Controlling defects in double-layer cuprates by chemical modifications

In-situ high temperature electrical conductivity and thermopower have been measured simultaneously on a number of ordered perovskite-like oxides containing double CuO_4/2 sheets. Equilibrium measurements have been conducted as a function of oxygen partial pressure, temperature and chemical substitution in order to understand the relationships between the chemical architecture and the transport and defect properties. Data for LaBa₂Cu₂NbO₈ and LaCa₂Cu₂GaO₇ are presented and compared with those of known triple perovskite superconductors, Y_1-xCa_xSr₂Cu₂GaO₇ and YBa₂Cu₃O_7-δ, and several quadruple perovskites, Ln′Ln″Ba₂Cu₂M₂O₁₁ (Ln = Lanthanide, Y; M = Sn, Ti). These materials belong to a general family of superconductors which are constructed from similar `active' layers (double perovskite blocks of square-pyramidal copper-oxygen sheets), and interleaved with fixed valence cations in perovskite-like `conditioning' layers. Similarities in the transport properties of the non-superconducting and superconducting materials at elevated temperatures are illustrated, and the amount and types of defects, including carrier concentrations, are correlated with the internal chemistry and inner architecture of each material.