Microstructural Aspects of the Ionic Transport Properties of Strontium-Substituted Lanthanum Cobaltites
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Microstructural Aspects of the Ionic Transport Properties of Strontium-Substituted Lanthanum Cobaltites W. Sitte, E. Bucher, W. Preis, I. Papst1, W. Grogger1, and F. Hofer1 Institute of Physical Chemistry, University of Leoben, A-8700 Leoben, Austria 1 Research Institute for Electron Microscopy, Graz University of Technology, A-8010 Graz, Austria ABSTRACT The ionic conductivity and microstructure of selected compositions of the solid solution La1-xSrxCoO3-δ (LSC) were examined with respect to possible vacancy ordering phenomena. Homogeneous samples of LSC were prepared by the glycine nitrate process. The ionic conductivity was obtained as a function of the oxygen partial pressure (-3.5 ≤ log[p(O2)/atm] ≤ 0.5) using a recently developed galvanostatic polarization technique. At 825°C the p(O2)dependence of the ionic conductivity of La1-xSrxCoO3-δ (x = 0.4 and 0.6) shows a distinct maximum. Although this behavior has yet to be explained unambiguously it is indicative of decreasing mobility of ionic charge carriers, e. g. due to cooperative vacancy ordering. From the temperature dependence of the ionic conductivity of La1-xSrxCoO3-δ (x = 0.6) activation energies at constant nonstoichiometry (0.20 ≤ δ ≤ 0.28) were obtained. As vacancy association and microstructure are presumed to play a significant role we combined the results of ionic conductivity measurements and electron microscopical investigations. HRTEM images revealed a superstructure within microdomains of about 100 nm in size.
INTRODUCTION The high ionic conductivity of (La,Sr)CoO3-δ (LSC) - superior to La1-xSrxMnO3-δ - together with its high surface exchange coefficient make this material attractive for application as intermediate temperature SOFC cathode [1,2]. Compared with the electronic conductivity [3-6] of these mixed-conducting perovskites there are only few investigations regarding the oxygen ion conductivity [7,8]. Whereas the oxygen vacancies are believed to have a tendency to disorder at elevated temperatures, the limits of ideal solution approximation and the necessity of considering interactions between point defects and clusters must be kept in mind at lower temperatures. Further, the ionic conductivity in the temperature range 700°-900°C is heavily influenced by the microstructure. For example, a sharp decrease in the temperature dependence of the ionic conductivity was reported, which has been interpreted in terms of an order-disorder type transition [7]. Vacancy ordering and formation of defect clusters in La1-xSrxCoO3-δ have been reported in a number of studies [9-12] and are frequently assumed to play an important role in controlling the oxygen ion conductivity. If association of defects takes place, an assigned fraction of oxygen vacancies should no longer be free to participate in the conduction process and thus the ionic conductivity would be lowered.
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The objective of this study was to obtain data on the ionic transport properties of two different compositions of La1-xSrxCoO3-δ (x = 0.4 and 0.6) and to elucidate the p
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