Oxygen nonstoichiometry in La2Ni(M)O4+δ (M = Cu, Co) under oxidizing conditions

The oxygen hyperstoichiometry of La2NiO4+δ, La2Ni0.9Co0.1O4+δ and La2Ni0.8Cu0.2O4+δ with K2NiF4-type structure was studied by thermogravimetric analysis and coulometric titration at 923-1223 K in the oxygen partial pressure range 8×10−5 to 0.6 atm. The p(O2)-T-δ diagrams can be described using a statistical thermodynamic approach, relating the strongly non-ideal behavior of lanthanum nickelate-based compounds to the coulombic repulsion of oxygen interstitials and interaction of holes localized on the B-site cations. The relationships between defect concentrations and chemical potentials were expressed analyzing the configuration probabilities via binomial distributions and using the corresponding number of states for the discrete Fermi-Dirac distribution. The results suggest that the distance between two interstitials cannot be shorter than the a parameter of K2NiF4-type unit cell. The repulsion of holes leads to similar phenomena, whilst the hole energy levels are determined by the interaction with other p-type charge carriers located in the second B-site cation coordination sphere. The sites occupied by nickel and copper cations in La2Ni0.8Cu0.2O4+δ seem essentially equivalent from an energetic point of view, within the limits of experimental error. In the case of La2Ni0.9Co0.1O4+δ, however, the description of the p(O2)-T-δ diagram requires also to consider hole trapping by cobalt, which forms stable Co3+ existing in equilibrium with the electronic subsystem. The processes resulting in formation of oxygen vacancies and Cu+ states in the perovskite planes have no statistically significant effects on the oxygen nonstoichiometry. Doping-induced lattice expansion of La2NiO4-based phases favors the oxygen de-intercalation processes due to metal-oxygen bond weakening.