Journal of the European Ceramic Society 25 (2005) 3619–3627 Sintering and electrical conductivity in fast oxide ion conductors La 2-x R x Mo 2-y W y O 9 (R: Nd, Gd, Y) Samuel Georges a , Franc ¸ois Goutenoire a , Philippe Lacorre a,∗ , Marlu C´ esar Steil b a Laboratoire des Oxydes et Fluorures, UMR CNRS 6010, Universit´ e du Maine, Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France b Laboratoire de Cristallochimie et de Physicochimie du Solide, UMR CNRS 8012, ENSCL, Universit´ e des Sciences et Technologies de Lille, B.P. 108, 59652 Villeneuve d’Ascq Cedex, France Received 6 July 2003; received in revised form 21 September 2004; accepted 24 September 2004 Available online 8 December 2004 Abstract Attrition and ball milling are used as mechanical means to reduce grain size of optimized fast oxide-ion conductors La 2-x R x Mo 2-y W y O 9 (R: rare earths). Dilatometry is used to determine the optimal sintering conditions in order to obtain high density samples (greater than 96% of relative density) with help of scanning electron microscopy to characterize their microstructure. The optimal sintering temperatures are highly dependent on the chemical composition, and therefore identical annealing temperatures do not warrant similar relative densities. Complex impedance spectroscopy show that above the transition temperature of La 2 Mo 2 O 9 at 580 ◦ C, the conductivity of all the studied compounds is lower than that of the parent compound, whereas just below the transition, in most cases the stabilization of the cubic phase increases conductivity. An interesting result is that tungsten substitution, which stabilizes La 2 Mo 2 O 9 against reduction, does not affect significantly the oxide ion conduction. © 2004 Elsevier Ltd. All rights reserved. Keywords: Milling; Sintering; Porosity; Ionic conductivity; Fuel cells; (La, Gd) 2 (Mo, W) 2 O 9 1. Introduction Several possibilities of substitution of lanthanum or molybdenum by other iso- or aliovalent elements in the new fast oxide-ion conductor La 2 Mo 2 O 9 1 have already been reported 2–9 leading to the so-called LAMOX family. Among other applications, this type of materials could be used as solid electrolytes in some electrochemical devices such as solid oxide fuel cells (SOFCs). This application requires stringent properties to be verified. Among them figure pure fast oxide- ion conduction, thermal and electrochemical stability, and high relative density to avoid gas diffusion through the elec- trolyte membrane. 10 Attempts to optimize the composition of the La 2-x R x Mo 2-y W y O 9 (R: Nd, Gd, Y) series relative to both con- ∗ Corresponding author. Tel.: +33 2 4383 2643; fax: +33 2 4383 3506. E-mail address: philippe.lacorre@univ-lemans.fr (P. Lacorre). ductivity and stability against reduction were partially successful. 6–8 If molybdenum reducibility can be overcome by tungsten substitution, 8 the real effect of chemical substi- tution on conductivity could not be measured because of the competing effects of relative density. 7 As a matter of fact, a correlation between the pellets’ density and conductivity in- cited us to follow a more careful approach in order to clearly identify and separate both effects. This concern, together with the necessity for an easy control and optimization of the electrolyte’s relative density, is at the origin of the current study. Attempts to improve the relative density of LAMOX materials through sintering of small particles powders have already been carried out successfully using different preparation techniques, such as for instance sol–gel, 11 freeze dried precursors 12 or impregnation of porous samples. 13 In order to avoid the problem of additional resistivity due to grain boundaries in sol–gel prepared samples, we have 0955-2219/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2004.09.029