M. Dubois et al. / Journal of Solid State Chemistry 184 (2011) 220–226
225
3
+
Moreover because of the presence of Th paramagnetic ions, the
KTb
Tb
the presence of alkaline fluorides or other elements such as aluminum
fluoride modifies the temperature of TbF into TbF conversion and
2
F
9
(TbF
17 formula, i.e. K
4
/KF¼2) is actually a mixed-valence compound with
III IV
linewidth of the S
FWHM are equal to 1218 Hz (2220 Hz for Li5.5Ce12
3170 Hz for Li5.5Ce12 50) for S and S , respectively. Considering
two Lorentzian lines for the fitting, the S /S ratio of the integrated
surfaces is equal to 1.1 for inserted Li2+xTh12 50. Of about 0.63 and
.69 were found for S /S in the cases of the as-synthesized
compounds, Li5.5Th12 50, respectively. Taking
into account the crystal structure, the S /S theoretical ratio
should be equal to 2/3.5 i.e. 0.57 [18]. As reported for the cerium
mixed-valence, because of the opened structure of Li5.5
1
line is affected in comparison with the S
2
line;
K
2
4
F
2 3
Tb Tb F17 [10,11]. Moreover it is to note that
F50) and 2320 Hz
(
F
1
2
4
3
1
2
this seems to favour the mixed-valence, in particular KF, RbF, and CsF
for the case of terbium fluoride. These observations about terbium
fluorides are useful to understand the appearance of mixed-valence in
actinide and lanthanide fluorides. By analogy with terbium
tetrafluorides, the presence of lithium ions may favour the mixed-
F
0
1
2
F
50 and Li5.5Ce12F
1
2
3
+
valence in thorium fluoride and Th may be formed in situ by
3
+
M
12
F
50
4
decomposition of the ‘‘free’’ ThF . The presence of Th ions instead of
4
+
fluoride, the insertion compound can exhibit large change in the
Th needs the compensation of the decrease of the positive charge.
3
x
+
4+
ꢁ
stoichiometry, better formulated Li2+xCe
of the S /S
underlines that the Th /Th ratio was significantly increased.
From NMR data, the S /S ratio is related to the ratio of the amount
of Li in the locked position and in the channels. As S /S
¼1.1 and
Ce12ꢁx
F
50. The increase
The vacancies of F has been first investigated, because it could result
+
1
2
, resulting from the electrochemical insertion of Li ions,
in an EPR response as hole center. Nevertheless, such hypothesis
cannot explain the changes after the electrochemical insertion. As a
3
+
4+
1
2
4
matter of fact, under our conditions, i.e. with an LiClO /PC electrolyte,
+
+
ꢁ
1
2
Li ions can be reversibly exchanged contrary to F . On the contrary,
the insertion of lithium ions into the opened channels of the structure
results in the compensation of anionic and cationic charges. Each time
+
3+
4+
the number locked Li ions is equal to 2 per Li2+xTh
x
Th12ꢁxF
50
+
formulae unit (extracted from the XRD structure), 1.82 Li ions are
located in the channels. The stoichiometry seen by Li NMR is
7
4+
3+
+
that a Th ion is replaced by Th , one additional Li ion is inserted
into the channel. This process is reversible and does not change
significantly the crystal structure. The existence of the opened
channel has been proved with the cerium mixed-valence
3
+
4+
Li2+1.82Th1.82Th10.18F50. Similar calculations can be done with the
amount of inserted or extracted Li ions (NLi+) taking into account
+
the current intensity and time, i.e. related to the area of the anodic or
cathodic peak of the voltammogram (Fig. 3). The number of Li ions,
Li+, is estimated of 1.77 and 1.35 Li ions per Li2+xTh12
unit during the fifth insertion and extraction processes, respectively.
+
Li5.5Ce12F50, which is unambiguously isostructural with the present
+
+
N
F50 formula
compound. Moreover the possible diffusion of Li ions along
the channel has been proved by the exchange of these ions by
water molecules.
7
For the comparison with the value as seen by Li NMR for the sample
obtained after an electroreduction until 1.5 V, the value during the
insertion process is considered and the estimated stoichiometry is
3
+
4+
Li2+1.77Th1.77Th10.23
F50. A good agreement between the two methods
is found and the stoichiometry of the inserted compound close to
4. Conclusion
3
+
4+
Li3.8Th1.8Th10.2
F
50 can be proposed.
