The reason for this behaviour seems to be the displacement of
oxygen ions in channels from the center into an interstitial position
for the systems with cation vacancies and/or oxygen excess. This
was shown by neutron diffraction studies and atomistic modeling, suggesting
the interstitial mechanism of ion transport in apatite systems
[22–29].
The amount of cation vacancies and oxygen excess can be controlled
by doping, which allow improving transport properties of the ATLS. In
this respect, different doping strategies such as lanthanum substitution
and doping on the Si sitewere proposed to increase the oxygen ion conductivity
of ATLS, samples doped on the Si site showing higher conductivity
than comparable samples doped on the La site [30–35]. In the
recent literature, it has been shown that the solubility limits for different
cations are high and heavy doping up to 30% on both sites (La as well
as Si) results to single phase apatite materials [30–35,37].
In temperature-programmed experiments, the dynamic degree of
exchange is determined by the ratio between the rate of the surface reaction
and the rate of bulk diffusion, the latter being controlled by oxygen
self-diffusion proportional to the ionic conductivity [37]. Hence, a
high dynamic degree of exchange for doped apatites suggests a high
specific rate of oxygen heteroexchange exceeding that for doped zirconia
oxides. Specific rates of oxygen heteroexchange determined by the
ability of surface sites to activate oxygen molecules are indeed much
higher for doped La silicates than those for doped zirconia samples
and close to that for doped ceria sample. While for doped ceria, the
rate of the surface reaction is determined by well-known ability of Ce
cations to activate O2 molecules, for doped apatites this role can be
played by doping cations (Fe, Al, Sr) as well as La cations known for
their ability to stabilize superoxide (O2)− species. This behaviour allows
the expectation of relatively high surface exchange rates during the selfdiffusion
experiments.
Results of dynamic oxygen heteroexchange suggest that not only
channel/interstitial oxygen atoms are involved in the oxide-ion migration
but those of tetrahedra as well. Particularly, it has been found
for powdered samples characterized in a static installation in the
temperature-programmed mode by the dynamic degree of exchange
that La silicate doping significantly increases the oxygen mobility
in the lattice [37]. In this study, for some single phase samples,
it has been shown that the dynamic degree of isotopic exchange
expressed as the fraction of bulk oxygen atoms 16O in solid exchanged
for 18O from a gas (Vx) is higher than that of channel oxygen atoms
including interstitial ones for the highly conductive La10Si5AlO26.5 and
La10Si5FeO26.5 samples and at higher temperatures for other samples
as well. This is in agreement with a complex mode of oxygen diffusion
in ATLS proposed by Kendrick et al. [34]. In this model, the oxide-ion
motion involves cooperative displacements of the silicate substructure
with concomitant formation of one Si–O bond and breaking of another
Si–O bond similar to cooperativemechanisms in solids with tetrahedral
moieties.
In the present work, the parameters of oxygen transport in a wide
temperature range of differently doped apatite type electrolyte materials,
namely the oxygen self diffusion coefficientD* and the oxygen surface
exchange coefficient k*, were determined by ion exchange depth
profile technique (IEDP) followed by the analysis of the induced 18O diffusion
profiles by secondary ion mass spectrometry (SIMS). The results
obtained are compared with the conductivity data, and the influence of
dopants on the conduction mechanism is analyzed.