Conductivity and oxygen diffusion in bixbyites and fluorites Ln6?xMoO12?δ (Ln = Er,Tm; x = 0, 0.5)

In the present work erbium and thulium molybdates Ln_5.5MoO_11.25?δ (Ln = Er and Tm) with fluorite and Ln₆MoO_12?δ (Ln = Er and Tm) with bixbyite structure have been studied. The materials have been obtained by mechanical activation method followed by sintering at 1600°С for 3 h. New compounds have been characterized by X-ray diffraction. The total conductivity was investigated using impedance spectroscopy method in dry and wet air. Oxygen diffusivity data was acquired by oxygen isotope exchange with C¹⁸O₂. The combination of different techniques allowed us to determine ionic conductivity components in these compounds. Er and Tm fluorites and bixbyites showed oxygen-ion conductivity in dry air and oxygen-ion and proton conductivity in wet air up to 550–600°С. In wet atmosphere Er and Tm fluorites and bixbyites have total conductivity of about 2?10^?6 S/cm at 500 °C. At higher temperatures they are mixed oxygen-electronic conductors in dry and wet atmosphere. At lower (T < 400 °C) temperatures bixbyites are slightly better ionic conductors compared to fluorites. A high oxygen-ion mobility in all compounds above 200°С has been confirmed by isotope exchange method with С¹⁸O₂: tracer diffusion coefficient values were ~10^?11 – 10^?10 cm²/s at 700 °C. Fluorites were demonstrated to have a higher oxygen mobility compared to bixbyites; the effect is more pronounced for Tm molybdates.