Trivalent Rare Earth Ion Conduction in the Rare Earth Tungstates with the Sc₂(WO₄)₃-Type Structure

Abstract

To realize a trivalent ion conduction in solids, the Sc 2 (WO 4 ) 3 -type structure was chosen on the basis of the mobile trivalent ions and the structure which reduces the electrostatic interaction between the framework and the mobile trivalent ionic species as much as possible. The typical conductivity of the rare earth tungstates R 2 (WO 4 ) 3 (R = Sc, Y, and Er−Lu) with the Sc 2 (WO 4 ) 3 -type structure was found to be on the order of 10 - 5 S cm - 1 at 600 °C. Among the rare earth tungstates, Sc 2 (WO 4 ) 3 (σ 600 ° C = 6.5 × 10 - 5 S cm - 1, E a = 44.1 kJ mol - 1 ) was found to be the most suitable size for the ionic conduction with regard to the relation between the mobile ion radius and the lattice size. The rare earth ion conducting characteristics were investigated by means of the rare earth concentration cell measurements and dc electrolyses. The electromotive force measurements with the Sc−Y binary alloy and the yttrium tungsten bronze as the electrodes strongly suggest the possibility of the trivalent ion conduction of rare earths such as Sc 3+ and Y 3+ . Furthermore, by the dc electrolysis, the mobile species was clarified to be the rare earth ions Sc 3+ and Y 3+ in the rare earth tungstates with the Sc 2 (WO 4 ) 3 -type structure.