Phase stability and ionic conductivity in substituted La2W2O9

Different substitutions, i.e. Sr2+, Ba2+, K+, Nb5+ and V5+, have been performed in the triclinic alpha-La2W2O9 structure in order to stabilise the high temperature and better ionic conductor cubic beta-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 beta-La2Mo2O9. Nanocrystalline materials obtained by a freeze-drying precursor method at 600 degrees C exhibit mainly the beta-La2W2O9 structure, however, the triclinic alpha-form is stabilised as the firing temperature increases and the crystallite size grows. Only high levels of Ba2+ and V5+ substitutions retained the cubic form at room temperature after firing above 1100 degrees C. However, these phases are metastable above 700 degrees C, exhibiting an irreversible transformation to the low temperature triclinic alpha-phase. The synthesis, structure, phase stability, kinetic of phase transformation and electrical conductivity of these materials have been studied in the present report. (c) 2007 Elsevier Inc. All rights reserved.