Doping- and Strain-Dependent Electrolyte-Gate-Induced Perovskite to Brownmillerite Transformation in Epitaxial...

Much recent attention has focused on the voltage-driven reversible topotactic transformation between the ferromagnetic metallic perovskite (P) SrCoO_3-δ and oxygen-vacancy-ordered antiferromagnetic insulating brownmillerite (BM) SrCoO_2.5. This is emerging as a paradigmatic example of the power of electrochemical gating (using, e.g., ionic liquids/gels), the wide modulation of electronic, magnetic, and optical properties generating clear application potential. SrCoO₃ films are challenging with respect to stability, however, and there has been little exploration of alternate compositions. Here, we present the first study of ion-gel-gating-induced P → BM transformations across almost the entire La_1-SrCoO₃ phase diagram (0 ≤ x ≤ 0.70), under both tensile and compressive epitaxial strain. Electronic transport, magnetometry, and operando synchrotron X-ray diffraction establish that voltage-induced P → BM transformations are possible at essentially all x, including x ≤ 0.50, where both P and BM phases are highly stable. Under small compressive strain, the transformation threshold voltage decreases from approximately +2.7 V at x = 0 to negligible at x = 0.70. Both larger compressive strain and tensile strain induce further threshold voltage lowering, particularly at low x. The P → BM threshold voltage is thus tunable, via both composition and strain. At x = 0.50, voltage-controlled ferromagnetism, transport, and optical transmittance are then demonstrated, achieving Curie temperature and resistivity modulations of ∼220 K and at least 5 orders of magnitude, respectively, and enabling estimation of the voltage-dependent Co valence. The results are analyzed in the context of doping- and strain-dependent oxygen vacancy formation energies and diffusion coefficients, establishing that it is thermodynamic factors, not kinetics, that underpin the decrease in the threshold voltage with x, that is, with increasing formal Co valence. These findings substantially advance the practical and mechanistic understanding of this voltage-driven transformation, with fundamental and technological implications.