The highest trivalent ion conductivity was successfully realized for the composite of 0.75Sc_2(WO_4)_3, 0.25ScTaO_4. Here, scandium tantalate was microscopically dispersed simultaneously during the preparation process to obtain a microscopically mixed composite and the enhancement of the Sc~(3+) ionic conduction was achieved. From the measurments of the electrical conductivity dependencies on oxygen partial pressure, the polarization behavior, and electrolysis, the mobile ion was shown to be Sc~(3+) ion and the other of the cations, i.e., Ta~(5+) and W~(6+) were found to operate as lattice forming ions with the oxide ions.
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