Pure Fluorine Intercalation into Brownmillerite Oxide Thin Films by Using Ionic Liquid Gating

Abstract

Electrolyte gating, exemplified by ionic liquid gating (ILG), has emerged as a promising technique for modifying the physical properties of transition-metal oxides through cation intercalations, such as H + and Li + . However, the anion insertion by the ILG remains largely unexplored, except for O 2− . In this study, we have developed an ILG-based F − insertion method to allow oxidative fluorine insertion without a loss of oxide ions. Employing a solution of tetramethylammonium fluoride tetrahydrate in 1-methyl-1propylpyrrolidinium bis-trifluorosulfonyl-imide as an electrolyte, fluoride ions were electrochemically inserted into brownmillerite SrCoO 2.5 films, resulting in the formation of a SrCoO 2.5 F 0.5 perovskite with the Co 3.5+ state. Similar to its oxide counterpart La 0.5 Sr 0.5 Co 3.5+ O 3 , the obtained SrCoO 2.5 F 0.5 is a ferromagnetic metal with a Currie temperature (T C ) of ∼110 K. Notably, this behavior was not observed in any SrCoO 3−x F y compounds prepared using conventional reactions. Additionally, this technique was applied to SrFeO 2.5 brownmillerite, leading to SeFeO 2.5 F 0.5 perovskite, which suppresses a charge disproportionation as found in the oxide counterpart La 1−x Sr x FeO 3 . The ILGbased fluorination reaction developed in this study opens new avenues for the exploration of functional oxyfluorides from oxygendeficient oxide hosts.

Mechanisms of Hysteresis and Reversibility across the Voltage-Driven Perovskite–Brownmillerite Transformation in Electrolyte-Gated Ultrathin La0.5Sr0.5CoO3−δ

“…It is now understood that this can be achieved via ionic liquids and gels, − solid electrolytes, ,− and ionic conductors, ,− utilizing these media to control the insertion/extraction of various chemical species into/out of target materials they are interfaced with. These species include oxygen ions/vacancies (O/V O ), − H ions, ,− ,,,,,,, Li ions, ,, N ions, , F ions, etc., inserted/extracted into/from a variety of target materials, spanning metals, − ,,,, oxides, − ,− ...…”