Voltage-Controlled Topotactic Phase Transition in Thin-Film SrCoOx Monitored by In Situ X-ray Diffraction

Topotactic phase transition of functional oxides induced by changes in oxygen nonstoichiometry can largely alter multiple physical and chemical properties, including electrical conductivity, magnetic state, oxygen diffusivity, and electrocatalytic reactivity. For tuning these properties reversibly, feasible means to control oxygen nonstoichiometry-dependent phase transitions in functional oxides are needed. This paper describes the use of electrochemical potential to induce phase transition in strontium cobaltites, SrCoOx (SCO) between the brownmillerite (BM) phase, SrCoO₂.₅, and the perovskite (P) phase, SrCoO₃₋δ. To monitor the structural evolution of SCO, in situ X-ray diffraction (XRD) was performed on an electrochemical cell having (001) oriented thin-film SrCoOx as the working electrode on a single crystal (001) yttria-stabilized zirconia electrolyte in air. In order to change the effective pO₂ in SCO and trigger the phase transition from BM to P, external electrical biases of up to 200 mV were applied across the SCO film. The phase transition from BM to P phase could be triggered at a bias as low as 30 mV, corresponding to an effective pO₂ of 1 atm at 500 °C. The phase transition was fully reversible and the epitaxial film quality was maintained after reversible phase transitions. These results demonstrate the use of electrical bias to obtain fast and easily accessible switching between different phases as well as distinct physical and chemical properties of functional oxides as exemplified here for SCO.