Doping- and Strain-Dependent Electrolyte-Gate-Induced Perovskite to Brownmillerite Transformation in Epitaxial La_1–<i>x</i>Sr_<i>x</i>CoO_3−δ Films

Abstract

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, <i>e.g.</i>, 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-<i>x</i>Sr_<i>x</i>CoO₃ phase diagram (0 ≤ <i>x</i> ≤ 0.70), under both tensile and compressive epitaxial strain. Electronic transport, magnetometry, and <i>operando</i> synchrotron X-ray diffraction establish that voltage-induced P → BM transformations are possible at essentially all <i>x</i>, including <i>x</i> ≤ 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 <i>x</i> = 0 to negligible at <i>x</i> = 0.70. Both larger compressive strain and tensile strain induce further threshold voltage lowering, particularly at low <i>x</i>. The P → BM threshold voltage is thus <i>tunable</i>, <i>via</i> both composition and strain. At <i>x</i> = 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 <i>x</i>, 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.