Solid State Ionics 107 (1998) 203–211 Grain boundary role in the electrical properties of La Sr Co Fe O perovskites 1 2 x x 0.8 0.2 3 2d a, a a b c * G. Garcia-Belmonte , J. Bisquert , F. Fabregat ,V. Kozhukharov , J.B. Carda a ` ´ Universitat Jaume I, Dpt. Ciencies Experimentals, E-12080 Castello, Spain b University of Chemical Technology and Metallurgy, Sofia-1756, Bulgaria c ´ ` ` ´ Universitat Jaume I, Dpt. Quımica Organica i Inorganica, E-12080 Castello, Spain Received 3 March 1997; accepted 7 November 1997 Abstract The electrical properties of mixed-conducting perovskite La Sr Co Fe O samples have been studied as function 12 x x 0.8 0.2 32d of composition, temperature and bias conditions. Attention is focused on the investigation of the grain boundary role in the conductivity reduction. Impedance spectroscopy and I –V characteristics are applied to observe the behaviour of the perovskite samples. Sr-substitution leads to an increase of the electronic conductivity of the samples and reduction of the E a value. It was established that the samples possess a typical semiconductor-like behaviour. The impedance spectra show that the charge carriers can be divided into two different categories: those which are blocked and those which are non-blocked by grain boundary related processes. In an attempt to discuss the formation and effect of depletion zones at grain boundaries, a model of back-to-back double Schottky barriers is suggested. It was shown that the grain boundaries have an influence (in our case a decrease of conductivity) on electrical properties in perovskite electronic ceramics. It was confirmed that the dielectric constant values decrease with increase of the frequency.  1998 Elsevier Science B.V. All rights reserved. Keywords: Strontium- and iron-doped lanthanum-cobaltite perovskites; Electrical properties; Grain boundary role; Schottky barrier; Composition and temperature influence 1. Introduction limitations of Co-containing perovskites, generally for SOFC cathodes application. As this simple According to Teraoka et al. [1] and Steele [2] the compound is close to the charge transfer insulator preferred perovskite for oxygen-ion conducting de- region, it does not posses any magnetic ordering, i.e. vices is LaCoO . LaCoO -based perovskite ma- it exhibits a spin-state transition as a function of the 3 32d terials are good candidates for application as oxygen temperature. To improve properties during high pumps, solid oxide full cells (SOFC) and semi- temperature fuel cell operation A- or B-type structur- permeable mixed conducting oxide membranes. al substitutions are very often used. It may be noted There is a study [3] about the advantages and also that magnetic moments appear when a Sr-dop- ing process is performed and thereby partly converts * Co(III)- to Co(IV)-state. Corresponding author. Tel.: 134 64 345700; fax: 134 64 345654; e-mail: garciag@uji.es In this context, investigations of the electrical 0167-2738 / 98 / $19.00  1998 Elsevier Science B.V. All rights reserved. PII S0167-2738(97)00541-9