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Oxygen non-stoichiometry profoundly impacts the electrical, magnetic, and catalytic properties of metal oxide. Limited by the low mass and volume of thin oxide films, conventional quantification methods, such as thermogravimetry, are not directly applicable. While chemical capacitance has been successfully applied to monitor oxygen non-stoichiometry in thin oxide films, detailed a-priori understanding of the defect chemistry is often very helpful in its interpretation. In this study, changes in non-stoichiometry in Pr doped CeO2 (PCO) thin films are measured by coulometric titration. I-V titration measurements are performed on electrochemical cells, over the temperature range from 550 to 700 ℃, oxygen partial pressure range from 10-4 to 0.21 atm, and bias range of -50 mV to 50 mV, to extract changes in stoichiometry. The results agree well with values obtained by chemical capacitance, demonstrating the utility in applying coulometric titration to investigate oxygen non-stoichiometry in oxide thin films.

Densification and sintering of CeO[subscript 2] nanoparticles and their electrical properties were simultaneously studied as a function of temperature in controlled atmosphere using a modified dilatometer. CeO[subscript 2] nanoparticles simultaneously shrink and become more resistive upon initial heating, associated with desorption of water. The electrical conductance G at 300–550 °C revealed a pO[subscript 2] dependence described by log (G) = A + n × log (pO[subscript 2]) with n ~ −1/6, consistent with n-type conduction. The results were analyzed with a defect equilibrium model based on the reduction of ceria and formation of doubly ionized oxygen vacancies and electrons. The activation energy was found equal to (1.3 ± 0.1) eV, which results in an enthalpy of reduction of (2.7 ± 0.4) eV, considerably lower than that for bulk ceria (~4.5 eV). The coarsening of particles during heat treatment at 800 °C were analysed assuming grain boundary diffusion-limited sintering. Although the coarsened powder shows a similar pO[subscript 2] dependence, the activation energy was considerably higher (1.9 ± 0.1) eV, leading to a reduction enthalpy of (4.5 ± 0.4) eV. The decrease in the enthalpy of reduction with decreasing particle size is consistent with the increasing fraction of oxide ions residing at the surface. Alternate interpretations based on space charge effects and surface adsorption/desorption were considered and found to be less consistent with the experimental results.

Key physical and chemical properties of functional materials depend largely on the existence of structural and electronic defects within their crystalline lattices or amorphous structure. This special issue on Defects & Relaxation Processes in Crystalline and Amorphous Solids is dedicated to the late Professor Arthur Stanley Nowick, a pioneer in the modeling and characterization of defect formation and transport by anelastic and dielectric relaxation behavior and ionic conductivity. Professor Nowick made seminal contributions to our understanding of fast oxygen and proton conduction in oxides, alkali metal ion transport in quartz and glasses, diffusion in metals and metal halides, dislocation motion and generation in metals, morphological stability, surface structure and the application of dilatometry and X-ray diffraction for the identification and monitoring of defects in solids. [1] He was highly regarded for the clarity of his writing, having authored two authoritative books in the defects field including, Anelastic Relaxation in Crystalline Solids, (coauthored with Brian S. Berry) in 1972, [2] and Crystal Properties via Group Theory in 1995. [3] His career was split between academia and industry, serving as Assistant/Associate Professor of Metallurgy, Yale Univ. (1951–57) and later as Professor of Metallurgy and Materials Science & Henry Marion Howe Professor, Columbia Univ. (1966–93). While at Columbia, he received the Great Teacher Award (1987). In the intervening years, he served as the Head of the metallurgy group at the IBM T. J. Watson Research Center. Outside of his scientific career, he was an accomplished pianist, with wide-ranging interests in classical music and jazz. An insightful description of his career and accomplishments was published in 2012 by Prof. Daniel N. Beshers, his longtime Columbia University colleague and collaborator [4].