In Situ Derived Impedance–Structure Correlation during LaNiO ₃ Decomposition

We demonstrate that in situ impedance spectroscopy is a marker method to follow LaNiO₃ decomposition upon hydrogen reduction and is highly potent for the in situ detection of bulk- and surface-located chemical and structural transitions. Combined with in situ X-ray diffraction (XRD), it simultaneously proved the possibility to assess the electrochemical properties of oxygen-deficient phases and the full decomposition products La₂O₃ and Ni. In situ correlation of impedance and differential thermoanalytic data allows quantitatively pinpointing distinct exothermic peaks to LaNiO_2.5 and La₂O₃ + Ni formation. The initial impedance increase at low temperatures is related by in situ near-ambient pressure X-ray photoelectron spectroscopy to near-surface redox transformations. Equilibrium impedance investigations revealed a pronounced kinetic delay in the structural transformations at low temperatures. In situ impedance spectroscopy upon redox cycling between reductive (H₂) and oxidative (O₂) conditions allowed us to clearly discriminate between reversible and irreversible transformations and demonstrated exceptional sensitivity to surface reorganization, including the reoccupation of oxygen vacancies and recompensation of structural defects. Frequency-dependent investigations demonstrate that LaNiO₃ exhibits an inductive reactance in O₂. Formation of oxygen-deficient LaNiO_2.5 and irreversible decomposition into La₂O₃ + Ni are reflected in the frequency-dependent investigations and expressed via increasing capacitance. p-type semiconduction profoundly influences the impedance behavior of NiO in oxidative and reductive atmospheres and was found to be the key conduction contribution of LaNiO₃ decomposition at 600 °C. Our work highlights the strength of in situ impedance spectroscopy as a noninvasive, highly responsive marker for surface chemistry, defect dynamics, and bulk structural transformations during redox experiments in perovskites, as evidenced for LaNiO₃.