Synthesis of Hexagonal Nanophases in the La₂O₃-MO₃ (M = Mo, W) Systems

We report a study of nanophases in the La2O3-MO3 (M = Mo, W) systems, which are known to contain a variety of good oxygen-ion and proton conductors. Mechanically activated La2O3 + MO3 (M = Mo, W) mixtures and the final ceramics have been characterized by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) with Rietveld refinement. The microstructure of the materials has been examined by scanning electron microscopy (SEM), and their conductivity in dry and wet air has been determined using impedance spectroscopy. In both systems, the formation of hexagonal La15M8.5O48 (phase II, 5H polytype) (M = Mo, W) nanophases is observed for the composition 1:1, with exothermic peaks in the DSC curve in the range similar to 480-520 degrees C for La15Mo8.5O48 and similar to 685-760 degrees C for La15W8.5O48, respectively. The crystallite size of the nanocrystalline tungstates is similar to 40 nm, and that of the nanocrystalline molybdates is similar to 50 nm. At higher temperatures (similar to 630-690 and similar to 1000 degrees C), we observe irreversible reconstructive phase transitions of hexagonal La15Mo8.5O48 to tetragonal gamma-La2MoO6 and of hexagonal La15W8.5O48 to orthorhombic beta-La2WO6. We compare the temperature dependences of conductivity for nanoparticulate and microcrystalline hexagonal phases and high-temperature phases differing in density. Above 600 degrees C, oxygen ion conduction prevails in the coarse-grained La18W10O57 (phase I, 6H polytype) ceramic. Low-density La15W8.5O48 and La15Mo8.5O48 (phase II, 5H polytype) nanoceramics exhibit predominantly electron conduction with an activation energy of 1.36 and 1.35 eV, respectively, in dry air.