Abstract
It has been known that the high oxygen conductivity in stabilized zirconia is realized through defect structures in the crystal lattice, which is caused by adding di- or tri-valent oxides such as lime, yttria, scandia etc. However, the detailed mechanism is still an open question. Recently we have been studying the thermodynamic properties of stabilized zirconia, focusing attention on the defect structures. In the present review, the results of heat capacity measurements on pure zirconia (ZrO2) and yttria stabilized zirconia (YSZ) {(ZrO2)1-x(Y2O3)x; x=0.04, 0.08, 0.10, 0.11} and of molecular dynamics calculations are described, and analyzed in detail. An excess heat capacity has been found in comparison with pure zirconia, which suggests soft vibration modes in stabilized zirconia. The excess heat capacity of YSZ decreased with increasing yttria content. The softening of these lattice vibration modes should be attributed to the stabilization of cubic/tetragonal form. The eightfold coordination of Zr ion in cubic/tetragonal stabilized zirconia is of over-coordination by oxygen ions comparing with monoclinic zirconia, in which the coordination number of Zr ion is 7, and should induce the softening. The formation of anion vacancy by additive doping of yttria leads the decrease in the average coordination number of Zr ion, and suppresses the softening. Molecular dynamics simulation confirmed the mechanism.
