Defect structure and redox energetics of NdBaCo2O6-δ

Abstract The oxygen content in the double perovskite NdBaCo2O6-δ was measured as a function of oxygen partial pressure (1–10−5 atm) at different temperatures (623–1323 K) using thermogravimetry analysis (TG) and coulometric titration technique. The defect structure model of the oxide studied was proposed and successfully verified using the pO2 – T – δ data, yielding the enthalpies and entropies of the defect reactions involved. Standard enthalpy of formation from elements ( Δ f H 298 ∘ ) was measured for quenched NdBaCo2O6-δ samples with different oxygen content. Regarding the Δ f H 298 ∘ (6-δ) dependences, two important observations were made. First, the Δ f H 298 ∘ (6-δ) consists of two linear fragments with different slopes reflecting different modes of oxygen exchange, accompanied by either reduction of cobalt from Co+3 to Co+2 or from Co+4 to Co+3 in the ranges 5.0 ≤ 6-δ ≤5.5 and 5.5  Δ f H 298 ∘ (6-δ) values and those calculated using the partial molar enthalpy of oxygen, which was evaluated using the defect structure model, agree well with each other. This can be regarded as an independent verification of both the model and the related defect reaction thermodynamics.