Characterisation of scheelite LaW0.16Nb0.84O4.08 ion conductor by combined synchrotron techniques. Structure, W oxidation state and interdiffusion

Scheelite-type materials such as LaNbO4 are increasingly attracting attention as a possible alternative to the most common fluorite and perovskite structure as ion conductors. However, they are much less used and investigated. The introduction of tungsten in lanthanum orthoniobate leads to conduction properties that are compatible with oxygen ion conductivity. In this paper, we studied the effect of the introduction of tungsten in the LaNbO4 structure. High resolution X-ray diffraction showed that in LaNb1-xWxO4þx/2 with x ¼ 0.16 the monoclinic distortion is largely suppressed and the tetragonal phase is predominant at room temperature. By XANES/EXAFS we proved that tungsten is in its 6þ valence state and no W5þ was detected. With X-ray microspectroscopy, we studied in detail with a submicrometre-probe the interdiffusion and degradation processes taking place between the material and LSM, a common electrode material, during their long-term contact at high temperatures.