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+δ, where nearly one tungsten (x ∼ 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 H2 separation membranes.