Investigation of Diffusion Mechanisms in ABO3-δ Perovskites and related Ordered Oxygen Vacancy Structures with Nudged Elastic Band Calculations

Abstract

Perovskite oxides are of interest as a redox material for thermochemical cycles, since their ABO3 stoichiometry allows to tailor them efficiently to the needs of various applications. Due to partial reduction, which reaches significant off-stoichiometries between 0 < δ < 0.5, perovskites promise good process efficiencies. Tailoring of perovskites requires knowledge of their thermodynamic and kinetic properties, however these are assumed to change with increasing degree of reduction. During the initial stages of reduction, isolated oxygen vacancies (VO) are introduced into the perovskite lattice. With increasing δ, perovskites undergo a reversible phase transition, since the VO are starting to interact and transform the perovskite into an ordered oxygen vacancy (OOV) structure. This Thesis investigates the thermodynamics of the perovskite CaMnO3-δ and the oxygen deficient variant CaMnO2.5+δ, using first-principle density functional theory (DFT) calculations. Additionally, oxygen diffusion kinetics are investigated, using a combination of DFT and the Nudged Elastic Band (NEB) method. At the initial stage of reduction with δ ≈ 0, formation energy and diffusion of isolated VO within the defect perovskite CaMnO3-δ are investigated. For this case, an oxygen vacancy formation energy of EVO,δ≈0 = 1.94 eV (187 kJ/mol) is found. The diffusion in defect perovskite is found to be approximately isotropic, due to similar barriers Em ≈ 0.7 eV (68 kJ/mol) in all three dimensions. At the final state of reduction with δ ≈ 0.5, different variants of OOV-structures are compared with respect to their configurational energy, to find preferable structures for CaMnO2.5. As a result, two promising structures are found: the structure being known as Brownmillerite, and a structure, which has been experimentally determined for CaMnO2.5. These OOV-structures are employed to investigate the formation energy and diffusion of an oxygen atom on an interstitial lattice site within oxygen deficient CaMnO2.5+δ. As a result, the diffusion mechanics and barriers in OOV-structures do not only differ from those of VO diffusion in perovskite, but also vary among the different OOV-structures, themselves. Finally, both thermodynamic properties and oxygen diffusion mechanisms are compared for the two limiting cases δ ≈ 0 and δ ≈ 0.5, with the aim to learn about the problems and chances, which arise by the similarities and differences between both cases. This knowledge is of high importance for the purposeful tailoring of perovskites, to employ them as suitable redox materials for thermochemical cycles.