Several mixed-valence terbium fluorides were investigated such
as Tb 15 compound [12,13], potassium containing terbium fluorides
Tb 17 and KTb 12 [14,15], and also aluminum containing terbium
fluorides Rb AlTb 16 [16] and RbAl Tb 22 [17]. In a general way, the
synthesis of mixed-valence compounds depends on the reaction
3
+
The present work underlines the existence of Th
mixed-valence fluorides and also completes the study of fluorides
with general formula Li2+x
coexistence of the M and M oxidation states in these compounds
has been confirmed in a previous paper by XRD, XPS, and solid
state NMR.
ions in
4
F
3
+
4+
K
2
4
F
3
F
M
x
M
12ꢁx
F
50 with M¼Ce and Th. The
3
+
4+
2
3
F
2
4
F
3
+
4+
atmosphere, reducing or oxidizing, but the case of M /M mixed-
valence fluorides (M corresponds to lanthanide or actinide element) is
particular and the processes are controlled by the decomposition
First of all, it has been shown that the thorium and cerium mixed-
valence fluorides are isostrutural. Their structure exhibits an opened
channels network, where a part of the lithium ions are located, the
4
temperature of the ‘‘free’’ MF tetrafluoride. For instance, the analysis
+
4+
3+
of KF–TbF –TbF ternary system and the corresponding three binary
3
4
other Li ions being locked. The M into M reduction and the
+
systems allows to better understand the conditions of appearance of
the mixed-valence in the terbium fluorides. Any compound with
concomitant insertion of Li into the channel can be electrochemically
performed, thanks to the liquid electrolyte. The thorium mixed-
valence fluoride appears then as an insertion compound with general
KF/TbF
4
ratio higher than 1, i.e. KTbF
5
, was highlighted in the KF–TbF
TbF , K Tb 31 and KTbF ).
4
3
x
+
4+
binary system (including K
3
TbF , K
7
2
6
7
6
F
5
stoichiometry Li2+xTh
Th12ꢁxF50. This thorium fluoride is very
Indeed, by the constraint for +4 terbium ions to be obligatorily
in eight coordination in fluorinated environment [6–11], the
combinations with potassium fluoride are limited. This obligation
original because, on one hand, only a few actinide or lanthanide
mixed-valence fluorides are reported in the literature and, on the
3
+
1
7
other hand, Th ions are very unusual in the solid state. H and Li
3
+
+
does not exist for Tb and a series of trivalent terbium fluorides
covers all the composition range K TbF –KTb 10 (the intermediate
fluorides are K TbF , KTbF , and KTb ). In KF–TbF binary system, an
enrichment of the reactive mixture above the limit of the TbF /KF
solid state NMR data evidence the possible exchange between Li and
3
6
3
F
H
2
O and explains the ambiguity about the real composition reported
in the literature about thorium fluoride with Th/F atomic ratio close to
4, i.e. LiTh 17 and Th O.
The electrochemical insertion–extraction process allows the
2
5
4
F
2 7
4
4
4
F
4
F
16 ꢀ H
2
molar ratio equal to 1 results in an excess of tetravalent element.
However this terbium tetrafluoride excess, non-associated with KF in
3
+
4+
control of the Th /Th ratio contrary to the solid state synthesis,
for which the mixed-valence appearance may be related to the
decomposition of ‘‘free’’ tetrafluoride. This decomposition depends
on the extent of the tetrafluoride excess, the temperature and the
cooling rate. This study suggests that the content of Li , and then
Th , can be increased or, on the contrary, decreased to synthesize
by an original way new fluorides with variable Li2+xTh
composition. The changes of the Li and Th content have been
followed by solid state Li NMR and EPR, respectively.
a defined tetrafluoride compound, thermally behaves like ‘‘free’’ TbF
Chilingarov et al. [25] showed that the decomposition of TbF into
TbF and atomic fluorine occurs at temperatures higher than 550 1C
even in fluorine atmosphere. If the synthesis conditions allow it, TbF
4
.
4
3
+
4
3
+
in excess releases by decomposition trivalent fluoride, which is
suitable to immediately combine with the other components of
3
x
+
4+
Th12ꢁxF
50
3
+
4+
+
3+
the reactive mixture. This forms Tb /Tb mixed-valence terbium
fluoride. In other words, an induced transformation occurs during the
7
thermal treatment from the KF–TbF
ternary system. The obtained mixed-valence compounds are thermo-
dynamically stable. Thus, the terbium fluorides initially formulated
4
binary into KF–TbF
3
–TbF
4
Taking into account the structure, the lower limit is close to
3
+
4+
Li
2
Th12
F
50, where x¼0 in Li2+xTh
x
Th12ꢁxF50. Works are in progress
to investigate to upper limit. Such an electrochemical synthesis would