| Summary: | We report a study of nanophases in the La<sub>2</sub>O<sub>3</sub>–MO<sub>3</sub> (M = Mo, W) systems, which are known to contain a variety of good oxygen-ion and proton conductors. Mechanically activated La<sub>2</sub>O<sub>3</sub> + MO<sub>3</sub> (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 La<sub>15</sub>M<sub>8.5</sub>O<sub>48</sub> (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 ~480–520 °C for La<sub>15</sub>Mo<sub>8.5</sub>O<sub>48</sub> and ~685–760 °C for La<sub>15</sub>W<sub>8.5</sub>O<sub>48</sub>, respectively. The crystallite size of the nanocrystalline tungstates is ~40 nm, and that of the nanocrystalline molybdates is ~50 nm. At higher temperatures (~630–690 and ~1000 °C), we observe irreversible reconstructive phase transitions of hexagonal La<sub>15</sub>Mo<sub>8.5</sub>O<sub>48</sub> to tetragonal γ-La<sub>2</sub>MoO<sub>6</sub> and of hexagonal La<sub>15</sub>W<sub>8.5</sub>O<sub>48</sub> to orthorhombic β-La<sub>2</sub>WO<sub>6</sub>. We compare the temperature dependences of conductivity for nanoparticulate and microcrystalline hexagonal phases and high-temperature phases differing in density. Above 600 °C, oxygen ion conduction prevails in the coarse-grained La<sub>18</sub>W<sub>10</sub>O<sub>57</sub> (phase I, 6H polytype) ceramic. Low-density La<sub>15</sub>W<sub>8.5</sub>O<sub>48</sub> and La<sub>15</sub>Mo<sub>8.5</sub>O<sub>48</sub> (phase II, 5H polytype) nanoceramics exhibit predominantly electron conduction with an activation energy of 1.36 and 1.35 eV, respectively, in dry air.
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