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 (Ia3̅) and rhombohedral (R3̅) polymorphs of the Ln6MoO12−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) Yb6MoO12−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) Ln6MoO12−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 Er6MoO12−δ, Tm6MoO12−δ, and Yb6MoO12−δ bixbyites, respectively, oxygen ion conductivity prevails in the range ∼200–500 °C. Oxygen mobility data for the rhombohedral Ln6MoO12−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 Ln6MoO12−δ (Ln = Er, Tm, Yb) bixbyites (Ia3̅) and rhombohedral phases Ln6MoO12−Δ (Ln = Er, Tm, Yb; Δ = δ1, δ2) (R3̅) in dry and wet air, the proton conductivity contribution exists only in Ln6MoO12−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 Ln6MoO12−δ (Ln = Er, Tm, Yb) bixbyites are mixed electron–proton conductors at these temperatures. © 2019 American Chemical Society

Keywords

Atmospheric chemistry, Oxygen, Electric conductivity, Phase transformations, Trigonal lattices, Rare earths

Citation

Shlyakhtina, A. V., Lyskov, N. V., Avdeev, M., Goffman, V. G., Gorshkov, N. V., Knotko, A. V., Kolbanev, I. V., Karyagina, O. K., Maslakov, K. I., Shcherbakova, L. G., Sadovskaya, E. M., Sadykov, V. A., & Eremeev, N. F. (2019). Comparative study of electrical conduction and oxygen diffusion in the rhombohedral and bixbyite Ln6MoO12 (Ln = Er, Tm, Yb) polymorphs. Inorganic chemistry, 58(7), 4275-4288. doi:10.1021/acs.inorgchem.8b03397