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

The structural stability and ion conductivity of (La 1.8 Dy 0.2 )(Mo 2− x W x )O 9 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 H 2 -reduced samples. Without tungsten, the H 2 -reduced surface of (La 1.8 Dy 0.2 )Mo 2 O 9 is engraved with deep notches at grain boundaries and shallow cuts in the grain interior. At a sufficient W level, the H 2 -reduced surface is similar to the surface without H 2 reduction. The XPS analysis concludes that 20% Mo on the (La 1.8 Dy 0.2 )Mo 2 O 9 surface is reduced to Mo 4+ and Mo 0 , whilst all Mo is detected at the oxidation state of + 6 in (La 1.8 Dy 0.2 )(Mo 1 W 1 )O 9 after 600 °C 3%H 2 reduction. No α–β phase transformation is experienced in any specimens of (La 1.8 Dy 0.2 )(Mo 2− x W x )O 9 , 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 (La 1.8 Dy 0.2 )(Mo 1 W 1 )O 9 exhibits a conductivity of 0.18 S cm − 1 at 800 °C, still higher than 0.08 S cm − 1 of La 2 Mo 2 O 9 . 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.