It was noted that the cubic, tetragonal and t' In2O3-doped ZrO2 phases of this system exhibited high ionic conductivities, while the ZrO2-doped In2O3 phases exhibited high electronic conductivities. These phases were in thermodynamic equilibrium at high temperatures. The ionic conductivity of ZrO2 depended upon the crystal symmetry for a given In2O3 concentration. At 1000C, the highest conductivities were obtained for cubic ZrO2 that was doped with 25mol%InO1.5. At lower concentrations, the ionic conductivity of cubic ZrO2 decreased due to a first-order phase transformation to the tetragonal (t') form. Point defect models were proposed for electronic conduction in ZrO2-doped In2O3. Two maxima in the electronic conductivity were found. One of these was in the 2-phase region, and one was in the InO1.5 single-phase region. In the heterogeneous 2-phase material, cubic ZrO2 plus InO1.5, the electronic conductivity increased sharply up to 104S/m with increasing InO1.5 concentration. This material was a 3-dimensional composite of ion-conducting and electron-conducting phases.

L.J.Gauckler, K.Sasaki: Solid State Ionics, 1995, 75, 203-10