Hydrogen induced phase transition and emergent properties in SrFeO_x

Ionic modulation, in particular that by hydrogen intercalation, has become a powerful method to tune the material’s properties in recent years. Though the SrFeO_x system is akin to SrCoO_x that can be protonated to the HSrCoO_2.5 phase, it remains a challenge for the hydrogenation of SrFeO_2.5. In this work, starting from the perovskite SrFeO_3-δ, we achieved the hydrogen intercalation and obtained a 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, i.e., the perovskite SrFeO_3-δ is the prerequisite of the hydrogen-induced phase transition. Then, by realization of the hydrogenation, the complete phase transition cycle among the perovskite SrFeO_3-δ, brownmillerite SrFeO_2.5 and the hydrogenated HSrFeO_2.5 phase, are finished. Upon hydrogenation, SrFeO_3-δ exhibits a remarkable 9.4% lattice expansion, and its electronic state undergoes a multi-step evolution, transforming from pristine helical antiferromagnetic insulator to a bad metal, and eventually returning to an antiferromagnetic insulator. Moreover, based on the obtained results we also successfully fabricated the microscale patterns with varied surface morphology and electric conductivity that can be used to construct electronic devices. This work provides a different pathway to modulate the properties of correlated and functional materials.