Pseudotetragonal structure of Li2+xCex 3+Ce12-x4+F50: The first mixed valence cerium fluoride

Article abstract of DOI:10.1021/ic901962k

The crystal structure of the new Li_5.5Ce₁₂F ₅₀ compound has been fully characterized by single-crystal and synchrotron powder X-ray diffraction. An accurate pseudotetragonal structure was described In the monoclinic P2₁ space group with 68 independent crystallography sites. The Li_5.5Ce₁₂F₅₀ composition belongs to the Li_2+xCe_x³⁺Ce _12-x⁴⁺F₆₀ solid solution. Its structure consists of an opened fluorine framework where a channel network allows the intercalation of relatively mobile lithium cations, inducing the formation of the mixed-valence cerium (the intercalation of Li⁺ leads to the reduction of a part of Ce⁴⁺ to Ce³⁺). One part of the lithium ions, necessary for the electroneutrality of the tetravalent equivalent cerium fluoride (Li₂Ce₁₂F₅₀ composition), is in a locked fluorine polyhedron. Only the supplementary x amount of lithium is able to be exchanged in Li_2+xCe_x³⁺Ce _12-x⁴⁺F₅₀. The structure of Li ^2+xCe_x³⁺Ce_12-x⁴⁺F ₅₀ is a rearrangement, due to lithium intercalation, of the base CeF₄ structure. Bond valence calculation on Ce sites, Ce coordination polyhedra volumes, and a calculated Ce cationic radius give the indication of a partial long-range ordering of trivalent and tetravalent cerium cations In specific slabs of the structure. ⁷Li NMR spectroscopy and XPS analyses have confirmed all of the structure details.

Full text of DOI:10.1021/ic901962k

686 Inorg. Chem. 2010, 49, 686–694
DOI: 10.1021/ic901962k
4þ
Pseudotetragonal Structure of Li2þxCex3þCe12-x F50: The First Mixed Valence
Cerium Fluoride
ꢀ ‡,§
ꢂ
Guillaume Renaudin,*,†,‡ Belto Dieudonne, Daniel Avignant,‡,§ Elise Mapemba,‡,§ Malika El-Ghozzi,‡,§
‡,§
ꢁ
ꢀ
ꢀ ^
Solene Fleutot, Herve Martinez, Radovan Cerny, and Marc Dubois
†
ꢀ
ꢀ
Clermont Universite, ENSCCF, Laboratoire des Materiaux Inorganiques, BP 10448, F-63000
‡
§
ꢁ
ꢀ
Clermont-Ferrand, France, CNRS, UMR 6002, F-63177 Aubiere Cedex, France, Clermont Universite,
ꢀ
ꢀ
Universite Blaise Pascal, Laboratoires des Materiaux Inorganiques, BP 10448, F-63000 Clermont-Ferrand,
ꢀ
ꢀ ꢀ
France, Universite de Pau et des Pays de l’Adour, IPREM CNRS UMR 5254, Helioparc Pau Pyrenees,
2 Avenue de President Angot, F-64053 Pau Cedex 9, France, and ^University of Geneva, Laboratory of
ꢀ
Crystallography, 24 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland
Received October 5, 2009
The crystal structure of the new Li5.5Ce12F50 compound has been fully characterized by single-crystal and synchrotron
powder X-ray diffraction. An accurate pseudotetragonal structure was described in the monoclinic P21 space group
4þ
with 68 independent crystallographic sites. The Li5.5Ce12F50 composition belongs to the Li2þxCex3þCe12-x F50 solid
solution. Its structure consists of an opened fluorine framework where a channel network allows the intercalation of
relatively mobile lithium cations, inducing the formation of the mixed-valence cerium (the intercalation of Liþ leads
to the reduction of a part of Ce4þ to Ce3þ). One part of the lithium ions, necessary for the electroneutrality
of the tetravalent equivalent cerium fluoride (Li2Ce12F50 composition), is in a locked fluorine polyhedron. Only
4þ
the supplementary x amount of lithium is able to be exchanged in Li2þxCex3þCe12-x F50. The structure
4þ
of Li2þxCex3þCe12-x
F is a rearrangement, due to lithium intercalation, of the base CeF4 structure. Bond valence
50
calculation on Ce sites, Ce coordination polyhedra volumes, and a calculated Ce cationic radius give the indication
of a partial long-range ordering of trivalent and tetravalent cerium cations in specific slabs of the structure. 7Li NMR
spectroscopy and XPS analyses have confirmed all of the structure details.
1. Introduction
high instability, justifying the low number of studies on
tetravalent terbium compounds.1-4 Nevertheless, Tb4þ is
more easily stabilized by fluorine anions than by oxide
anions. The tetravalent Ce4þ cation can be stabilized in either
an oxygenated or fluorinated medium.5-7 Up to now, very
few mixed-valence terbium fluorides were reported in
the literature. Among them, the Tb4F15 compound was
synthesized by Brekhovskikh et al.8 and characterized by
Popov et al.9 Other mixed-valence terbium fluorides have
In spite of their usual “rare earths” name, the lanthanide
elements are now largely used in advanced technologies. The
applications relate to optics with the development of solid
lasers, optical fibers for telecommunications, and devices
with electrochromic or electroluminescent diodes and so
on. The remarkable magnetic properties of some inorganic
fluorides result also in interesting potentialities. The diversity
and the high added value of these applications justify the
numerous research projects undertaken on these lanthanides
and actinides fluorides. In particular, the crystal chemistry of
terbium fluorides has been extensively studied because of the
half-filled shell of the 4f7 electronic configuration of terbium,
which stabilizes the tetravalent cation. Compounds exhibit-
ing mixed valences are subject to many studies because of
their potential applications related to the electronic exchange
and magnetism.
