High temperature electrical properties and defect chemistry of La_2-xCa_xCuO_4-y superconductors - II. Defect structure modeling

The defect structure of La_2-xCa_xCuO_4-y superconductors was examined by fitting electrical property and oxygen nonstoichiometry data to a variety of models. Those involving charged oxygen vacancies, whether isolated or associated, were inadequate for simultaneously describing the dependence of both properties on doping level and oxygen partial pressure. Neutral oxygen vacancies and their association with dopants were invoked, resulting in a successful reconciliation of all existing experimental data. The defect types and configurations can be rationalized in terms of the bond-mismatch between the (LaO)₂ and CuO₂ layers in the La₂CuO₄ structure. The defect associates proposed may be responsible for the suppression in T_c relative to the La_2-xBa_xCuO_4-y and La_2-xSr_xCuO_4-y systems.