Effects of A- and B-site (co-)acceptor doping on the structure and proton conductivity of LaNbO4

B-site and A- and B-site co-doped lanthanum niobates, La(1-x)DA(x)Nb(1-y)DB(y)O(4-δ), where DA stands for Ca or Ba and DB for Ga, Ge or In (x = 0 or 0.01; y = 0.01), have been synthesized via the solid-state route. Essentially single-phase materials were obtained after 10 h at 1500 °C with monoclinic Fergusonite structure at room temperature. The unit cell volume increases slightly with the presence of the acceptor. The electrical conductivity of the materials, characterized as a function of the temperature from 300 °C to 900 °C under wet reducing conditions with AC impedance spectroscopy, was dominated by protons. The maximum conductivity of 4.1 · 10−4 S · cm−1 at 900 °C was obtained for La0.99Ca0.01Nb0.99Ga0.01O4-δ, which is a factor of ~ 2.5 lower than the highest conductivity reported for the A-site Ca-doped LaNbO4. Based on a semi-quantitative evaluation it was shown that the hydration of the B-site and A- and B-site co-doped LaNbO4 is slightly more exothermic and that the proton mobility has higher activation enthalpies than Ca-doped LaNbO4. This is accounted for by a stronger association between the acceptor and the proton for B-site than for A-site doping.