(1) Cunningham, B. B; Fray, D. C.; Rollier, M. A. J. Am. Chem. Soc.
1954, 76, 3361–3363.
€
(2) Hoppe, R.; Rodder, K. M. Z. Anorg. Allg. Chem. 1961, 312, 277–278.
(3) Avignant, D.; Cousseins, J.-C. C. R. Acad. Sci. Paris, Ser. C 1974, 278,
613–616.
(4) Avignant, D.; Cousseins, J.-C. Rev. Chim. Miner. 1978, 15, 360–367.
(5) Laligant, Y.; Le Bail, A.; Ferey, G.; Avignant, D.; Cousseins, J.-C.
Eur. J. Solid State Inorg. Chem. 1989, 25, 551–563.
(6) El-Ghozzi, M.; Avignant, D.; Cousseins, J.-C. Eur. J. Solid State
Inorg. Chem. 1992, 29, 981–992.
Contrary to the cerium ions for which the þ4 oxidation
(7) Gaumet, V.; Avignant, D. Acta Crystallogr., Sect. C 1997, C53, 1176–
1178.
(8) Brekhovskikh, M. N.; Popov, A. I.; Yu, M.; Il’inskii, A.; Fedorov,
V. A. Russ. J. Inorg. Chem. 1989, 34, 573–577.
state can be stabilized in solution, the Tb4þ cation exhibits a
*To whom correspondence should be addressed. Tel.: 00 33 4 73 40 73 36.
Fax: 00 33 4 73 40 71 08. E-mail: guillaume.renaudin@ensccf.fr.
(9) Popov, A. I.; Val’kovskii, M. D.; Fedorov, P. P.; Yu Kiselev, M. Russ.
J. Inorg. Chem. 1991, 36, 476–480.
r
pubs.acs.org/IC
Published on Web 12/18/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 687
been synthesized and characterized in our laboratory:
KTb3F1210,11 and K2Tb4F1712 present a long-range ordering
between Tb3þ and Tb4þ. For others, such as the aluminum-
valence has been investigated by XPS experiments in order to
extract the Ce4þ/Ce3þ ratio.
13
2. Experimental Section
containing terbium fluoride Rb2AlTb3F16 and RbAl2Tb4-
F22,14 Tb3þ and Tb4þ are statistically distributed into the
same crystallographic sites. Mixed-valence ternary terbium
fluoride appears at a molar ratio of TbF4/AF > 1 (A = Li, K,
Rb, Cs). Chilingarov et al.15 showed that the decomposition of
TbF4 into TbF3 and atomic fluorine occurs at temperatures
higher than 550 ꢀC, even in a fluorine atmosphere. If the
synthesis conditions allow it, TbF4 in excess transforms to
trivalent fluoride by decomposition, which is suitable to im-
mediately combine with the other components of the reactive
mixture. This forms Tb3þ/Tb4þ mixed-valence terbium fluor-
ide. In other words, a thermally induced transformation occurs
from the AF-TbF4 binary system into the AF-TbF3-TbF4
ternary system. The obtained mixed-valence compounds are
thermodynamically stable. Nevertheless, the reaction tempera-
ture and the cooling rate must be optimized. Thus, the
terbium fluoride initially formulated, KTb2F9 (TbF4/KF = 2),
is actually a mixed-valence compound with the K2Tb4F17
formula, that is, K2Tb3þTb4þ3F17.11
2.1. Synthesis. Samples, used for single-crystal and synchro-
tron powder measurements, were synthesized by solid-state reac-
tions involving a mixture of LiF (0.058 g), CeF3 (0.444 g), and
CeF4 (1.459 g) starting materials in a 1:1:3 ratio. Pure CeF4
tetrafluoride has been prepared by the following route: (i)
dissolution of CeO2 in a HF solution (40%), (ii) after evaporation
of the solution, the dried resulting powder was fluorinated under
a pure F2 gas stream at 500 ꢀC. CeF3 was a commercial product
(99.9% Aldrich). LiF (99.9%, Aldrich) was dehydrated by heat-
ing at 110 ꢀC overnight in a primary vacuum. The 1:1:3 mixture
was loaded into a platinum capsule (40 mm of length, 5 mm of
diameter). The airtight platinum capsule was heated at 300 ꢀC for
24 h, and then a temperature of 550 ꢀC was applied over 72 h.
2.2. X-Ray Diffraction. Single-crystal X-ray diffraction in-
tensity data were collected from two orange single crystals at
room temperature by the use of a Stoe IPDS II image plate
diffractometer. Both data were similar. The data set from the
crystal given the best Rint was used to solve and refine the
structure. The details of data collection and structure refinement
are given in Table 1.
In the LiF-MF4 (M = Ce, Th, and U) binary systems,
compounds formulated as LiM4F17 were reported,16-19 but
no structural study confirmed their real stoichiometries. By
analogy with terbium tetrafluorides, their location above the
MF4/LiF molar ratio equal to unity increases the ambiguity
about their real stoichiometry. Indeed, the initial “LiTh4F17”
Synchrotron powder diffraction data of a freshly synthesized
compound was obtained at the Swiss-Norwegian Beamline
(SNBL) at the ESRF Grenoble (six analyzer crystal detector).
