Hydrogen in Brownmillerite Perovskites: First-Principles Insights into Energetics and Induced Electronic-Magnetic Changes

Abstract: Hydrogen uptake in brownmillerite perovskites A2B2O5 offers an (electro)chemically accessible route to tune functional properties, but mechanistic understanding and design rules for hydrogen-responsive oxides remain limited. Here we employ density functional theory (DFT) to quantify how H absorption affects electronic structure, magnetic exchange, and anisotropy in representative Sr2Fe2O5 and Sr2Co2O5 oxides. We find that hydrogenation introduces a localized electron that stabilizes near the proton, with B-site-dependent preference. The resulting lattice distortions and redistribution of charge density modify exchange coupling and cant the Neel vector, giving rise to weak ferromagnetism. We also show that absorption energies are highly sensitive to proton-electron arrangements and magnetic order, varying by up to 1 eV across different settings. This sensitivity demands consistent treatment of charge localization and spin states, together with careful choice of computational parameters. Extending to a variety of experimentally reported A2B2O5 compositions, we identify candidates with favorable H uptake and uncover a trend linking more favorable absorption to a higher B-site d-electron count. We also demonstrate that the preferred proton absorption site in these materials is governed by local O-O separations and lattice flexibility, which describe the ability of the framework to accommodate proton-induced distortions. Finally, benchmarks of universal machine-learning interatomic potentials reveal uncertainties of about 1 eV for site-resolved absorption energies, motivating descriptor-based surrogate models and targeted DFT validation. Together, these results establish practical design rules for hydrogen-responsive oxides relevant to iono-electronic devices, sensors, and electrically tunable spin functionality.