Orientation-dependent electrochemical reduction and proton evolution in the oxygen-deficient perovskite SrFeO2.5+y



 Electrochemical reactions with insertions of ions in solids depend on crystallographic orientations. We investigated electrochemical responses of (100), (110), and (111)-oriented oxygen-deficient perovskite SrFeO2.5+y
 epitaxial films in electric-field-effect transistor structures with the proton-conducting electrolyte Nafion as a gate insulator. We found that only (100)-oriented SrFeO2.5+y
 films exhibit changes associated with gate-voltage-induced electrochemical reductions. Furthermore, elastic recoil detection analysis shows that electrochemically reduced (100) films can accommodate protons, forming the proton-containing oxide H0.11SrFeO2.5+y
 . Our results show that oxygen vacancies form preferentially along the {100} axes and ion diffusion in electrochemical reactions occurs dominantly along the {100} directions in SrFeO2.5+y
 .