Comparative Study of Electrical Conduction and Oxygen Diffusion in the Rhombohedral and Bixbyite Ln6MoO12 (Ln = Er, Tm, Yb) Polymorphs

Electrical conduction_ and oxygen diffusion mobility in the bixbyite (Ia3) and rhombohedral (R3) polymorphs of the Ln(6)MoO(12-Delta) (Ln = Er, Tm, Yb; Delta = delta, delta 1, delta 2; delta 1 > delta 2) 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 (Ia3) Yb6Mo012_6 phase estimated by the temperature-programmed heteroexchange with C1802 was shown to be much higher than that for rhombohedral (R3) RI (with large oxygen deficiency) and (R3) RII (with small oxygen deficiency) Ln(6)MoO(12-Delta) (Ln = Tm, Yb; A = 61; 61 > 62) oxides. According to the activation energy for total conduction in ambient air, 0.99, 0.93, and 1.01 eV in Er6MoO12-6, Tm6MoO12-6 and Yb6Mo012-6 bixbyites, respectively, oxygen ion conductivity prevails in the range--,200-500 C. Oxygen mobility data for the rhombohedral Ln(6)MoO(12-Delta) (Ln = Er, Tm, Yb; A = 61, 62) phases RI and RH 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 Ln6Mo0,2_,5 (Ln = Er, Tm, Yb) bixbyites (Ia3) and rhombohedral phases Ln(6)MoO(12-Delta) (Ln = Er, Tm, Yb; A = 61, 62) (R3) in dry and wet air, the proton conductivity contribution exists only in Ln(6)MoO(12-Delta) (Ln = Er, Tm, Yb) bixbyites up to 450-600 degrees 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 degrees C temperature range make it possible to confirm unequivocally that Ln(6)MoO(12-Delta) (Ln = Er, Tm, Yb) bixbyites are mixed electron-proton conductors at these temperatures.