Conductivity and oxygen diffusion in bixbyites and fluorites Ln(6-x)MoO(12-delta) (Ln = Er, Tm; x=0, 0.5)

In the present work erbium and thulium molybdates Ln(5.5)MoO(11.25-delta) (Ln = Er and Tm) with fluorite and Ln(6)MoO(12-delta) (Ln = Er and Tm) with bixbyite structure have been studied. The materials have been obtained by mechanical activation method followed by sintering at 1600 degrees C 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 (CO2)-O-18. 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 degrees C. In wet atmosphere Er and Tm fluorites and bixbyites have total conductivity of about 2.10(-6) S/cm at 500 degrees C. At higher temperatures they are mixed oxygen-electronic conductors in dry and wet atmosphere. At lower (T < 400 degrees C) temperatures bixbyites are slightly better ionic conductors compared to fluorites. A high oxygen-ion mobility in all compounds above 200 degrees C has been confirmed by isotope exchange method with (CO2)-O-18: tracer diffusion coefficient values were similar to 10(-11) - 10(-10) cm(2)/s at 700 degrees C. Fluorites were demonstrated to have a higher oxygen mobility compared to bixbyites; the effect is more pronounced for Tm molybdates. (C) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.