Journal of Solid State Chemistry 181 (2008) 253–262 Phase stability and ionic conductivity in substituted La 2 W 2 O 9 D. Marrero-Lo´pez à , J. Pen˜a-Martı´nez, J.C. Ruiz-Morales, P. Nu´n˜ez à Departamento de Quı´mica Inorga´nica, Universidad de La Laguna, Avda. Astrofı´sico Francisco Sa´nchez s/n, 38200-La Laguna, Tenerife, Spain Received 11 October 2007; received in revised form 21 November 2007; accepted 27 November 2007 Available online 27 December 2007 Abstract Different substitutions, i.e. Sr 2+ , Ba 2+ ,K + , Nb 5+ and V 5+ , have been performed in the triclinic a-La 2 W 2 O 9 structure in order to stabilise the high temperature and better ionic conductor cubic b-phase. This approach has been used to try to obtain a new series of ionic conductors with LAMOX-type structure without molybdenum and presumably better redox stability compared to b-La 2 Mo 2 O 9 . Nanocrystalline materials obtained by a freeze-drying precursor method at 600 1C exhibit mainly the b-La 2 W 2 O 9 structure, however, the triclinic a-form is stabilised as the firing temperature increases and the crystallite size grows. Only high levels of Ba 2+ and V 5+ substitutions retained the cubic form at room temperature after firing above 1100 1C. However, these phases are metastable above 700 1C, exhibiting an irreversible transformation to the low temperature triclinic a-phase. The synthesis, structure, phase stability, kinetic of phase transformation and electrical conductivity of these materials have been studied in the present report. r 2007 Elsevier Inc. All rights reserved. Keywords: La 2 Mo 2 O 9 ; Solid electrolyte; Nanocrystalline materials; Phase transition 1. Introduction In recent years there has been a growing interest in phases derived from La 2 Mo 2 O 9 , the so-called LAMOX compounds [1–3]. Such compounds possess high oxide ion conductivity at intermediate temperatures and comparable to doped ceria [4]. Hence, they have potential electro- chemical applications, such as oxygen sensors, dense ceramic membranes for oxygen separation, oxygen pumps and fuel cells components [5,6]. Non-substituted La 2- Mo 2 O 9 exhibits two different crystallographic polymorphs, a and b, with a reversible structural phase transition at 560 1C associated with long-range oxygen vacancy ordering [2,3]. The high temperature b-polymorph is a better conductor than the low temperature a-polymorph and it crystallises in a cubic symmetry (s.g. P2 1 3) [2]. On the other hand, the a-polymorph is an ordered superstructure relative to the more symmetric b-form with a slight monoclinic distortion (s.g. P2 1 ) [7]. The main limitations for practical applications of non- substituted La 2 Mo 2 O 9 as solid electrolyte are the phase transition and low stability under reducing conditions. The phase transition produces a drastic drop in the conductivity below 560 1C and possibly mechanical failure due to the high thermal expansion of the unit cell volume between the high and low temperature polymorphs. A wide range of substitutions have been investigated in order to stabilise the b-polymorph, such as: La 3+ by Bi 3+ [8], Ca 2+ [9,10], Ba 2+ [10–12],K + [12,13],Y 3+ [14] and rare earth elements [15–17], whereas Mo 6+ has been substituted by Nb 5+ [18,19],V 5+ , Fe 3+ , Al 3+ [20], Cr 6+ [21] and W 6+ [21–26]. Most of these substitutions stabilise the b-polymorph at room temperature, although generally they do not improve the ionic conductivity. In addition, b-La 2 Mo 2 O 9 exhibits also a limited stability range under reducing conditions due to the presence of Mo 6+ , which is partially reduced to lower oxidation states, increasing the non-desirable n-type electronic conductivity and phase degradation [27–29]. The redox stability of La 2 Mo 2 O 9 -based materials can be improved by partial substitution of Mo 6+ by W 6+ [24–26]. The similar ionic radius of Mo 6+ and W 6+ , 0.59 and 0.60 A ˚ , respectively, permits high levels of substitution, up to 80% [22], maintaining the cubic b-La 2 Mo 2 O 9 ARTICLE IN PRESS www.elsevier.com/locate/jssc 0022-4596/$ - see front matter r 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jssc.2007.11.024 à Corresponding authors. Fax: +34 922318461. E-mail addresses: damarre@ull.es (D. Marrero-Lo´ pez), pnunez@ull.es (P. Nu´n˜ez).