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Operando Tracking of Oxygen-Vacancy Dynamics and Negative Capacitance in Ca-Doped BiFeO3
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DOI：10.1002/adfm.77423.png)
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Abstract
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Recent advances in mixed ionic-electronic conductors have highlighted the critical yet poorly understood role of non-stationary defect dynamics in shaping electronic functionality, motivating the need for operando methods capable of resolving ionic motion and electrical response on matched timescales. Here, we address this challenge by combining operando electrocoloration with snapshot electrochemical impedance spectroscopy (EIS) to track oxygen-vacancy transport in lateral Bi0.7Ca0.3FeO3-δ thin-film devices under direct current (DC) bias. Time-resolved optical imaging reveals the drift of oxygen vacancies and the emergence of vacancy-poor (p-type), vacancy-rich (n-type), and transient filamentary regions, which correlate directly with distinct transformations in the impedance spectra. The early response follows the Jamnik-Maier framework for mixed conductors with ion-blocking electrodes, while progressive spectral distortions require an additional resistor-capacitor (RC) branch associated with filamentary conduction, supported by Gaussian-process distribution-of-relaxation-times analysis. After the p- and n-type fronts merge, the system enters a new regime marked by a sign reversal in the low-frequency imaginary impedance and a robust negative-capacitance element (Cneg ≈ −3 × 10−7 F), consistent with a frozen, over-screened space-charge configuration in a strongly asymmetric internal p-n landscape. These results establish operando EIS-electrocoloration as a powerful platform for probing non-stationary ion-electron dynamics and negative capacitance phenomena in solid-state electrochemical oxides.
Keywords:
distribution of relaxation times
electrocoloration
equivalent circuits
negative capacitance
operando electrochemical impedance spectroscopy
oxygen vacancies
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