Defect Chemistry of Pr Doped Ceria Thin Films Investigated by in Situ Optical and Impedance Measurements

Abstract

While oxygen defect equilibrium in functional oxide thin films has a strong impact on device performance and longevity, few methods exist for evaluating it in situ. In this study, simultaneous in situ optical transmission and electrochemical impedance spectroscopy measurements are demonstrated as powerful and convenient means for this purpose, utilizing Pr x Ce 1−x O 2−δ (PCO) with multiple oxidation states for Pr (3+ and 4+) as a model system. The Pr 4+ color center optical absorptivity increased linearly with the Pr 4+ concentration, each independently derived from optical and chemical capacitance measurements, respectively, at elevated temperatures (550−700 °C) and under controlled pO 2 (10 −4 −1 atm), validating the use of optical absorption as a convenient means for monitoring defect concentrations and oxygen nonstoichiometry. The extracted extinction coefficient for Pr 4+ (ε Pr 4+ = 5.86 ± 0.12 × 10 −18 cm 2 with y-axis intercept of 927.5 ± 207.3 cm −1 ) can now be utilized to study defect equilibria of PCO, and by extension other relevant oxide films, by optical means alone. This enables defect characterization at reduced temperatures where other characterization techniques, for example, chemical capacitance, may not be feasible.

Oxygen surface exchange kinetics measurement by simultaneous optical transmission relaxation and impedance spectroscopy: Sr(Ti,Fe)O3-x thin film case study

“…The oxygen partial pressures were measured with the aid of a homemade Nernst-type yttria-stabilized zirconia sensor operated at a higher temperature in a separate tube furnace connected to the exhaust gas line from the OTR setup furnace. Because this second furnace used a large tube, its gas flush time was much longer than that of the OTR setup [18]. For this reason, time dependences of measured oxygen partial pressures shown later in the results section do not represent the exact time dependence of the gas environment in the OTR furnace.…”

“…In a third study we apply the OTR method to understand the relationship between microstructure and surface exchange kinetics, as well as their coupled evolution over time, in STF35 films [15,16]. The OTR method has recently been applied to measure kinetics [17] and point defect concentrations [18] in films of the fluorite-structured Ce 0.9 Pr 0.1 O 2-x , and so the present work contributes to demonstrating wider applicability of this technique on a system with a different crystal, electronic, and point defect structure. Prior to these recent thin film studies, OTR had been developed to study reaction kinetics and solid state diffusion primarily in various bulk or single-crystal materials [19–23].…”

Active Tuning of Optical Constants in the Visible–UV: Praseodymium‐Doped Ceria—a Model Mixed Ionic–Electronic Conductor

“…[ 29 ] Although such measurements have been reported in the literature for some time, [ 26–28 ] there have been limited reports on how the complex refractive index of MIEC oxides can be correlated to their applicable thermodynamic and kinetic defect models. Earlier studies, correlating variations in optical absorption with corresponding changes in Pr valence and driven in turn by changes in oxygen stoichiometry, [ 30–32 ] were instrumental in demonstrating the ability to use optical absorption, in a limited light wavelength range, to monitor changes in defect properties, [ 33 ] as well as monitoring surface oxygen exchange and diffusion kinetics. [ 34,35 ] These earlier studies serve as the basis for the here reported chemical control of PCO's complex refractive index within the visible–UV spectrum.…”

“…First, the previously published thermodynamic defect model for PCO is reviewed [ 31,33,36 ] and is further developed to relate to PCO's optical properties over a wide non‐stoichiometry range. Then, results relating to the dependence of optical transmittance, and the corresponding complex refractive index, on temperature and PCO's oxidation state are reported and analyzed.…”

Effect of Pr on the electrical and chemical properties of cerium oxide prepared by combustion method

“…The deviations of the oxygen stoichiometry with oxidation state −2 are given in order to conserve the electroneutrality of CeO 2 when Pr 3+ cations are introduced in high concentration at the positions of the Ce 4+ ions. In this way, the specific defects established in the Ce 1−x Pr x O 2 systems result in the improvement of the electrochemical, catalytic and mechanical properties, achieving improved performance and greater longevity in the direct conversion of chemical energy to electrical energy in SOFC devices …”