Trivalent Al³⁺ Ion Conduction in Aluminum Tungstate Solid

Ionic conduction of trivalent aluminum in solid aluminum tungstate, Al 2 (WO 4 ) 3, has been directly and quantitatively demonstrated. Trivalent ions, especially for aluminum where the polarizability is considerably low, are strongly bonded in a crystal lattice and have been believed to be unlikely to exhibit migration in solids because of high electrostatic interaction with the surrounding skeleton structure. The material, Al 2 (WO 4 ) 3, which has the Sc 2 (WO 4 ) 3 structure, was selected in order to reduce the interaction between the framework and the mobile species, Al 3+, as much as possible. The ionic conduction characteristics of Al 2 (WO 4 ) 3 were investigated by means of electrolysis and electromotive force measurement by constructing an aluminum concentration cell. The typical electrical conductivity of Al 2 (WO 4 ) 3 was, approximately 2 × 10 - 5 S cm - 1 at 800 °C. The dc electrolysis data strongly support the conclusion that ionic Al 3+ is the mobile species in Al 2 (WO 4 ) 3 . By further electrochemical measurements, it was confirmed that the mobile species was aluminum ion in a trivalent state with no contribution due to electronic conduction. The ionic transference number was determined to be unity from the oxygen−air gas concentration cell and the aluminum concentration cell method.