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Abstract

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 500 degrees C 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. (c) 2005 Elsevier Inc. All rights
reserved.