Chemistry Reference
In-Depth Information
using the superposition model, proposed by Bureau et al.
60
This model takes into account
the cation-fluorine bond distances and assigns isotropic chemical shifts to fluorine crystal-
lographic sites through the use of phenomenological parameters.
The proportions of fluoride ion environments determined by the
19
F NMR study
(Table 8.9) are clearly different from those predicted by the statistical calculations
on the basis of a random distribution of anions in the 8c sites and cations in the 4a sites
(Table 8.10). The contents of the fluoride ion environments are equal to the relative line
intensities (Table 8.9) multiplied by
y
2
(y corresponds to the F molar content).
The comparison of these values shows that the more numerous are the Ca
2
þ
cations
in the vicinity of F
ions, the higher is the discrepancy between experimental and predicted
contents. Based on the bond lengths determined from the Brown and Altermatt model
(Table 8.8), the average value of the (Ce,Ca)-(O,F) distances in the FCa
4
n
Ce
n
environments can be calculated with a linear combination of F-Ca and F-Ce bond lengths.
These distances increase from 2.37 to 2.50
˚
when n increases from 0 to 4. Moreover,
this latter distance appears underestimated considering isotropic chemical shift calcula-
tions (see above). The more numerous the Ce
4
þ
cations in the vicinity of F
ions, the larger
the average F-(Ce,Ca) distances, the stronger the constrains for the network and the
less stable the environments of fluoride ions. This is confirmed by the absence of
FCe
4
sites that would create too many constrains for the network of the
studied Ce
1
x
Ca
x
O
2
x
y/2
F
y
compounds where the average distance (Ce,Ca)-(O,F) is
around 2.35
˚
.
The observed contents of the fluoride ion environments may be understood as the result
of a compromise between the strong affinity of fluoride ions in fourfold coordination for
Ca
2
þ
cations in the fluorite structure, the F and Ca contents and the network constrains
induced by longer Ce-F bonds.
High F and Ca contents lead to the segregation of the F
and Ca
2
þ
ions into the network
of the studied Ce-Ca oxyfluorides and finally to the formation of CaF
2
as observed in
sample e which has the same Ca/Ce ratio as sample a (Table 8.5) but has been prepared as
samples b, c and d (see experimental section).
Since the proportions of FCa
4
environments are similar for samples a and b, it may be
inferred that a longer duration of maturation of the oxyhydroxides (sample a), allows the
synthesis of a sample with a more homogeneous distribution of Ca
2
þ
cations correspond-
ing to a larger content of FCa
4
avoiding thus the formation of CaF
2
during the fluorination.
The highest F content of sample a can also be explained by the increase of the proportions
of the more stable environments for fluorine ions (FCa
3
Ce and FCa
2
Ce
2
) and the decrease
of the proportions of the less stable environment for fluorine ions (FCaCe
3
) compared with
sample b.
Table 8.9 shows that for samples a and b corresponding to the highest Ca and F contents,
the proportion of FCa
4
environments remains stable around 13 %. Moreover, the propor-
tions of FCa
2
Ce
2
and FCa
3
Ce sites tend towards 50 % and 25 % of the total F sites
respectively. Then, the proportion of FCa
4
sites seems to reach a limit rate from the atomic
ratio Ca/Ce
¼
1/3 corresponding to the Ce
0.75
Ca
0.25
O
1.67
F
0.17
composition whereas the
anionic vacancy content reaches a limit equal to 1/12 starting from Ca/Ce atomic ratio
around 1/4 with a lower F content (Table 8.5). Then, the Ca and F contents as well as the
proportion of each fluorine environment seem to be independent of the creation of anionic
vacancies into the ceria matrix.
