Ionic conductivity of tetragonal ZrO2 polycrystal doped with TiO2 and GeO2

The effects of the co-doping and the resultant co-segregation of 2 mol% TiO2 and 2 mol% GeO2 on the ionic conductivity and on the chemical bonding state in a tetragonal ZrO2 polycrystal were investigated. The conductivity data and grain boundary microstructure showed that the doped Ti4+ and Ge4+ cations segregate along the grain boundary, and this segregation causes a reduction in the conductivity of both the grain interior and grain boundary and an increase in the activation energy of the grain boundary conductivity. Overall, the data indicate that the segregation retards the diffusion of oxygen anions. A first-principle molecular orbital calculation explains the retarded diffusion of the oxygen anion from a change in the covalent bonds around the dopant cations; an increase in the strength of the covalent bond between the oxygen and doped cation should work to suppress the diffusion of the oxygen anion.