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. SrCoO 3 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−x Sr x CoO 3 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.

Materials with tunable infrared refractive index changes have enabled active metasurfaces for novel control of optical circuits, thermal radiation, and more. Ion‐gel‐gated epitaxial films of the perovskite cobaltite La1−xSrxCoO3−δ (LSCO) with 0.00 ≤ x ≤ 0.70 offer a new route to significant, voltage‐tuned, nonvolatile refractive index modulation for infrared active metasurfaces, shown here through Kramers–Kronig‐consistent dispersion models, structural and electronic transport characterization, and electromagnetic simulations before and after electrochemical reduction. As‐grown perovskite films are high‐index insulators for x < 0.18 but lossy metals for x > 0.18, due to a percolation insulator‐metal transition. Positive‐voltage gating of LSCO transistors with x > 0.18 reveals a metal‐insulator transition from the metallic perovskite phase to a high‐index (n > 2.5), low‐loss insulating phase, accompanied by a perovskite to oxygen‐vacancy‐ordered brownmillerite transformation at high x. At x < 0.18, despite nominally insulating character, the LSCO films undergo remarkable refractive index changes to another lower‐index, lower‐loss insulating perovskite state with Δn > 0.6. In simulations of plasmonic metasurfaces, these metal‐insulator and insulator‐insulator transitions support significant, varied mid‐infrared reflectance modulation, thus framing electrochemically gated LSCO as a diverse library of room‐temperature phase‐change materials for applications including dynamic thermal imaging, camouflage, and optical memories.