Nanoscale phase mixture and multi-field-induced topotactic phase transformation in SrFeOx.

Nanoscale phase mixtures in transition metal oxides (TMOs) often render these materials susceptible to external stimuli (electric field, mechanical stress, etc.), which can lead to rich functional properties and device applications. Here, direct observation and multi-field manipulation of a nanoscale mixture of brownmillerite SrFeO2.5 (BM-SFO) and perovskite SrFeO3 (PV-SFO) phases in SrFeOx (SFO) epitaxial thin films are reported. The mixed-phase SFO film in its pristine state exhibits a nanoscaffold structure consisting of PV-SFO nanodomains embedded in the BM-SFO matrix. This nanoscaffold structure produces grid-like patterns in the current and electrochemical strain maps, owing to the starkly different electrical and electrochemical properties of BM-SFO and PV-SFO. Moreover, an electric-field control of reversible topotactic phase transformation between BM-SFO and PV-SFO is demonstrated by the electric field-induced reversible changes in surface height, conductance and electrochemical strain response. In addition, it is also shown that the BM-SFO --> PV-SFO phase transformation can be enabled by applying mechanical stress. This study therefore not only identifies a strong nanometric structure-property correlation in the mixed-phase SFO, but also offers a new paradigm for the multi-field control of topotactic phase transformation.