Composition Dependent Anomaly in Oxygen Vacancy Formation Energy of La0.6Sr0.4Co1-YFey O3-Δ Thin Films

Abstract

Introduction Oxygen nonstoichiometry is an important parameter for perovskite type (ABO3) oxides such as La0.6Sr0.4Co1-yFeyO3-δ (LSCF) in considering their electrochemical or mechanical properties. Because LSCF is sometimes used as a thin film, it is necessary to understand the oxygen nonstoichiometry in the film. Previous reports [1][2] claimed that oxygen nonstoichiometry in a film of LSCF deposited by pulsed laser deposition (PLD) did not always match with that of the bulk. It is considered that partial molar enthalpy of oxygen, h O, was different between the bulk and the film, but its mechanism was not clarified. Careful comparison of the previous results suggests that the deviation of the enthalpy (=hO,film –hO,bulk ) is dependent on Co to Fe ratio as well as the deposition process. In this study, we have made systematic measurements of the oxygen nonstoichiometry in LSCF films deposited by PLD with the process parameters unchanged but changing the Co to Fe ratio. The results will be compared with those in the bulk, and the composition dependence of the deviation of h O will be discussed. Method The oxygen vacancy concentrations in La0.6Sr0.4Co1-yFeyO3-δ (y=0,0.2,0.4,0.6,0.8,1) films were determined from the electrochemical capacitance of the LSCF film electrode deposited on sintered Ce0.9Gd0.1O1.95 electrolyte. LSCF dense films were prepared by pulsed laser deposition with 4th harmonic of Nd:YAG laser (266 nm) under pure oxygen atmosphere of 1 Pa on heated substrates at 973K for 3h. The resulting thickness was about 500-750 nm. Counter electrode (CE) was porous La0.6Sr0.4CoO3-δ sintered before the film deposition. Reference electrode (RE) was porous Pt sintered after PLD. Electrochemical impedance spectroscopy measurements were performed at 773-973K in O2-Ar mixed gases (p(O2) = 101-105Pa). The results were well fitted to R-C parallel equivalent circuit. Results and discussions Kuhn et al. [3] modeled the nonstoichiometry of Co-rich La0.6Sr0.4Co1-yFeyO3-δ (LSCF) with an itinerant electron model [4], and Fe-rich LSCF, with a localized electron model taking into account the disproportionation of Fe [5]. The itinerant electron model is characterized by partial molar enthalpy of oxygen, h O, and the deviation from ideal solution, a. The localized electron model is characterized by the standard enthalpy change of oxygen incorporation and charge disproportionation reaction, Δ HOX and Δ Hi . The same model was applied to LSCF films in this study. Fig.1 shows the partial molar enthalpy of oxygen in terms of the difference from the standard state,Δ hO =hO -h0 O , for Co-rich LSCF, and the standard enthalpy change of oxygen incorporation reaction,Δ hO , for Fe-rich LSCF. TheΔ hO andΔ HOX of LSCF bulk are taken from Kuhn et al [3]. In Co-rich LSCF,Δ hO of the film was 20~30 kJ/mol greater than that of bulk. On the other hand, in Fe-rich LSCF,Δ HOX of the film was almost the same as that of bulk. This result suggests that Co ion in B site may play an important role in making difference between the film and the bulk. The mechanism of the increase in theΔ hO in the LSCF film will be discussed.