Room‐Temperature AFM Electric‐Field‐Induced Topotactic Transformation between Perovskite and Brownmillerite SrFeO

Reversible structural transformations between perovskite (PV) ABO3−δ and brownmillerite (BM) ABO2.5 (A = Ca2+, Sr2+; B = Fe4+/3, Co4+/3+) oxides can be induced by topotactic oxygen exchange at moderate temperatures under reducing/oxidizing conditions. The combination of a large oxide-ion conductivity and a small free energy difference between the 4+/3+ oxidation states of many 3d transition metal ions enables these topotactic transformations. Herein, it is demonstrated that the electric field produced by a voltage-biased atomic force microscopy tip can induce such transformation between PV SrFeO3−δ and BM SrFeO2.5 at room temperature and with sub-micrometer spatial resolution. Interestingly, the structural transformation is kept after the electric field is removed, allowing a nonvolatile control of the local chemical, electrical, optical, and magnetic properties. Thus, the results presented in this paper open the door for the fabrication of stable ionic-based devices through the electric field patterning of different crystallographic phases.