0.15) annealed in oxygen also possess the perovskite structure and show high-T/sub c/ superconductivity. The superconducting transition temperature seems to generally increase with the nominal Cu/sup 3+//Cu/sup 2+/ ratio in the superconducting copper oxides. Control of the Cu/sup 3+//Cu/sup 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-T/sub c/ copper oxides