Journal of New Materials for Electrochemical Systems 7, 51-57 (2004) © J. New. Mat. Electrochem. Systems *To whom correspondence should be addressed: Fax:+33(0)2 4383- 2643; e-mail: philippe.lacorre@univ-lemans.fr 51 1. INTRODUCTION Solid Oxide Fuel Cells (SOFCs) are thought to have specific advantages over the other types of fuel cells, among which their high performance rate. Although the high working temperature necessary to make oxide ions mobile enough in the electrolyte (around 1000°C for stabilized zirconias) is not necessarily a handicap in cogeneration processes, lowering it down to about 700°C is a current challenge. It can be achieved either by reducing the current electrolytes thickness or by finding more efficient electrolytes. Research is very The LAMOX Family of Fast Oxide-Ion Conductors: Overview and Recent Results S. Georges a , F. Goutenoire a , O. Bohnke a , M.C. Steil b , S.J. Skinner c , H.-D. Wiemhöfer d and P. Lacorre a,* a Laboratoire des Oxides et Fluorures, UMR CNRS 6010, Université du Maine, 72085 Le Mans cedex 9, France b Laboratoire de Cristallochimie et de Physicochimie du Solide, UMR CNRS 8012, ENSCL, Université de Science et Technologie de Lille, B.P. 108, 59652 Villeneuve d’Ascq Cedex, France c Centre for Ion Conducting Membranes, Department of Materials, Imperial College London, Prince Consort Road, London SW7 2BP, UK d Institute of Inorganic and Analytical Chemistry and SFB 458, University of Münster, Wilhelm-Klemm-Str. 8, 48149, Münster, Germany (Received June 3, 2003 ; received revised January 9, 2004) Abstract: A short review of our recent works and original results on the new LAMOX family of fast oxide-ion conductors based on La 2 Mo 2 O 9 are presented. The order/disorder phase transition of La 2 Mo 2 O 9 , accompanied by an increase of anion conductivity, is suppressed by most of the substituants which stabilize the highly conducting cubic phase down to room temperature. The series La 2-x R x Mo 2-y W y O 9 (R=Nd,Gd,Y) was studied with respect to the optimisation of anion conduction and stability against reduction. Tungsten substitution is a good way to minimize oxygen loss under reducing atmosphere, a drawback in applications such as electrolytes for solid oxide fuel cells. Ball-milling and dilatometric studies allowed an efficient control and improvement of the samples relative density, for a better characterisation of the oxygen diffusion properties, a first account of which is presented here. The LAMOX family exhibits among the highest oxygen tracer diffusion coefficients. A specific curvature in the conductivity curves of cubic substituted phases is interpreted as resulting from a VTF-type conduction mechanism above a certain temperature, an indication of the existence of a dynamic disorder at high T, which would freeze while cooling down. Key words: Oxygen ion conductor; SOFC; Solid electrolyte; Ionic transport number; SIMS active in the field. For instance, we have discovered [1] a new family of fast oxide-ion conductors, the so called LAMOX family, based on the parent compound La 2 Mo 2 O 9 whose anion conduction is higher than those of the best stabilized zirconias in the intermediate temperature range. Beside high anion conductivity however, other specific properties are to be fulfilled by fast oxide-ion conductors in order for them to be useful as solid electrolytes. Among them figure structural and electrochemical stability, and low reducibility. Since hexavalent molybdenum is known to be readily reducible in low oxygen pressures, we have undertaken compositional optimisation relative to anion conductivity and reducibility in the LAMOX family. Densification of these ceramics has