More details are given in Table 1. Synchrotron powder diffrac-
tion was used to check the symmetry of the compound, that is,
the weak lattice distortion unresolved with the image plate
diffractometer. More details are given in ref 20.
composition has been corrected as Th6F24 H2O, a hydrated
3
7
1
2.3. 7Li MAS NMR Spectroscopy. The Li and H spectra
were recorded with magic angle spinning at room temperature
using a Bruker MSL300 spectrometer operating at frequencies
of 116.6 and 300.1 MHz for 7 Li and 1 H nuclei, respectively. The
spinning rate was equal to 14 kHz using a 4 mm Bruker probe. A
single π/2 pulse sequence was used (τ-acquisition with τ = 6 and
5 μs for 7Li and 1H nuclei, respectively). A total of 1000 and 128
scans were recorded with a recycling time of 1 and 5 s for 7Li and
1H, respectively. The 1H chemical shift refers to tetramethylsi-
lane by using adamantane as an external reference. The 7Li
chemical shifts are given with respect to solid LiCl.
thorium tetrafluoride, thanks to a structural study which has
been carried out on a single crystal by Cousson et al.19
Nevertheless, these authors mentioned that lithium ions are
necessary for the synthesis of the hydrated tetrafluoride,
although they are absent from the structure.
With the dual aim of unambiguously determining the real
stoichiometry of the “LiM4F17” (M = Ce, Th, and U)
compounds and investigating whether the mechanism of
formation of mixed-valence terbium fluorides is transposable
to LiF/MF4 systems, a compound of nominal composition
1LiF, 1CeF3, and 3CeF4 was synthesized and characterized.
The synthesized Li5.5Ce12F50 compound has a pseudotetra-
gonal symmetry. A first paper describes the weak mono-
clinic distortion of the lattice.20 The present paper gives its
2.4. XPS. XPS measurements were carried out with a Kratos
Axis Ultra spectrometer using focused monochromatized Al KR
radiation (hν = 1486.6 eV). For the Ag 3d5/2 line, the full width
at half-maximum (fwhm) was 0.58 eV under the recording
conditions. The analyzed area of the samples was 300 ꢀ
700 μm2. Peaks were recorded with a constant pass energy of
20 eV. The pressure in the analysis chamber was ca. 5 ꢀ 10-8 Pa.
To prevent the samples from moisture/air exposure on the
analysis site, the XPS spectrometer was directly connected
through a transfer chamber to a nitrogen drybox. Short acquisi-
tion time control spectra were recorded at the beginning and at
the end of each experiment to check the nondegradation of the
samples. The binding energy scale was calibrated from the
carbon contamination using the C 1s peak at 285.0 eV. Core
peaks were analyzed using a nonlinear Shirley-type back-
ground.21 The peak positions and areas were optimized with a
weighted least-squares fitting method using 70% Gaussian/30%
Lorentzian lineshapes. Quantification was performed on the
basis of Scofield’s relative sensitivity factors.22
7
complete crystallographic description together with a Li
magic-angle spinning (MAS) NMR investigation. The mixed
(10) Avignant, D.; Largeau, E.; Gaumet, V.; Dugnat, P.; El-Ghozzi, M.
J. Alloys Compd. 1998, 275-277, 1–5.
(11) Largeau, E.; El-Ghozzi, M.; Avignant, D. J. Sol. State Chem. 1998,
139, 248–258.
(12) Largeau, E. Ph.D. Thesis, Blaise Pascal University, Clermont-
Ferrand, France, 1998.
(13) Josse, M.; Dubois, M.; El-Ghozzi, M.; Avignant, D. Solid State Sci.
2003, 5, 1141–1148.
(14) Josse, M.; Dubois, M.; El-Ghozzi, M.; Avignant, D. J. Alloys
Compd. 2004, 374, 213–218.
(15) Chilingarov, N. S.; Rau, J. V.; Sidorov, L. N.; Bencze, L.; Popovic,
A.; Sukhoverkhov, V. F. J. Fluorine Chem. 2000, 104, 291–295.
(16) Thoma, R. E.; Insley, H.; Landau, B. S.; Friedman, H. A.; Grimes,
W. R. J. Phys. Chem. 1959, 63, 1266–1274.
(17) Barton, C. J.; Friedman, H. A.; Grimes, W. R.; Insley, H.; Moore,
R. E.; Thoma, R. E. J. Am. Ceram. Soc. 1958, 41, 63–69.
(18) Cousson, A.; Pages, M.; Cousseins, J. C.; Vedrine, A. J. Cryst.
Growth 1977, 40, 157–160.
3. Structure Solution and Refinement
3.1. Structure Solution from Single-Crystal Data. A
reciprocal lattice observed from both single crystals
(19) Cousson, A.; Pages, M.; Chevalier, R. Acta Crystallogr., Sect. B
1979, 35, 1763–1765.
(20) Renaudin, G.; Mapemba, E.; El-Ghozzi, M.; Dubois, M.; Avignant,
(21) Shirley, D. A. Phys. Rev. B 1972, 5, 4709–4714.
(22) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129–137.
ꢂ
ꢀ
D.; Cerny, R. Z. Kristallogr. 2007, 26, 455–460.

688 Inorganic Chemistry, Vol. 49, No. 2, 2010
Renaudin et al.
