Local Structure of Proton-Conducting Lanthanum Tungstate La_28–xW_4+xO_54+δ: a Combined Density Functional Theory and Pair Distribution Function Study

Lanthanum tungstate (La 28– x W 4+ x O 54+δ ) is a good proton conductor and exhibits a complex fluorite-type structure. To gain further understanding of the short-range order in the structure we correlate the optimized configurations obtained by density functional theory (DFT) with the experimental atomic pair distribution function analysis (PDF) of time-of-flight neutron and synchrotron X-ray data, collected at room temperature. The local atomic arrangements cannot be described by means of any average symmetric structure. Tungsten forms WO 6 octahedra in alternating directions, La1 is mainly 8-fold coordinated in relatively symmetric cubes, and La2 is coordinated with 6 or 7 oxygens in heavily distorted cubes. Both DFT and PDF confirm that the excess tungsten ( x ) is incorporated in La2 ( 1 / 4, 1 / 4, 1 / 4 ) sites in the La 27 W 5 O 55.5 composition. This additional tungsten can be considered as a donor self-dopant in the material and has implications to the conducting properties and the defect structure.