Kinetic aspects of the synthesis of Ln6âˆ’Ñ MoO12−δ (Ln = Sm, Ho -Yb; x = 0, 0.5) rare-earth molybdates using mechanical activation of oxides

The electrical conductivity of the samples with different % of tetragonal phase has been measured by impedance spectroscopy in dry and wet air. The Arrhenius plot of conductivity for pure tetragonal phase has the form of straight line over the entire temperature range studied, 440-900 °C (Еа = 1.37 eV, σ 600°C = 1 × 10−5 S cm−1) in dry and wet air. The absence of electrode dispersions at low frequencies and conductivity growth in a wet air makes us assume the predominantly electronic conductivity type of the Ho10Mo2O21 tetragonal phase in the temperature range 440-900 °C. In multiphase samples surface electronic conductivity is observed up to 620-650 °С in wet air, which is associated with the presence of defects at grain boundaries of crystallographically related phases (bixbyite, rhombohedral and tetragonal phases).