Table 1. Parameters of Data Collection and Treatment for Single-Crystal and Powder Sample of Li5.5Ce12F50
single crystal used for solution and refinement
powder used for lattice determination
formula
molar mass (g/mol)
space group
Li5.5Ce12F50
2669.61
P21
˚
a [A]
8.855(2)
22.968(5)
8.855(2)
8.82391(7)
˚
b [A]
22.9188(2)
8.85384(7)
90.6093(2)
1790.44(3)
˚
c [A]
β [deg]
90.0(-)
3
V [A ]
˚
1800.94(2)
Z
wavelength [A]
2
˚
0.71073 (Mo KR)
8.63
0.37504
1.40
μ [mm-1
cryst size [mm]
]
0.103 ꢀ 0.095 ꢀ 0.060
0.2
glass capillary diameter [mm]
analytical for cylindrical shape
4.95
SNBL powder diffractometer
2θ step [deg]
absorption correction
density (calcd) [g cm-3
data collection
detector distance [mm]
exposure time [min]
numerical from crystal shape and size
4.92
]
Stoe IPDS II
100
0.002
0.5
3
time/step [s]
ω range; increment [deg]
2θ interval [deg]
range in hkl
reflns measured
reflns unique
Rint
reflns with I > 4σ(I)
data/params
structure solution
structure refinement
S
0-180; 1.0
2.29-59.53
1.0-17.5
h = (12; -28 < k < þ31; l = (12
15972
8776
0.042
6312 (Rσ = 0.039)
19.3
SIR928 SHELXS-97 (Sheldrick, 1997a)
SHELXL-97 (Sheldrick, 1997b)
FullProf.2k 10
1.51
0.075
0.142
13.8
0.040
0.072
χ2
RB (all reflections)
Rwp (background corrected)
RF (all reflns)
RwF2 (all reflns)
extinction coefficient
residual in difference
electron-density map
0.00049(3)
-3.43, 2.63
˚
indicates a tetragonal lattice with a = 8.855 (2) A and c=
and all cerium atoms presented a disordered nine-fluorine
coordination. Finally, the best solution was found in the
P41 space group, and refinement led to RF = 0.083,
RwF = 0.155, and S=1.49. Lithium atoms were located
by Fourier difference maps. The structure refinement was
acceptable with a refined composition of Li2Ce6F25, with
two-thirds of cerium atoms coordinated in 8-fold coordi-
nation and one-thirdin 9-fold coordination. Because of the
presence of some observed but normally systematically
absent reflections, and the presence of numerous incon-
sistent equivalents, a supplementary resolution work was
˚
22.968(5) A. Analysis of the diffraction data set under-
lines a 91% probability of a centrosymmetric structure
with the 4/m Laue symmetry (Rint = 0.061) and a pri-
mitive lattice. Automatic analysis of the extinction con-
ditions using the Autostart Module of WinGX23 did not
lead to the determination of the space group. The first
attempt to solve the structure was performed in the P42/n
space group. The starting atomic parameters of heavy
cerium atoms were deduced from an automatic interpre-
tation of direct methods with SIR92.24 Fluorine atoms
were subsequently located from difference Fourier maps,
and the structure was refined using SHELXL97.25 The
solution obtained in the P42/n space group seemed to be
correct (with R =0.197), but refinement did not decrease
below RF =0.159. Lithium atoms were not located in the
structure by Fourier difference maps, and the refined
composition corresponded to CeF4 (all cerium atoms in
an eight-fluorine coordination with a square-based anti-
prism neighboring). Refinement of the solution obtained
with the P4 space group brought a weak improve-
ment with an RF =0.154 and a refined composition of
CeF4.167. Nevertheless, lithium atoms were not located,
performed in monoclinic 2/m Laue symmetry (Rint
0.053). After transformation of the diffraction data set in
=
a monoclinic lattice with unique axis bB (instead of the
measured c ), the Autostart Module of WinGX23 assigned
B
the centrosymmetricP21/c space group, but solutionin this
P21/c symmetry was not acceptable. Finally, the structure
of this new fluoride compound was solved and refined in
the noncentrosymmetric P21 space group with a final
RF =0.050and afinal refinedcompositionof Li5.5Ce12F50,
indicating the presence of the Ce4þ/Ce3þ mixed valence.
3.2. Synchrotron Powder Diffraction. Synchrotron
powder diffraction data have been recorded on a freshly
prepared powdered sample in order to check the mono-
clinic symmetry of the lattice. Numerous diffraction line
splittings were observed, clearly indicating a lowering of
the previously supposed tetragonal lattice. Figure 1 shows
the synchrotron powder diffraction data and Rietveld
(23) WinGX, version 1.70.01: Farrugia, L. J. J. Appl. Crystallogr. 1999,
32, 837-838.
(24) SIR92: Altomare, A; Cascarano, G.; Giacovazzo, C.; Gualiardi, A.
J. Appl. Crystallogr. 1993, 26, 343-350.
(25) Sheldrick, G. M. SHELXL-97, release 97-2; University of Goettingen:
Goettingen, Germany, 1997.

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 689
the 68 independent atomic positions of the structure are
listed in Table SI1 (in the Supporting Information), and
the anisotropic displacement parameters of cerium atoms
are given in Table SI2 (in the Supporting Information). In
the final run, 328 independent parameters were allowed to
vary. Selected interatomic distances are listed in Table 2.
4. Discussion
4.1. Structure Description. The structure of Li5.5-
Ce12F50 is built from stacking of the cerium polyhedra
with lithium cations inserted in cavities and channels.
Figure 2 shows a general view of the structure along the [101]
direction, and the projections of four details along the
bB monoclinic axis. Cerium atoms present two coordina-
tion numbers: 8-fold coordinated by fluorine for eight Ce
sites (CN8 for Ce81 to Ce88 with a square antiprism
polyhedron) and 9-fold coordinated by fluorine for four
Ce sites (CN9 for Ce91 to Ce94 with a monocapped
square antiprism polyhedron). Interatomic Ce-F dis-
tances, indicated in Table 2, are quite homogeneous
Figure 1. Rietveld plot of the fluoride sample (Rwp = 0.117 and χ2 =
13.7 for global contributions, RBragg = 0.040 and RF = 0.046 for the new
fluoride phase). Observed (a, dots) and calculated (a, solid line) synchro-
˚
˚
˚
around each Ce site: 2.20 A e dCe81-F e 2.35 A, 2.27 A
˚
tron(SNBL) powder diffractionpattern(λ =0.37504A) areshownwitha
˚
˚
˚
˚
e dCe82-F e 2.39 A, 2.22 A e dCe83-F e 2.36 A, 2.29 A
difference curve (b) and Bragg peak positions for Li2þxCe12F50 (c1; 83(1)
refined weight%)andimpuritiesCeF3 (c2) andCeF4 (c3). The insetshows
the splitting from the monoclinic distortion of the lattice.
