Chemistry Reference
In-Depth Information
for high Ca content, the Ca
2
þ
ions are probably not sufficiently homogeneously distributed
into the network. So, the introduction of fluorine ions in the mixture leads to the segrega-
tion of Ca atoms and finally to the formation of CaF
2
. In the latter conditions, the duration
of the oxide's maturation induced a better homogeneity of Ca
2
þ
ions into the network of
ceria. As F
ions were introduced into the Ca-Ce matrix, the formation of CaF
2
was
avoided, leading to Ca-Ce oxyfluorides with high Ca/Ce and F/Ce atomic ratios.
Coming from NaOH addition, Na
þ
ions have been detected in samples with the lowest
Ca contents, so that (Ca
þ
Na)/Ce atomic ratio is always lower than 0.10. It can be thus
considered a first step that alkaline and alkaline-earth cations are substituted for Ce
4
þ
cations. In a recent
23
Na NMR study, which will be published later, on such Ce-based
compounds adopting fluorite-type structure, the recorded spectra present characteristic
features of electric-field gradient and isotropic chemical distributions (asymmetry of the
central line and broadening of the spinning side bands). This is in agreement with the
occurrence of Na
þ
disordered environments with various numbers of O
2
and F
anions at
different distances surrounding Na
þ
cations.
To confirm the Ce valence state in Ce-Ca based oxides and oxyfluorides after the
coprecipitation and the thermal treatment at 600 C under air, magnetic measurements
were carried out. In Table 8.3, molar susceptibilities at 300 K of these samples after
diamagnetic corrections are listed as well as those of Ce
2
Sn
2
O
7
,
46
CeF
3
, commercial
CeO
2
from Alfa Products and CeO
2
data reported in the literature.
47
In CeF
3
and
Ce
2
Sn
2
O
7
compounds, cerium ions are totally in trivalent state whereas they are totally
in tetravalent state in CeO
2
. The magnetic susceptibility of cerium trifluoride follows a
Curie-Weiss law versus the temperature (100 K £ T £ 300 K) with a theoretical effective
magnetic moment equal to 2.54 m
B
/Ce atom (2.43 m
B
/Ce atom for the experimental value).
CeO
2
is assumed to be diamagnetic; its magnetic susceptibility is equal to 0,6.10
4
emu/
mol and is almost 50 times lower than those of CeF
3
and Ce
2
Sn
2
O
7
compounds. The
comparison of their magnetic susceptibilities with paramagnetic and diamagnetic refer-
ences in Table 8.3 reveals that the Ce
3
þ
content is so low that it cannot be determined.
Moreover, the
19
F MAS-NMR study, mentioned in the following section also confirms
the absence of any dipolar broadening between fluorine ions and Ce
3
þ
ions leading to
consider that the majority of Ce ions are in the tetravalent state. Therefore, as the cerium is
considered only in tetravalent state in Ce-Ca based oxides and oxyfluorides, the following
formula
can
be
proposed
for
these
systems:
Ce
1
x
Ca
x
O
2
x
for
the
oxides
and
Ce
1
x
Ca
x
O
2
x
y/2
F
y
for the oxyfluorides.
Table 8.3 Room-temperature magnetic susceptibility of Ce-vased oxides and
oxyfluorides as well as Ce
2
Sn
2
O
7
, CeF
3
and CeO
2
compounds as references
Compositions
m
(emu/mol)
1,1.10
4
O1 Ce
0.91
Na
0.09
O
1.87
1,3.10
4
O6 Ce
0.75
Ca
0.25
O
1.75
1,2.10
4
F5Ce
0.75
Ca
0.25
O
1.67
F
0.17
3,1.10
3
Ce
2
Sn
2
O
7
[24]
2,1.10
3
CeF
3
0,6.10
4
CeO
2
[25]
0,5.10
4
CeO
2
(Alfa Products)
