Nitrogen-doped zirconia: A comparison with cation stabilized zirconia

The conductivity behavior of nitrogen-doped zirconia is compared with that of zirconia doped with lower-valent cations and discussed in the framework of defect–defect interactions. While nominally introducing the same number of vacancies as yttrium, nitrogen dopants introduced in the anion sublattice of zirconia lead to substantially different defect kinetics and energetics. Compared to the equivalent yttrium doping nitrogen doping in the Y–Zr–O–N system substantially increases the activation energy and correspondingly decreases the conductivity at temperatures below image in the vacancy range below 4 mol%. The comparison of N-doped zirconia and zirconia systems doped with size-matched cation stabilizers, such as Sc, Yb and Y, shows that elastically driven vacancy–vacancy ordering interactions can phenomenologically account for the temperature- and composition-dependence. It is striking that materials with superior high-temperature conductivities due to weak dopant–vacancy interactions undergo severe deterioration at low temperature due to the strong vacancy-ordering. The analysis also explains qualitatively similar effects of Y co-doping in Yb-, Sc-, and N-doped zirconia. Small amount of Y in N-doped zirconia as well as in Sc-doped zirconia appears to hinder the formation of the long-range ordered phase and thus enhance the conductivity substantially.