Oxygen non-stoichiometry and defect thermodynamics in La2Ni0.9Fe0.1O4+δ

The oxygen hyperstoichiometry of K2NiF4-type La2Ni0.9Fe0.1O4+δ, studied by thermogravimetric analysis and coulometric titration in the oxygen partial pressure range 6×10−5–0.7 atm at 923–1223 K, is considerably higher than that of undoped lanthanum nickelate. The p(O2)-T-δ diagram of iron-doped lanthanum nickelate can be adequately described by introducing point-defect interaction energy in the concentration-dependent part of defect chemical potentials and accounting for the site-exclusion effects. The critical factors affecting the equilibrium oxygen incorporation process include coulombic repulsion of interstitial anions, trapping of the p-type electronic charge carriers by iron, and interaction between Fe3+ and holes localized on nickel cations. Due to low chemical expansion of La2Ni0.9Fe0.1O4+δ lattice, the thermodynamic functions governing oxygen intercalation, site-blocking factors and hole mobility are all independent of the defect concentrations. The predominant 3+ state of iron cations under oxidizing conditions was confirmed by the Mössbauer spectroscopy. The stability of La2NiO4-based phase in reducing atmospheres is essentially unaffected by doping.