˚
e dCe84-F e 2.45 A, 2.09 A e dCe85-F e 2.31 A, 2.17 A e
˚
˚
˚
˚
˚
˚
dCe86-F e 2.36 A, 2.10 A e dCe87-F e 2.32 A, 2.15 A e
˚
˚
dCe88-F e 2.31 A, 2.12 A e dCe91-F e 2.41 A, 2.22 A
˚
˚
˚
plot performed with FullProf26 by using the atomic
positions from the single-crystal data analysis (P21
symmetry). Refined monoclinic lattice parameters are
˚
˚
e dCe92-F e 2.45 A, 2.14 A e dCe93-F e 2.37 A, and 2.22
˚
˚
˚
A e dCe94-F e 2.48 A. Nevertheless, average Ce-F
distances around each Ce site, or the volume of each Ce
polyhedron, show three cases: Ce91 to Ce92 present a
similar behavior corresponding to 9-fold Ce4þ, Ce85 to
Ce88 present a similar behavior corresponding to 8-fold
Ce4þ, and Ce81 to Ce84 present a gradual evolution
(sequence Ce81, Ce83, Ce82, Ce84) corresponding to an
8-fold mixed valence (see Table 2 and Figure 3b). The
calculated cerium cationic radius (considering the aver-
˚
˚
˚
a =8.82391(7) A, b = 22.9188(2) A, c=8.85384(7) A,
and β =90.6093(2)ꢀ. Cerium trifuoride CeF3 and cerium
tetrafluoride CeF4 impurities are present in the sample
with, respectively, 13(1) and 4(1) refined wt %.
3.3. Structure Refinement. Structure parameters of
Li5.5Ce12F50 have been refined in the monoclinic P21
space group using single-crystal data. All relevant details
concerning data collection and treatment are listed in
Table 1. Structure solution has indicated 12 independent
cerium positions. A total of 50 independent fluorine
positions were subsequently easily discovered with Four-
ier difference maps. All of these atomic coordinates were
refined (using full-matrix least-squares refinement on F2)
with anisotropic atomic displacement parameters for
cerium atoms. Careful analysis of new Fourier difference
maps, in accordance with geometric considerations, al-
lowed the localization of lithium atoms. This led to six
refined independent lithium atomic positions. In the final
runs of refinement, two fluorine atomic displace-
˚
age Ce-F distances and an anionic radius of 1.285 A for
F- 27) agrees fairly well with these observations: around
4þ
0.96 A for CN8 Ce , around 1.01 A for CN9 Ce4þ, and
˚
˚
between 0.97 A and 1.14 A for CN8 mixed Ce3þ/Ce4þ
(Figure 3a). These crystal chemical indications of the
mixed valence correlate with the refined composition
˚
˚
4þ
(Li11Ce73þCe17
F
per unit cell) and the orange color
100
of our crystals. Bond valence sums (BVS28) have been
calculated for cerium sites by considering the values of
2.036, respectively 1.995, for the bond valence parameter
Rij assuming Ce3þ, respectively Ce4þ (Table 2). Similar
29
2
˚
BVS calculations have been performed on CeF3 and
ment parameters were constrained to Uiso = 0.005 A
CeF430 structures to validate the method. The calculated
valences correlate well with our previous observations:
tetravalent Ce4þ cations are located in the four 9-fold
coordinated Ce91 to Ce94 sites and in the four 8-fold
coordinated Ce85 to Ce88 sites; mixed Ce3þ/Ce4þ va-
lences are located in the four 8-fold coordinated Ce81 to
Ce84 sites. Polyhedra volumes, calculated cationic radii,
and BVS calculations for the two Ce82 and Ce84 sites
because of an unrealistic quasi-zero refined value
(F09 and F16). Due to similar geometric environments,
Li1 and Li2 sites have been restrained to have the same
atomic displacement parameter, as have the four similar
Li3, Li4, Li5, and Li6 sites. Occupancy parameters have
been checked for all of the atomic positions of the
structure. The occupancy of the lithium position Li4 only
deviated significantly from the unity at a value close to
half occupancy. So, in the final run, its occupancy was
fixed at 1/2, while all of the other sites were refined with
the ideal occupancy corresponding to the composi-
tion Li5.5Ce12F50 (Z=2). The positional parameters of
(27) Shannon, R. D. Acta Crystallogr. Sect. A 1976, 32, 751–767.
(28) Brese, N. E.; O’Keeffe, M. Acta Crystallogr., Sect. B 1991, 47, 192–
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(29) Cheetham, A. K.; Fender, B. E. F.; Fuess, H.; Wright, A. F. Acta
Crystallogr., Sect. B 1976, 32, 94–97.
(30) Schmidt, R.; Mueller, B. G. Z. Anorg. Allg. Chem. 1999, 625, 605–
608.
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(26) Rodriguez-Carvajal, J. FullProf.2k, version 3.30; Laboratoire Leon
Brillouin (CEA-CNRS): France, 2005.

690 Inorganic Chemistry, Vol. 49, No. 2, 2010
Renaudin et al.
