Tuning Electrochemical Response in Ion-Gel-Gated La_1-<i>x</i>Sr<i>_x</i>CoO_3-δ Films via Sr-doping and strain

Electrolyte gating has proven powerful for controlling electronic, magnetic, and optical properties of materials such as oxides. Understanding the true gating mechanisms (i.e., electrostatic vs. electrochemical), however, particularly in oxides, where O vacancy formation and diffusion is often facile, remains challenging. Here, we present a study of the Sr-doping and strain dependence of the ion-gel gate response of La_1-xSr_xCoO_3-δ (LSCO) films (0 < x < 0.7), using transport, magnetometry, and operando synchrotron X-ray diffraction. Consistent with prior work, small negative gate biases induce reversible electrostatic hole accumulation, whereas positive biases induce oxygen vacancies (V_O) [1,2]. The window of reversibility and bias threshold for V_O formation are found to systematically decrease with x, consistent with higher electrochemical activity of Co⁴⁺ than Co³⁺; tensile strain similarly promotes electrochemical response. At sufficiently high x, a topotactic transformation to a brownmillerite phase occurs. These findings are placed in the context of prior literature and discussed in terms of x-dependent variations in V_O formation enthalpy and diffusivity.