Analysis of the Atomic-Scale Defect Chemistry at Interfaces in Fluorite Structured Oxides by Electron Energy Loss Spectr
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ANALYSIS OF THE ATOMIC-SCALE DEFECT CHEMISTRY AT INTERFACES IN FLUORITE STRUCTURED OXIDES BY ELECTRON ENERGY LOSS SPECTROSCOPY Y. Ito, Y Lei1, N.D. Browning1 and T.J. Mazanec2 Department of Physics, Northern Illinois University, DeKalb, IL 60115 1 Department of Physics (M/C 273), University of Illinois at Chicago, Chicago, IL 60607 2 BP Amoco Chemicals, Naperville, IL 60566-7011 ABSTRACT Gd3+ doped Ce oxides are a major candidate for use as the electrolyte in solid oxide fuel cells operating at ~500 ˚C. Here, the effect of the atomic structure on the local electronic properties, i.e. oxygen coordination and cation valence, at grain boundaries in the fluorite structured Gd0.2Ce0.8O2-x ceramic electrolyte is investigated by a combination of atomic resolution Zcontrast imaging and electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM). In particular, EELS analyses from grain boundaries reveals a complex interaction between segregation of the dopant (Gd3+), oxygen vacancies and the valence state of Ce. These results are similar to observations from fluorite-structured Yttria-Stabilized Zirconium (YSZ) bicrystal grain boundaries. INTRODUCTION Gd3+ doped Ce oxides are highly attractive candidates as electrolytes for solid oxide fuel cells operating at ~500 ˚C [1,2]. Like YSZ, the high oxygen conductivity has a structural origin, i.e. the fluorite structure has the highest capacity for oxygen vacancies while the cation sublattice holds the cubic structure of the bulk without collapsing [3]. For their successful commercial implementation, a full understanding of the defect chemistry in the bulk and at grain boundaries is essential. In particular, the contribution of the grain boundaries to the total ionic conductivity through such effects as the segregation of dopants, vacancies and impurities is of crucial importance [4]. The route to characterizing oxide materials on this level is afforded by the combination of Zcontrast imaging [4] and EELS [5] in the STEM. These correlated techniques [6] allow direct images of crystal and defect structures to be obtained, the composition to be quantified and the effect of the structures on the local electronic properties (i.e., oxygen coordination and cation valence) to be assessed [7]. Here the effect of the atomic structure on the local electronic properties, i.e. oxygen coordination and cation valence at grain boundaries of the fluorite structured Gd0.2Ce0.8O2-x ceramic membrane material is investigated by a combination of Zcontrast imaging [5] and EELS [6] in the JEOL 2010F STEM [7]. These results are compared with atomic resolution Z-contrast imaging and EELS analyses of a model symmetric [001] tilt grain boundary YSZ [8]. The use of the model system for comparison is required to obtain a detailed understanding of the atomic scale phenomena in ceramics, as the polycrystalline nature of Gd0.2Ce0.8O2-x ceramic membrane material makes it almost impossible to locate the ideal zone-axis orientation required to image the boundary plane.
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