Theoretical calculations of the interaction between neighboring oxygen ions for selected clusters were performed by the density functional method (DFT/B3LYP) in order to investigate the stable effect of single and mixed doped c-ZrO2. The Mulliken charge population numbers of oxygen ions for undoped, cation-doped, anion-doped and co-doped c-ZrO2 were discussed. Theoretical results indicate that the stable ability of doped c-ZrO2 could be described as the Coulomb repulsive force between neighboring oxygen ions (f(c)), which agrees well with the experimental data. Moreover, the effect of third phase cation and/or anion-doped zirconia complex materials was successfully expounded by this method, especially on the conductivity decrease of the 8YSZ system with increasing SiC content. (C) 2007 Elsevier B.V. All rights reserved.