Valentina I. Voronkova

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 La₁₅M_8.5O₄₈ (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 ~480–520 °C for La₁₅Mo_8.5O₄₈ and ~685–760 °C for La₁₅W_8.5O₄₈, 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 La₁₅Mo_8.5O₄₈ to tetragonal γ-La₂MoO₆ and of hexagonal La₁₅W_8.5O₄₈ to orthorhombic β-La₂WO₆. 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 La₁₈W₁₀O₅₇ (phase I, 6H polytype) ceramic. Low-density La₁₅W_8.5O₄₈ and La₁₅Mo_8.5O₄₈ (phase II, 5H polytype) nanoceramics exhibit predominantly electron conduction with an activation energy of 1.36 and 1.35 eV, respectively, in dry air.

Oxymolybdates La₂MoO₆, both pure and doped with magnesium, have been obtained by the flux crystallization method and solid-phase synthesis as single crystals and polycrystals, respectively. The compounds are described by X-ray diffraction analysis, thermogravimetry, and differential scanning calorimetry; their conducting properties have been investigated. A model of the structure of magnesium-doped La₂MoO₆ single crystals is proposed. Incorporation of divalent magnesium atoms into the sites of hexavalent molybdenum atoms in the La₂MoO₆ structure leads to the formation of oxygen vacancies and a slight increase in the conductivity of doped samples in comparison with pure crystals. The compounds under study have hygroscopic properties.

Li _x Ln _{5– x} Mo ₃ O _{16.5–1.5 x} F _x (0.8 ≤ x ≤ 1.4 (Ln = La), 0.4 ≤ x ≤ 1 (Ln = Pr), 0.6 ≤ x ≤ 1 (Ln = Nd)) solid solutions on the Ln ₅ Mo ₃ O ₁₆ –LiLn ₄ Mo ₃ O ₁₅ F joins of the Ln ₂ O ₃ –MoO ₃ –LiF ternary systems, similar to the fluorite‐related cubic compound Nd ₅ Mo ₃ O ₁₆ , have been prepared by solid‐state reactions. Substitutions of lithium on the rare‐earth site and fluorine on the oxygen site have been shown to lead to a reversible phase transition accompanied by an anomaly in permittivity and a jump in conductivity by one and half orders of magnitude. In a wet atmosphere, the Li _x Ln _{5– x} Mo ₃ O _{16.5–1.5 x} F _x compounds may have proton conductivity.

Undoped and Mg-doped Pr₂MoO₆oxymolybdate polycrystals and single crystals have been prepared by solid-state reactions and flux growth. The compounds have been characterized by powder X-ray diffraction, energy-dispersive spectroscopy, inductively coupled plasma mass spectrometry, scanning transmission electron microscopy, single crystal X-ray structure analysis, differential scanning calorimetry and thermogravimetry. The (MgO)_x(Pr₂O₃)_y(MoO₃)_z(x+y+z= 1) solid solution series has been shown to extend tox= 0.03. The structure of the Mg-doped Pr₂MoO₆single crystals can be represented as superimposed lattices of the main matrix (Pr₂MoO₆) and lattices in which Mo atoms are partially replaced by Mg. The incorporation of Mg atoms into the structure of Pr₂MoO₆results in the disordering of the praseodymium and oxygen lattices. Both the polycrystalline and single-crystal Mg-doped samples are hygroscopic.