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 (y < 0) due to oxygen interstitials predominates as x→ 0, oxygen deficit (y > 0) 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.