Structure-Property Relations in High T_c Oxide Superconductors: Search for Systematics

Some interesting aspects of the structure and stoichiometry of YBa 2 Cu 3 O 7+δ and other high- T c oxide superconductors are discussed. The nature of Cu and the importance of the Cu 3+ /Cu 2+ ratio in these copper oxides are pointed out. Marginal metallicity of the non-superconducting phase in these oxides as an important factor in the mechanism is indicated. Effect of Y substitution in YBa 2 Cu 3 O 7+δ by other rare earths is commented upon.