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Proton migration in the triple conducting perovskite BaFeO3−δ is investigated using first-principles density functional theory calculations. Oxygen-deficient BaFeO3−δ exhibits pronounced lattice distortions that entail different chemical environments of lattice oxygen ions and thus different proton migration pathways. We systematically sampled these proton pathways and identified key structural parameters determining the height of the migration barrier. The calculated average migration barrier for proton transfer in Jahn–Teller distorted BaFeO3 is 0.22 eV. Analysis of geometric changes and chemical bonding in individual proton trajectories indicates that proton transfer occurs as a two-step process: an early stage where the energy change is mainly governed by the approach of donor and acceptor oxygen ions (the O–H bond is hardly stretched), and a second stage near the transition state where the O–H bond is broken. The calculated average migration barrier in oxygen deficient BaFeO2.75 is 0.18 eV, with a broad range of different barriers due to the increased lattice distortions caused by oxygen vacancies. The decrease in migration barrier with increasing oxygen deficiency could be attributed to the annihilation of oxygen (ligand) holes rather than to volume expansion upon reduction. Considering all calculated barriers in BaFeO3 and BaFeO2.75 we find important correlations of the migration barrier height with the initial separation of donor and acceptor oxygen ions, and the O–H bond length. While this co-dependence reflects the two-step nature of proton transfer, it is also helpful for the optimization of triple conducting oxides for various electrochemical applications. [ABSTRACT FROM AUTHOR]

Oxides with mixed protonic–electronic conductivity are relevant as oxygen electrodes for protonic ceramic fuel cells and electrolyzers. We investigate the modification of the electronic structure of (Ba,La)(Fe,Zn,Y)O3−δ when iron is partially replaced by Zn2+ or Y3+ and when it changes between 3+ and 4+ formal oxidation states. With a combination of X-ray Raman scattering (O K-edge) and X-ray absorption near edge structure (Fe, Zn, and Y K-edges) and the corresponding simulations reproducing the spectroscopic data, we quantitatively analyze the degree of covalency of the Fe–O bonds in reduced (Fe mainly 3+) and oxidized (Fe mainly 4+) materials. The simulations employ two semi-empirical parameters to account for the electronic perturbations arising from Y and Zn doping and/or a change of the iron oxidation state. In the case of large structural and electronic rearrangements, the explicit consideration of 5-fold coordinated iron proved necessary. The observed lower Fe–O bond covalence in Zn- and Y-doped samples can be related to their increased proton uptake. The results discussed and the methodology used for quantifying the covalency of the bond between transition metals and oxygen is expected to be applicable also for other perovskites. It can thus serve as a tool for further optimization of oxygen electrode materials for protonic ceramic fuel and electrolyzer cells. [ABSTRACT FROM AUTHOR]