Role of Hidden Grenier Phases in Topotactic Phase Transitions in La_1–<i>x</i>Sr_<i>x</i>CoO_3−δ

The perovskite cobaltites La_1-<i>x</i>Sr_<i>x</i>CoO_3-δ have been proposed as promising candidates for neuromorphic devices, since in this class of materials a topotactically driven metal-insulator transition (MIT) can be triggered by a moderate applied voltage. The control and minimization of such voltages is important to optimize the efficiency of neuromorphic devices. By using a combination of density functional theory calculations and operando X-ray diffraction measurements on electrolyte-gated epitaxial films, here we investigate the impact of hidden Grenier phases on the nonmonotonic change in threshold voltage for varying Sr concentrations. We show that the threshold voltage for the reduction of La_1-<i>x</i>Sr_<i>x</i>CoO₃ to brownmillerite La_1-<i>x</i>Sr_<i>x</i>CoO_2.5 is influenced by the presence of intermediate La_1-<i>x</i>Sr_<i>x</i>CoO_2.67 Grenier phases, which are challenging to detect. We discuss how the stability of these Grenier phases depends on the Sr concentration, cation ordering and the epitaxial strain applied to La_1-<i>x</i>Sr_<i>x</i>CoO_3-δ films. In particular, our calculations show that by applying a biaxial strain of varied strength, one may obtain either metallic or insulating intermediate Grenier phases, which in turn may be controlled by the choice of the substrate. Our findings provide fresh insights into the importance of atomic scale control of topotactic transitions in La_1-<i>x</i>Sr_<i>x</i>CoO_3-δ films for neuromorphic computing applications.