Structural stability and ion conductivity of the Dy and W substituted La2Mo2O9

The structural stability and ion conductivity of (La1.8Dy0.2)(Mo2−xWx)O9 are studied using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and impedance spectroscopy. The structural stabilization effect of W is demonstrated by comparing the microstructures and the oxidation states of ions of as-sintered and H2-reduced samples. Without tungsten, the H2-reduced surface of (La1.8Dy0.2)Mo2O9 is engraved with deep notches at grain boundaries and shallow cuts in the grain interior. At a sufficient W level, the H2-reduced surface is similar to the surface without H2 reduction. The XPS analysis concludes that 20% Mo on the (La1.8Dy0.2)Mo2O9 surface is reduced to Mo4+ and Mo0, whilst all Mo is detected at the oxidation state of + 6 in (La1.8Dy0.2)(Mo1W1)O9 after 600 °C 3%H2 reduction. No α–β phase transformation is experienced in any specimens of (La1.8Dy0.2)(Mo2−xWx)O9, when heated from 300 to 800 °C. The room-temperature lattice parameter of β-phase increases with increasing W content, reaches a maximum at x = 1.0, then decreases. Considering the W structural stabilization effect, its unfavorable influence on conductivity is tolerable because the sample of (La1.8Dy0.2)(Mo1W1)O9 exhibits a conductivity of 0.18 S cm− 1 at 800 °C, still higher than 0.08 S cm− 1 of La2Mo2O9. The temperature dependence of ion conductivity in this doubly substituted LAMOX is correlated to the Arrhenius form from 350 to 450 °C and the Vogel–Tamman–Fulcher form from 450 to 800 °C, and discussed.