Defect chemistry and ion transport in perovskite oxides probed by atomistic simulations

Energy storage and conversion devices have become essential to modern society and play a crucial role in the transition toward a more sustainable energy supply. Solid-state ionic conductors, such as perovskite oxides, are key materials in these devices. Despite significant progress in recent decades, many fundamental questions about ion transport in solids remain unanswered. In particular, the interplay between chemical composition, defect chemistry, lattice symmetry, and external conditions (e.g., temperature, oxygen partial pressure), and how these factors influence ion diffusion, is still not fully understood. This thesis addresses some of these open questions using atomistic simulations to study ion diffusion in perovskite oxides, with an emphasis on practical implications for energy storage and conversion applications. A compact guide for performing molecular dynamics simulations of ion transport in solids is presented, including practical advice on simulation setup and analysis, as well as a discussion of common pitfalls and how to avoid them. The poor stability of the superlative oxide-ion conductor Ba0.5Sr0.5Co0.8Fe0.2O2.5 (BSCF) severely limits its applicability. An alternative composition, in which Ba is replaced by Ca, was investigated with respect to structural properties and oxygen-vacancy diffusion using molecular dynamics simulations. The resulting material, Ca0.5Sr0.5Co0.8Fe0.2O2.5 (CSCF), is predicted to exhibit improved stability while maintaining similar oxygen-vacancy diffusivity, making it a promising candidate for experimental validation. Exsolution is a strategy for generating catalytically active nanoparticles on the surfaces of oxide host materials, yet the mechanisms underlying this process are not fully understood. A new model for the thermodynamics and kinetics of metal exsolution from perovskite oxides is proposed, based on the reduction of the transition-metal ion within the host lattice rather than at the surface. This model accounts for a wider range of experimental observations than previous approaches. The diffusion of oxygen in polycrystalline (La,Sr)FeO3−δ (LSF) was investigated using molecular dynamics simulations. The results show that at lower temperatures, faster grain-boundary diffusion of oxide ions occurs due to the near-complete depletion of oxygen vacancies in the bulk regions.