A combined approach for comprehensive explanation of defect’s thermodynamics in highly non-stoichiometric oxides: case of RBaCo2O6-δ family

The thermodynamics of defect formation in layered perovskite-like cobaltites RBaCo2O6-δ (R = La, Nd, Pr, Sm, Gd, Eu, Ho and Y) was investigated via two distinct methods – (I) processing experimental data on oxygen non-stoichiometry using a quasichemical model and (II) employing a combined approach within the framework of density functional theory (DFT) under general gradient (GGA) and quasiharmonic Debye (QHDA) approximations. The outcomes of the first-principles calculations exhibit a plausible agreement with the fitting results, thereby confirming the validity of the developed model. The study reveals that key thermodynamic parameters governing defect formation, such as the enthalpy and entropy of oxygen exchange, as well as the enthalpy of oxygen disordering over nonequivalent crystallographic positions, exhibit correlations with the size of the rare-earth element R. In contrast, the enthalpy of charge disproportionation is demonstrated to be independent of the nature of the R atom. In Addition, it is suggested that when R = Eu, partial reduction of the latter may be observed in the lattice of EuBaCo2O6-δ. The findings indicate that, at a first approximation, the parameters of defect formation in RBaCo2O6-δ remain relatively independent on temperature and oxygen non-stoichiometry δ. The employed combined approach has been demonstrated to be reliable for assessing the thermodynamic functions of defect formation at elevated temperatures.