This trend does not match to the expected behaviour based on the
ionic radii of the dopantswho are increasing in the order Al b Fe leading
to higher oxygen displacement and thus higher values expected for the
oxygen diffusion in the same manner. Iron in its Fe3+ formhas a slightly
bigger ionic radius when it is in the same coordination as Al3+ and
should lead to higher diffusivities than the aluminium-doped sample.
As we do not know the oxidation state of the iron in the samples, it is
not trivial to explain the diffusion behaviour. That is probably the reason
for the high activation enthalpy in the iron-doped samples. It is reported
by Tolchard et al. [36] that the picture is not clear when introducing
transition metals into the crystal as the oxidation state of the dopant
is not known. The activation enthalpies are for tracer diffusion in the
order LMSO ≤ LSO b LASO b LFSO (Table 3) and confirm the predicted
behaviour for the iron-doped ATLS.
For the incorporation reaction, the activation enthalpies are in
the order LMSO b LSO ≈ LASO b LFSO (Table 4). They are rather low
and in agreement with the quite high oxygen incorporation rates and
surface concentrations for pure ionic conducting materials like ATLS.
The diffusivities of the ATLS electrolyte materials are higher compared
to yttria stabilized zirconia which is an industrial standard electrolyte
material. In Fig. 6, oxygen self-diffusion data in 10YSZ are
plotted together with the diffusivities of the ATLS samples. It is obvious
that the diffusivities of the ATLS are especially at lower temperatures
higher. The activation enthalpy for oxygen diffusion in YSZ is higher as
compared to the ATLS used in this study.