Effects of the La/W ratio and doping on the structure, defect structure, stability and functional properties of proton-conducting lanthanum tungstate La28-xW4+xO54+δ. A review

The present review focuses on characteristics of lanthanum tungstate in the compositional region 25-30 mol% La2O3 in the La2O3-WO3 phase diagram. These tungstates represent an interesting family of materials for technological applications being proton or mixed proton-electron conductors depending on conditions. The material family was traditionally identified as La6WO12. However, recent efforts have shown that lanthanum tungstate can more correctly be represented by the formula La28-xW4+xO54+delta, where nearly one tungsten (x similar to 1) dissolves in lanthanum sites to form a stable composition. In the present contribution, the importance of the crystal structure and the effect of the La/W ratio on stability, defect chemistry and, accordingly, transport properties of this material are reviewed. A revisited phase diagram in this compositional region is presented. Reported doping strategies on both the A- and B-sites are discussed in view of the applicability of these materials as dense ceramic H-2 separation membranes.