Table 2. Selected Interatomic Distances around Cerium Atoms, Polyhedron Volumes, and Bond Valence Sum Assuming Trivalent and Tetravalent Cerium Cations
BVS
3
polyhedron volume (A )
˚
distance (A)
˚
central atom
Ce81b
fluoride ligands
CNa
Ce3þ
Ce4þ
valence
III/IV
F37
F47
F46
F48
F27
F04
F25
F42
<Ce81-F>
F43
F29
F18
F26
F38
F07
F10
F12
<Ce82-F>
F45
F35
F20
F44
F19
F17
F21
F15
<Ce83-F>
F41
F33
F24
F40
F39
F13
F11
F08
<Ce84-F>
F11
F28
F16
F29
F46
F03
F22
F19
<Ce85-F>
F22
F13
F43
F48
F09
F30
F15
F05
<Ce86-F>
F10
F32
F41
F20
F04
F14
F23
F01
<Ce87-F>
F23
F07
F33
F44
F06
F25
F31
F02
2.20 (2)
2.20 (1)
2.24 (2)
2.25 (2)
2.31 (2)
2.33 (2)
2.34 (2)
2.35 (2)
2.28
2.27 (1)
2.28 (1)
2.29 (1)
2.32 (2)
2.36 (1)
2.37 (1)
2.38 (1)
2.39 (1)
2.33
2.22 (2)
2.23 (1)
2.25 (1)
2.29 (1)
2.33 (2)
2.35 (2)
2.36 (2)
2.36 (2)
2.30
2.29 (2)
2.32 (2)
2.32 (2)
2.32 (2)
2.38 (1)
2.40 (1)
2.45 (1)
2.45 (1)
2.37
2.09 (1)
2.17 (1)
2.24 (1)
2.25 (2)
2.25 (1)
2.29 (2)
2.31 (1)
2.31 (2)
2.24
2.17 (1)
2.20 (1)
2.24 (1)
2.25 (2)
2.25 (1)
2.28 (1)
2.28 (2)
2.36 (2)
2.25
2.10 (2)
2.21 (1)
2.21 (2)
2.23 (1)
2.24 (2)
2.28 (1)
2.30 (1)
2.32 (2)
2.24
2.15 (1)
2.20 (2)
2.22 (2)
2.25 (1)
2.25 (1)
2.26 (2)
2.27 (1)
2.31 (2)
2.24
0.642
0.642
0.576
0.561
0.477
0.452
0.440
0.428
4.22
0.531
0.517
0.503
0.464
0.417
0.405
0.395
0.384
3.62
0.608
0.592
0.561
0.503
0.452
0.428
0.417
0.417
3.98
0.503
0.464
0.464
0.464
0.395
0.374
0.327
0.327
3.32
0.864
0.696
0.576
0.561
0.561
0.503
0.477
0.477
4.72
0.696
0.642
0.576
0.561
0.561
0.517
0.517
0.417
4.49
0.841
0.625
0.625
0.592
0.576
0.517
0.490
0.464
4.73
0.735
0.642
0.608
0.561
0.561
0.546
0.531
0.477
4.66
0.575
0.575
0.516
0.502
0.427
0.404
0.394
0.383
3.77
0.476
0.463
0.451
0.415
0.373
0.363
0.353
0.344
3.24
0.544
0.530
0.502
0.451
0.404
0.383
0.373
0.373
3.56
0.451
0.415
0.415
0.415
0.353
0.335
0.292
0.292
2.97
0.774
0.623
0.516
0.502
0.502
0.451
0.427
0.427
4.22
0.623
0.575
0.516
0.502
0.502
0.463
0.463
0.373
4.02
0.753
0.559
0.559
0.530
0.516
0.463
0.439
0.415
4.23
0.658
0.575
0.544
0.502
0.502
0.489
0.476
0.427
4.17
8
20.47
22.13
20.77
Ce82
Ce83b
Ce84
Ce85
Ce86
Ce87
Ce88
8
III
8
III/IV
8
8
22.91
19.49
III
IV
8
8
8
19.93
19.30
19.39
IV
IV
IV
<Ce88-F>

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 691
Table 2. Continued
BVS
3
polyhedron volume (A )
˚
distance (A)
˚
central atom
Ce91
fluoride ligands
CNa
Ce3þ
Ce4þ
valence
F30
F08
F21
F03
F26
F34
F16
F49
2.12 (1)
2.15 (1)
2.27 (2)
2.31 (2)
2.33 (2)
2.35 (1)
2.38 (1)
2.40 (1)
2.41 (2)
2.30
2.22 (1)
2.23 (1)
2.24 (1)
2.24 (2)
2.28 (1)
2.30 (2)
2.33 (1)
2.43 (1)
2.45 (1)
2.30
2.14 (1)
2.14 (1)
2.23 (2)
2.31 (2)
2.33 (2)
2.33 (1)
2.36 (2)
2.37 (1)
2.37 (1)
2.29
2.22 (2)
2.22 (1)
2.24 (2)
2.24 (1)
2.26 (1)
2.29 (2)
2.29 (2)
2.40 (1)
2.48 (1)
2.29
0.797
0.735
0.531
0.477
0.452
0.428
0.395
0.374
0.364
4.55
0.608
0.592
0.576
0.576
0.517
0.490
0.464
0.345
0.327
4.50
0.755
0.755
0.592
0.477
0.452
0.452
0.417
0.405
0.405
4.71
0.608
0.608
0.576
0.576
0.546
0.503
0.503
0.374
0.301
4.60
0.713
0.658
0.476
0.427
0.404
0.383
0.353
0.335
0.326
4.07
0.544
0.530
0.516
0.516
0.463
0.439
0.415
0.309
0.292
4.02
0.676
0.676
0.530
0.427
0.404
0.404
0.373
0.363
0.363
4.22
0.544
0.544
0.516
0.516
0.489
0.451
0.451
0.335
0.270
4.11
9
9
24.39
24.33
IV
IV
F37
<Ce91-F>
F18
F34
F39
F17
F28
F05
F47
F09
Ce92
Ce93
Ce94
F49
<Ce92-F>
F31
F12
F42
F01
F40
F35
F50
F36
9
23.75
IV
F14
<Ce93-F>
F27
F36
F24
F38
F32
F45
F02
F06
9
23.82
IV
F50
<Ce94-F>
a Cerium coordination number. b Mixed valence cerium sites.
correspond to a trivalent Ce3þ cation (by comparison of
the BVS values for Ce4þ and Ce3þ in Table 2).
