Point defect modeling of La₂CuO₄-based superconductors

A point defect modeling study of the oxygen nonstoichiometry and high-temperature electrical properties of La_2-xAE_xCuO_4-y (AE = Ba, Sr, and Ca) systems indicates that, whereas oxygen excess (y0) results primarily from dopant-vacancy associates at large x; isolated oxygen vacancies are apparently minority defect species. The onset of oxygen deficiency due to dopant-vacancy associates occurs at x≈0.15 in the AE = Ba and Sr systems but at x≈0.05 in the AE = Ca system. Ideal ionic point defect behavior persists to x≈0.4 in the AE = Ba and Ca systems and to x≈0.5 in the AE = Sr system. A premature flattening of the electrical properties above x≈0.2 occurs in the AE = Ba and Sr systems. Reasons for the extensive range of ideal ionic behavior and for the onset of degenerate electronic behavior are discussed, as are ramifications for high-T_c superconductivity in these materials.