Structure Specificity of Nanocrystalline Praseodymia Doped Ceria
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Structure Specificity of Nanocrystalline Praseodymia Doped Ceria Vladislav A. Sadykov1, V.I. Voronin2, A. N. Petrov3, Yu. V. Frolova1, V.V. Kriventsov1, D.I. Kochubei1, V. I. Zaikovskii1, H. Borchert1, S. Neophytides4. 1 Boreskov Institute of Catalysis SB RAS, Novosibirsk, Russia; 2 Institute of Metals Physics, Ural Branch of RAS, Ekaterinburg, Russia. 3 Chemical Department of Ural State University, Ekaterinburg, Russia; 4 Institute of Chemical Engineering & High Temperature Processes, Patras, Greece.
ABSTRACT The features of the structure of nanorystalline Ce1-xPrxO2-y system (0 ≤ x ≤ 0.5) prepared via a complex polymerized precursor (Pechini) route have been elucidated by using a combination of spectroscopic (XANES, XPS) and structural (TEM, neutron diffraction) methods. Within the studied range of composition, the structure of all samples air annealed at 500 oC corresponds to single-phase fluorite-like solid solution. The relative content of Pr3+ both in the bulk and in the surface layer appears to be as high as 20-50%. The Rietveld refinement revealed non-monotonous variation of structural parameters (lattice parameter, domain size, microstrain density, Ce-O and O-O distances) and residual lattice hydroxyls concentration with Pr content. Clustering of defects along with variation of the mean Pr cation radius/charge state and disordering of the surface layer/ domain boundaries appear to be responsible for the observed features. INTRODUCTION Praseodymia doped ceria (PCO) possessing a high mixed conductivity is of a great interest for such application as fuel cell electrodes, oxygen separation membranes, sensors etc. Nanocrystaline samples exhibit enhanced chemical diffusivities with a low activation energy probably associated with fast diffusion pathways along domain boundaries enriched by Pr3+ cations [1]. However, direct evidence of those cations existence by spectroscopic methods was still lacking. For oxidized samples sintered at high (1400 oC) temperatures, a model of PCO defect structure based upon the impedance measurements revealed Pr cations to be exclusively in 4+ state [2]. This implies absence of anion vacancies in the lattice of these samples. Nevertheless, even for undoped nanoceria a much larger population of defects (vacancies, electrons) was suggested as compared with that for bulk ceria [3]. Hence, for nano-PCO characterization of the bulk real structure and the surface layer composition is required. This work presents results of the structure specificity of nanocrystalline Ce1-xPrxO2-y system (0 ≤ x ≤ 0.5) prepared by polymerized precursor (M.P. Pechini: U.S. Patent 3,330,697 (1967)) route. Combination of spectroscopic (XANES, XPS) and structural (TEM and neutron diffraction) methods are applied for this purpose. EXPERIMENTAL Dispersed samples of ceria-based solid solutions with Pr content in the range of 0–50 at.% were prepared by a polymerized complex precursor route as in [4] and calcined at 500 oC. As reagents, metal nitrates solutions, ethylene glycol, citric acid and ethyl
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