Effect of acceptor size and hole degeneracy on oxygen nonstoichiometry of La2NiO4 + δ

It is conjectured that ever-hyperstoichiometric La2NiO4 + δ tends to incorporate oxygen interstitials in the rocksalt layers to mitigate the tensile/compressive stress at the LaO–rocksalt/LaNiO3–perovskilte layer interface due to their lattice mismatch, and the unusual positive deviation from ideal of its defect structure is due to the degeneracy pressure of charge compensating holes. In order to confirm these conjectures, oxygen nonstoichiometry (δ) has been measured, by coulometric titrometry, on the systems of La1.9A0.1NiO3.95 + δ, doped with the same-valence, but different-size acceptor impurities A = Ca2 + and Sr2 +, against oxygen activity across their entire stability ranges at 800°, 900°, and 1000 °C, respectively. It is found that the smaller Ca2 + enhances oxidation leading to larger oxygen excess over the larger Sr2 +, confirming the tensile stress effect on δ-increase. The positive deviation of δ and defect structure is quantitatively attributed to the hole degeneracy, confirming it to be the entire culprit for the positive deviation. Defect chemical parameters, partial molar quantities of component oxygen and phase-stability limits of La1.9A0.1NiO3.95 + δ are documented in comparison with the undoped host.