Hydrogen-Induced Phase Transition and Emergent Properties in SrFeO_x

In recent years, ionic modulation, particularly hydrogen intercalation, has gained attention as a powerful method for tuning the properties of materials. Although the SrFeO_x system is similar to SrCoO_x, which can be protonated to the HSrCoO_2.5 phase, it remains a challenge for the hydrogenation of SrFeO_2.5. In this study, starting from the perovskite SrFeO_3-\delta, we achieved hydrogen intercalation and obtained stable hydrogenated brownmillerite-phase HSrFeO_2.5 via Pt-catalyzed H-spillover at room temperature. The results indicate that the hydrogenation process is accompanied by the simultaneous oxygen ionic release, that is, perovskite SrFeO_3-\delta is the prerequisite for the hydrogen-induced phase transition. Subsequently, upon hydrogenation, the entire phase transition cycle among the perovskite SrFeO_3-\delta, brownmillerite SrFeO_2.5, and the hydrogenated HSrFeO_2.5 phase, is completed. Furthermore, SrFeO_3-\delta exhibits a remarkable 9.4% lattice expansion, and its electronic state undergoes a multi-step evolution, transforming from a pristine helical antiferromagnetic insulator to a bad metal, eventually returning to an antiferromagnetic insulator. Based on the obtained results, we fabricated microscale patterns with varied surface morphologies and electrical conductivities that can be used in fabricating electronic devices. This study presents a novel approach for modulating the properties of correlated and functional materials.