Crystal structure and ionic conductivity of novel rare-earth niobates Ln NbO 4 ( Ln = Nd, Sm, Eu, Gd) by substituting Nb with W

W-substituted rare-earth niobates ( Ln Nb 1 − x W x O 4 + δ ( Ln = Nd, Sm, Eu, Gd)) have been systematically synthesized by a solid-state reaction method. Powder X-ray diffraction at room temperature revealed that the monoclinic fergusonite phase structure of unsubstituted samples gradually changes to a tetragonal scheelite phase structure with increasing W substitution. The crystal structures of the samples with x ≥ 0.3 can be refined well (goodness-of-fit indicator S ≤ 1.5) as the tetragonal scheelite structure. The W substitution improves the ionic conductivities of the four rare-earth niobates, and all of their optimum W substitution amount ( x ) are 0.2. The activation energy of the conductivity for the substituted samples is almost constant regardless of the type of rare-earth ion, the temperature, and the W substitution amount.