Anionic conductors Ln 2/3 [Bi 12 O 14 ](MoO 4 ) 5 with Ln = La, Nd, Gd, Ho, Yb. Synthesis–spark plasma sintering–structure–electric properties Jean Galy a, ⁎ , Philippe Salles a , Patrick Rozier a , Alicia Castro b a Centre d'Elaboration de Matériaux et d'Etudes Structurales (CEMES/CNRS),29 rue Jeanne Marvig, BP 94347, 31055 Toulouse Cedex, France b Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain Received 15 May 2006; received in revised form 18 July 2006; accepted 31 July 2006 Dedicated to Professor Hartmut Fuess on the occasion of his 65th birthday Abstract A new series of columnar phases Ln 2/3 □ 1/3 [Bi 12 O 14 ](MoO 4 ) 5 (□ vacancy) with Ln = La, Nd, Gd, Ho and Yb have been synthesized and structurally characterized. They feature the same formula as the molybdate phase Bi 2/3 □ 1/3 [Bi 12 O 14 ](MoO 4 ) 5 and crystallize in the monoclinic system, space group P2/c. These phases are isostructural with the prototype structure Bi[Bi 12 O 14 ](MoO 4 ) 4 (VO 4 ). Pellets of this rare-earth series obtained by spark plasma sintering and measured by impedance spectroscopy show a good anionic conductivity with a parabolic evolution whose maximum is raised by the Gd species at a determined value σ =6.6×10 - 3 S cm - 1 at 980 K. © 2006 Elsevier B.V. All rights reserved. Keywords: Anionic conductors; Spark plasma sintering; LnBiMo oxides; X-rays; Impedance spectroscopy 1. Introduction Systematic studies of oxide systems based on Bi 2 O 3 and MoO 3 oxides associated with other elements like V, Ge, Si, etc. substituted for Mo [1,2] and the so-called lone pair (designed by E) elements, As(III), Pb(II), Sb(III) and Te(IV) or alkaline earth like Sr and Ba substituted for Bi [3–6], have shown their potential as good anionic conductors [2,3,7–13]. An original phase, Bi[Bi 12 O 14 ](MoO 4 ) 4 VO 4 , was found in the Bi 2 O 3 – MoO 3 –V 2 O 5 system whose synthesis, stoichiometry and crystal structure were clearly established [11–14]. In the monoclinic cell, the atomic architecture exhibits typical [Bi 12 O 14 ] columns developed along [010], surrounded by MoO 4 and VO 4 tetrahedra, the electrical balance of the network being achieved by isolated Bi atoms (Fig. 1). This prototype structure was also confirmed by creating a pure molybdate phase, Pb[Bi 12 O 14 ] (MoO 4 ) 5 , in which Pb atoms play the role of the isolated Bi [14]. These phases exhibit a typical X-ray powder pattern directly comparable to those of the phases evidenced in a large domain of the Bi 2 O 3 –MoO 3 system for stoichiometries mBi 2 O 3 :1MoO 3 with 1.267 ≤ m b 1.75. These phases were synthesized at temperatures above 700 °C. Recently, syntheses within the Solid State Ionics 177 (2006) 2897 – 2902 www.elsevier.com/locate/ssi ⁎ Corresponding author. E-mail address: galy@cemes.fr (J. Galy). Fig. 1. A polyhedral picture of HT phases projected onto the (010) plane: [Bi 12 O 14 ] column in red bordered by Bi with their lone pairs E and the “isolated” Bi (yellow balls) and (MoO 4 ) tetrahedra in blue. 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.07.059