Cation coordination and interstitial oxygen occupancy in co-doped zirconia from first principles

The fast oxygen-conducting cubic phase of zirconia is commonly stabilized by the substitution of group III cations such as yttrium and scandium, which creates vacancies in the oxygen sublattice. While scandia stabilized zirconia can achieve higher conductivity in freshly prepared specimens, it has previously been shown that yttria provides better stabilization characteristics than scandia, especially after extended operation at high temperatures. Density functional simulations of yttria and scandia zirconia show that oxygen ions neighboring vacancies can move into interstitial locations, effectively causing partial occupancy of neighboring anion sites, particularly in specimens containing scandia. Such partial occupancies can stabilize the substance in an ordered arrangement, causing age-related degradation over time, and can explain the diffuse scattering seen in X-ray diffraction studies of aged electrolytes. This tendency is reduced by increasing the yttria content of the mixture. Comparisons against rigid ion models show that the bonding is primarily ionic, but when cations become lower coordinated, the strength of the ionic bond increases, which can pull oxygen ions away from their ideal cubic lattice sites into interstitial locations that are shown to be stable at low temperature, a phenomenon which is more common in proximity to scandium ions than near yttrium ions.