Preparation and characterisation of ionic conductors based on LaNbO4

The lanthanum niobate system belongs to the newest systems studied as proton conductors, as its proton conductivity has been reported only recently [1]. In particular, LaNbO4-based materials are promising because of their high conductivity while being almost pure proton conductors, and for their stability in CO2-containing atmospheres and water vapour environment. The highest proton conductivity to date in the LaNbO4 system is for alkaline earth-doped lanthanum niobate, reaching the value of 10-3 Socm-1 at 800°C for the composition La0.99Ca0.01NbO4 [1]. Still, this value is one order of magnitude lower than for state-of-the-art proton conductors based on BaCeO3 and BaZrO3. These compounds crystallise in two polymorphs, a low temperature monoclinic phase and a high temperature tetragonal scheelite phase, the phase transformation occurring around 500°C [2]. In the scheelite structure, they show lower activation energy for proton conductivity, thus being favourable for applications. This work is aimed at the preparation and characterisation of powders and ceramics in the LaNbO4 system, with the final purpose of obtaining high conductivity samples by retaining the scheelite structure down to room temperature. In particular, we present results on the synthesis of powders by solid-state reaction, preparation of the corresponding ceramics and their electrical characterisation. Un-doped as well as A-site doped LaNbO4 were prepared, together with a study of the effect of non-stoichiometry on the material. In order to retain the scheelite structure, aliovalent elements were incorporated on the B-site, and, in view to increase the conductivity, co-doping was also explored. The Project FIRB2012: "INCYPIT - INnovative Ceramic and hYbrid materials for Proton conducting fuel cells at Intermediate Temperature: design, characterisation and device assembly", H.C. Starck and Treibacher companies are gratefully acknowledged. [1] R. Haugsrud and T. Norby, Nature Materials, 56, 193 (2006). [2] V.S. Stubican, Journal of the American Ceramic Society, 47, 55-58 (1964).