Substitution behavior and stable charge carrier species in long-bond length layered cuprates

The stabilities of charge carriers, the transport properties, and the defect structures of the layered, quadruple perovskite La₂Ba₂Cu₂Sn₂O₁₁ have been investigated by chemical substitutions, powder X-ray diffraction, and simultaneous high-temperature electrical conductivity and thermopower measurements. The in-plane copper-oxygen bond lengths, in cooperation with the copper-coordination environment, are observed to control the chemical solubilities, stable charge carrier species, and oxygen defects. Potential n-type substitutions are successful via the substitution of niobium for tin. However, oxygen defects are demonstrated to be important compensation species for the incorporation of the substituted cations and in the mediation of the charge carrier concentration with the ambient atmosphere in the near-synthesis temperature region. These materials are intrinsic semiconductors and display crossover from n-type to p-type behavior with increasing oxygen partial pressure, from 10^-5 to 1 atrn pO₂, in the temperature range from 600 to 800 °C. Furthermore, the transport parameters are shown to be composition-dependent for these compounds and asymmetric with respect to electron and electron-hole conduction, in contrast to other layered copper oxides. Specifics of the inner architecture of layered copper oxides, particularly residual internal stresses between layers, are discussed with respect to their control over the redox behavior of copper-oxygen networks, and the implications of these findings are elaborated on with respect to the realization of new n-type superconductors.