(Ce81 to Ce84) fully compose the opened B and D slabs.
These cerium sites are in the vicinity of the mobile Liþ
sites (Li3 to Li6). A long-range ordering of Ce3þ and Ce4þ
sites is then present in Li1þxCe6F25 (i.e., with a prefer-
ential location of Ce3þ in the four Ce81 to Ce84 sites when
Liþ cations are intercalated), as already observed in the
The connection of the 8- and 9-fold coordinated poly-
edra forms an opened tridimentional framework. The
structure is composed of a stacking of two kinds of slabs
along the monoclinic axis. One kind of slab (labeled B and
D in Figure 2 composed of Ce81 and Ce82 and of Ce83
and Ce84 polyhedra) is opened due to the presence of
perpendicular channels along the [101] and [101] direc-
tions. Two-thirds of the lithium sites (Li3 and Li4 in slab
B and Li5 and Li6 in slab D) are located at the crossing of
this channel network. These lithium cations seem to be
relatively mobile along the channels. The second kind of
slab (labeled A and C in Figure 2, respectively composed
by Ce85, Ce86, Ce91, and Ce92 and Ce87, Ce88, Ce93,
and Ce94 polyhedra) is compact and contains locked
lithium cations in the Li1 and Li2 sites, respectively in
slab A and C. Off-center Li1 and Li2 sites are 8-fold-
coordinated by fluorides. Compact A and C slabs are
composed of tetravalent Ce4þ cations only. Each of these
Ce polyhedra (Ce85 to Ce88 and Ce91 to Ce94) are
linked to seven neighboring polyhedra (mainly corner-
but also edge-sharing). The mixed valence cerium sites
10,11
Tb-based definite mixed-valence compounds KTb3F12
and K2Tb4F17.12 An explicit formula of the studied com-
4þ
pound is Li2þxCex3þCe12-x F50, indicating a minimum
amount of lithium (two per unit formulas) and a progres-
sive reduction of cerium (from Ce4þ to Ce3þ) when inter-
calating the x supplementary lithium cations.
The structure of Li5.5Ce12F50 is related to CeF4
(monoclinic C2/c symmetry20). The CeF4 structure pre-
sents a succession of opened and locked slabs also.
“Opened” slabs in CeF4 show similarity with the opened
slabs B and D in Li5.5Ce12F50. Locked slabs are different.
They are composed of 8-fold coordinated Ce atoms only.
Adjacent locked slabs are corner-sharing across the
opened slab region, blocking the “opened” slabs. The
Li5.5Ce12F50 structure is a rearrangement of the CeF4
structure, allowing the insertion of lithium cations into
channels of the opened slabs. Large similitude with the

692 Inorganic Chemistry, Vol. 49, No. 2, 2010
Renaudin et al.
Figure 2. General representation (along [101]) and details (along [010]) of the Li5.5Ce12F50 structure. Dark polyhedra and light polyhedra correspond
respectively to cerium atoms with a CN9 and CN8 coordination number. Small white, large light gray, dark gray, and black balls correspond respectively to
fluoride, cerium with CN8, cerium with CN9, and lithium atoms. Drawings A, B, C, and D are four details corresponding to the four independent structural
slabs constituting the whole structure. The inset in the right top of the figure shows the fluoride neighboring of cations (coordination of lithium atoms,
distorted cubic environment for Li1 and Li2, and bonds for Li3 to Li6 are not represented in the general and detailed views).
good indicators, but, in the present case, cannot be con-
sidered as definitive). The presence of two different
environments for lithium cations has been checked with
7Li MAS NMR spectroscopy, and the simultaneous
presence of Ce3þ and Ce4þ has been investigated using
XPS analysis (which allows qualitative and quantitative
studies).
4.2. 7Li NMR MAS Spectroscopy. Figure 4 displays
7
the Li NMR spectrum of the Li5.5Ce12F50 compound.
The NMR spectrum exhibits two peaks centered at 0 and
21.1 ppm noted S1 and S2, respectively, indicating the
presence of two types of lithium ions. Although two types
of lithium ions with different environments are high-
lighted by the structural study, the significant separation
of the NMR lines is probably ascribable to the presence of
Ce3þ paramagnetic ions, which results in the shift of the
S2 line. The paramagnetism effect on the chemical shifts
has been discussed for lithium in several works, in parti-
cular for various compositions of the Li(Ni,Co)O2 solid
solution31 and for nonstoichiometric LiCoO2,32 where
Figure 3. 7Li MAS NMR spectrum of the Li5.5Ce12F50 compound
(spinning speed of 14 kHz, O and / mark the spinning sidebands).
the presence of the Ni3þ or Co2þ cations in the low spin
state is accompanied by the appearance of new resonance
thorium fluoride Th6F24 H2O16 is evidenced. The hy-
drate compound Th6F24 H2O, I4/m symmetry, shows a
lines in addition to the line at 0 ppm. Paramagnetic
“external” species such as adsorbed oxygen molecules
result in a shift for zeolites with a high silica content.33
3
3
similar network with all thorium cations 8-fold-coordi-
nated (square antiprism) and water molecules located
into the channel network of the opened slabs, at the
location of the mobile lithium cations in the Li5.5Ce12F50
structure.
