Trivalent Aluminum Ion Conducting Characteristics in Al2(WO4)3 Single Crystals

Single crystals of the trivalent Al³⁺ ion conductor Al2(WO4)S were grown by the Czochralski (CZ) method. The ionic conductivity in the a-, b-, and c-axis directions was determined and Al³⁺ ion conduction in the direction of the b-axis was concluded to be the most suitable pathway for ion migration in the tungstate grains. The ionic conductivities in the a- and c-axis directions were 0.3 and 10^-2 times lower than the conductivity in the b-axis. Consistent with this observation, the lowest activation energy (Ea) for Al³⁺ ion migration was obtained for the b-axis direction. The Ea of the conductivity in the direction of the c-axis was almost comparable to that of the polycrystalline samples and the Ea of the Al³⁺ ionic conduction in the grains of this material was explicitly verified to be controlled by the Al³⁺ ionic migration in the c-axis direction. The Al³⁺ ion conductivity of the polycrystalline sample was higher in the higher temperature region, indicating that the conductivity in the grain boundaries enhances the total Al³⁺ ion conductivity to a considerable extent. From the oxygen pressure dependencies of the electrical conductivity and the polarization behavior, the single crystals were demonstrated to be pure Al³⁺ ionic conductors showing an anisotropic ion conducting behavior.