Doping- and Strain-Dependent Electrolyte-Gate-Induced Perovskite to Brownmillerite Transformation in Epitaxial La_1-xSr_xCoO_3-δ Films

The reversible topotactic transformation between perovskite (P) SrCoO_3-δ and oxygen-vacancy-ordered brownmillerite (BM) SrCoO_2.5 has attracted much attention recently due to wide modulation of electronic, magnetic, and optical properties, particularly when voltage-triggered in electrolyte gating. SrCoO₃ is relatively unstable, however, and there has been little exploration of alternate compositions. Here, we present the first study of ion-gel-gating-induced P to BM transformations across almost the entire La_1-xSr_xCoO_3-δ phase diagram (0 < x < 0.7). Electronic transport, magnetometry, and operando synchrotron X-ray diffraction establish the P to BM transformation at nearly all x, including x < 0.5, where both P and BM are highly air-stable. The transformation threshold voltage decreases significantly with x, but also with epitaxial strain (both tensile and compressive), providing desirable tunability for devices. These results are analyzed in the context of oxygen vacancy formation energies, diffusion coefficients, and recent theory, establishing that thermodynamics, not kinetics, underpins the decrease of the threshold voltage with x. These findings substantially advance the understanding of this voltage-driven transformation, with both fundamental and technological implications.