Investigation of Nonstoichiometry in Oxide Thin Films by Simultaneousin SituOptical Absorption and Chemical Capacitance Measurements: Pr-Doped Ceria, a Case Study

Simultaneous in situ optical absorption and electrochemical impedance spectroscopy measurements were performed, for the first time, at elevated temperature on a metal oxide thin film exhibiting oxygen nonstoichiometry, utilizing Pr 0.1 Ce 0.9 O 2−δ ( 10 PCO) as a model system. Chemical capacitance measurements, capable of providing explicit values of δ, were used to determine the optically absorbing center (Pr 4+ ) concentration and thereby the optical extinction coefficient for Pr 4+ . The absorption coefficient was found to exhibit a linear dependence on Pr 4+ concentration, validating the use of optical absorption to examine defect concentration trends and allowing derivation of the extinction coefficient ε Pr 4+ = 5.01 ± 0.14 × 10 –18 cm 2 . Values of Pr 4+ concentration derived from the chemical capacitance and corresponding trends in optical absorption were found to be self-consistent, validating the thin film defect model for 10PCO, thereby confirming that the oxygen reduction enthalpy in thin film 10PCO is lower than that in the bulk. The non-contact optical absorption technique thus provides an additional in situ method for investigating the defect equilibria of thin films and is expected to aid in confirming whether and under what conditions the defect thermodynamics of films differ from that of their bulk counterparts.