Trivalent Al³⁺ Ion Conduction in Aluminum Tungstate Solid

Abstract

Ionic conduction of trivalent aluminum in solid aluminum tungstate, Al2(WO4)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, Al2(WO4)3, which has the Sc2(WO4)3 structure, was selected in order to reduce the interaction between the framework and the mobile species, Al3+, as much as possible. The ionic conduction characteristics of Al2(WO4)3 were investigated by means of electrolysis and electromotive force measurement by constructing an aluminum concentration cell. The typical electrical conductivity of Al2(WO4)3 was, approximately 2 × 10-5 S cm-1 at 800 °C. The dc electrolysis data strongly support the conclusion that ionic Al3+ is the mobile species in Al2(WO4)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.