Comparative Study of Electrical Conduction and Oxygen Diffusion in the Rhombohedral and Bixbyite Ln₆MoO₁₂ (Ln = Er,...

Electrical conduction and oxygen diffusion mobility in the bixbyite ( Ia3̅) and rhombohedral ( R3̅) polymorphs of the Ln₆MoO_12-Δ (Ln = Er, Tm, Yb; Δ = δ, δ1, δ2; δ1 > δ2) heavy lanthanide molybdates, belonging to new, previously unexplored classes of potential mixed (ionic-electronic) conductors, have been studied in the range of 200-900 °C. The oxygen self-diffusion coefficient in bixbyite ( Ia3̅) Yb₆MoO_12-δ phase estimated by the temperature-programmed heteroexchange with C¹⁸O₂ was shown to be much higher than that for rhombohedral ( R3̅) RI (with large oxygen deficiency) and ( R3̅) RII (with small oxygen deficiency) Ln₆MoO_12-Δ (Ln = Tm, Yb; Δ = δ1; δ1 > δ2) oxides. According to the activation energy for total conduction in ambient air, 0.99, 0.93, and 1.01 eV in Er₆MoO_12-δ, Tm₆MoO_12-δ, and Yb₆MoO_12-δ bixbyites, respectively, oxygen ion conductivity prevails in the range ∼200-500 °C. Oxygen mobility data for the rhombohedral Ln₆MoO_12-Δ (Ln = Er, Tm, Yb; Δ = δ1, δ2) phases RI and RII indicate that the oxygen in these phases exhibits mobility at much higher temperatures, such as those above 600-700 °C. Accordingly, below 600-700 °C they have predominantly electronic conductivity. As shown by total conductivity study of Ln₆MoO_12-δ (Ln = Er, Tm, Yb) bixbyites ( Ia3̅) and rhombohedral phases Ln₆MoO_12-Δ (Ln = Er, Tm, Yb; Δ = δ1, δ2) ( R3̅) in dry and wet air, the proton conductivity contribution exists only in Ln₆MoO_12-δ (Ln = Er, Tm, Yb) bixbyites up to 450-600 °C and decreases with a decreasing of the lanthanide ionic radius. The obtained data on the mobility of oxygen and the presence of proton contribution in bixbyites in the 300-600 °C temperature range make it possible to confirm unequivocally that Ln₆MoO_12-δ (Ln = Er, Tm, Yb) bixbyites are mixed electron-proton conductors at these temperatures.