Phase stability and ionic conductivity in substituted La2W2O9

Different substitutions, i.e. Sr2+, Ba2+, K+, Nb5+ and V5+, have been performed in the triclinic α-La2W2O9 structure in order to stabilise the high temperature and better ionic conductor cubic β-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 β-La2Mo2O9. Nanocrystalline materials obtained by a freeze-drying precursor method at 600 °C exhibit mainly the β-La2W2O9 structure, however, the triclinic α-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 °C. However, these phases are metastable above 700 °C, exhibiting an irreversible transformation to the low temperature triclinic α-phase. The synthesis, structure, phase stability, kinetic of phase transformation and electrical conductivity of these materials have been studied in the present report.