Proton migration barriers in BaFeO3−δ – insights from DFT calculations

Abstract

Proton migration in the triple conducting perovskite BaFeO_3−δ is investigated using first-principles density functional theory calculations. Oxygen-deficient BaFeO_3−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 BaFeO_2.75 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 BaFeO_2.75 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.