Hydrogen Orientation Induces Coordination Reconstruction and Magnetic Competition in Protonated Brownmillerite HSrCoO2.5

Abstract Hydrogen insertion has emerged as a powerful route to magneto-ionic control in complex oxides, yet the atomic-level mechanisms by which hydrogen modifies magnetic order remain incompletely understood. In the brownmillerite oxide SrCoO2.5, protonation to form HSrCoO2.5 has been reported experimentally to induce weak ferromagnetic signals below ∼100 K, while prior first-principles studies predict an antiferromagnetic ground state at stoichiometric hydrogen loadings. Using density-functional theory combined with systematic configurational sampling, we identify hydrogen orientation within the oxygen-vacancy channels as a possible intrinsic mechanism that can reconcile the calculated antiferromagnetic ground state with experimentally observed weak ferromagnetic signals, without requiring changes in hydrogen concentration or extrinsic defects. We identify a low-energy hydrogen configuration in which rotation of the O–H groups toward the tetrahedral CoO4 layers draws an apical oxygen away from a neighboring CoO6 octahedron, stabilizing a square-planar, low-spin Co2+ site. This hydrogen-induced coordination reconstruction introduces ferromagnetic superexchange pathways via orbital-orthogonality mechanisms, placing antiferromagnetic and ferromagnetic-like states in near energetic degeneracy. The resulting magnetic competition provides a microscopic route to weak ferromagnetic responses at experimentally relevant temperatures.