Oxygen nonstoichiornetry in La2Ni(M)O4+delta (M = Cu, Co) under oxidizing conditions


The oxygen hyperstoichiometry of La2NiO4+delta, La2Ni0.9Co0.1O4+delta and La2Ni0.8Cu0.2O4+delta with K2NiF4-type structure was studied by thermogravimetric analysis and coulometric titration at 923-1223 K in the oxygen partial pressure range 8 x 10(-5) to 0.6 atm. The p(O-2)-T-delta 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+delta seem essentially equivalent from an energetic point of view, within the limits of experimental error. In the case of La2Ni0.9Co0.1O4+delta, however, the description of the p(O-2)-T-delta 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. (c) 2005 Elsevier SAS. All rights reserved.



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Chemistry; Physics


Naumovich, EN; Patrakeev, MV; Kharton, VV; Yaremchenko, AA; Lopinovich, DI; Marques, FMB

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