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

It is conjectured that ever-hyperstoichiometric La2NiO4+delta 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 (delta) has been measured, by coulometric titrometry, on the systems of La(1.9)A(0.1)NiO(3.95+delta), doped with the same-valence, but different-size acceptor impurities A = Ca2+ and Sr2+, against oxygen activity across their entire stability ranges at 800 degrees, 900 degrees, and 1000 degrees 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 delta-increase. The positive deviation of delta 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 La(1.9)A(0.1)NiO(3.95+delta) are documented in comparison with the undoped host. (C) 2012 Elsevier B.V. All rights reserved.