New crystal structure and characterization of lanthanum tungstate “La6WO12” prepared by freeze-drying synthesis

Abstract

Lanthanum tungstates with a La/W atomic ratio between 6 and 4.8 have been synthesized as polycrystalline materials using the freeze-drying wet-chemical precursor method. Our results show that a single phase material is obtained when the La/W ratio is between 5.3 and 5.7 (T = 1500 °C). Outside this compositional range, segregation of either La₂O₃ (La/W ≥ 5.8) or La₆W₂O₁₅ (La/W ≤ 5.2) are found. We have solved the crystal structure for the composition with a La/W nominal atomic ratio of 5.6 by combining powder X-ray and powder neutron diffraction techniques. This structure substantially differs from that previously reported for Ln₆WO₁₂ (Ln = Y, Ho). The main differences between the two structure types are the crystal symmetry, the different coordination environment of the cations and the formula unit. The formula unit can be written as La_6.63W_1.17O_13.43 (Z = 4; calculated density = 6.395 g/cm³), well in accordance with the diffraction techniques, He-pycnometry and electron probe microanalysis. These materials can be described as a face centred cubic structure with space group F3m. Lattice parameters vary between 11.173 and 11.188 Å, depending on composition. Dense ceramic materials are obtained at 1400 °C, and microanalyses measurements indicate that no significant tungsten evaporation occurs compared to the nominal values. Compositions with La₂O₃ segregation show similar conductivity values as the single phase ones, but those containing segregation of W-rich phases show a considerable drop in conductivity with increasing content of the secondary phase.