Considering the location of the Liþ ions in the Li5.5
-
Ce12F50 structure, two-thirds of them (Li3, Li4, Li5,
and Li6) are localized in the B and D layers, which contain
The effective formation of a cerium mixed-valence
fluoride is mainly supported by the refined Liþ amount
(which presents a large uncertainty due to its extremely
weak scattering contrast, especially in the presence of
cerium) and by crystal chemistry consideration (which are
(31) Marichal, C.; Hirschinger, J.; Granger, P.; Menetrier, M.; Rougier,
A.; Delmas, C. Inorg. Chem. 1995, 34, 1773–1778.
(32) Levasseur, S.; Menetrier, M.; Suard, E; Delmas, C. Solid State Ionics
2000, 128, 11–24.
(33) Accardi, R. J.; Lobo, R. F. Microporous Mesoporous Mater. 2000, 40,
25–34.

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 693
Figure 5. XPS Ce3d core level peaks of Li5.5Ce12F50
.
Figure 4. Environments of the 12 independent Ce sites: Calculated
cerium ionic radius (a) and polyhedron volumes (b).
Table 3. Labeling and Mean Positions of the 10 Components of Li5.5Ce12F50
spin-orbit
characteristic
binding energy (eV)
Ce3þ trivalent cerium ions. The other Liþ-including Li1
and Li2 ions are in the A and C layers, which are
exclusively built with tetravalent cerium ions. One may
assume that only the nuclear resonance of the first type of
lithium is affected by the presence of paramagnetic Ce3þ
because these ions are located within channels delimited
by the mixed Ce3þ/Ce4þ ions in statistical distribution. In
addition to the “paramagnetic” shift, the line width is also
affected in the case of the S2 line with a broadening; the
fwhm’s are equal to 2220 and 3170 Hz for S1 and S2,
respectively.
1
6
3
8
2
7
4
9
5
10
3d5/2
3d3/2
3d5/2
3d3/2
3d5/2
3d3/2
3d5/2
3d3/2
3d5/2
3d3/2
shake down of 3
shake down of 8
883.1
900.5
886.7
905.2
884.2
902.2
887.7
906.2
900.0
918.4
Ce3þ
satellite
satellite
satellite
satellite
Ce4þ
energetic window (Binding Energy), 880-920 eV. Note
that XPS spectra of cerium compounds are known to
exhibit rather complex features due to shake-up or -down
lines, multiplet splitting or hybridization with ligand
orbitals, and fractional occupancy of the valence 4f
orbitals.34 The Ce3d XPS spectrum of Li5.5Ce12F50
(Figure 5) exhibits 10 components (labeled from 1 to
10) with the assignment of each component defined in
Table 3. The theoretical basis for all of the components
has been reported in the literature.35
Four peaks labeled 1 and 3 (for 3d5/2) and 6 and 8 (for
3d3/2) are related to the Ce3þ oxidation state. The satellite
components 1 and 6 are related to the ligand-to-metal
charge transfer phenomenon by the primary photoioni-
zation process. Six peaks labeled 2, 4, and 5 (for 3d5/2
and 7, 9, and 10 (for 3d3/2) referring to the three pairs of
Taking into account the assignment of the S1 and S2
lines and the number of involved lithium ions, the theo-
retical ratio of their integrated surfaces should be equal to
2:3.5, that is, 0.57. Indeed, even if the lithium nucleus is
quadrupolar, integrated surfaces of the lines are almost
proportional to the relative concentration of the Liþ
cations. The experimental ratio of 0.69 has been obtained
by considering two Lorentzian lines for the fitting. The
weak difference between the theoretical and experimental
values could be explained by the particular structure of
the compound. Indeed, the opened structure of Li5.5-
Ce12F50 fluoride can be compared to an intercalation
4þ
compound with stoichiometry Li2þxCex3þCe12-x F50,
where x can vary in large proportions. It is worthy of note
that 1H MAS NMR confirms the absence of water
molecules within the opened channels of the Li5.5Ce12F50
structure. As a matter of fact, no significant signal was
obtained for its spectrum (not shown here).
)
(34) Shyu, J. Z.; Otto, K.; Watkins, W. L. H.; Graham, G. W.; Belitz, R.
K.; Gandhi, H. S. J. Catal. 1988, 114, 23–33.
(35) (a) Fujimori, A. J. Magn. Magn. Mater. 1985, 47-48, 243–247.
(b) Allen, J. W. J. Magn. Magn. Mater. 1985, 47-48, 168–174. (c) Fujimori, A.
Phys. Rev. B 1983, 28, 2281–2283. (d) Fujimori, A. Phys. Rev. B 1983, 27,
3992–4001.
4.3. X-Ray Photoelectron Spectroscopy (XPS). The
more relevant peak to identify the simultaneous presence
of Ce3þ and Ce4þ is the Ce3d core peak, located in the

694 Inorganic Chemistry, Vol. 49, No. 2, 2010
Renaudin et al.
spin-orbit doublets are characteristic of Ce4þ final
states.36,37 The literature reports that the peak labeled 10
arises exclusively from Ce4þ due to a transition from the 4f0
initial state to the 4f0 final state. This fact is pretty interest-
ing, as this component is isolated from the others and
located around 918 eV. Then, this peak could be used as a
diffraction and high-resolution synchrotron powder diffrac-
tion data. The synchrotron powder diffraction pattern has
been essential in determining the effective monoclinic P21
symmetry of the lattice, due to a weak distortion of a pseudo-
tetragonal lattice. The structure of Li5.5Ce12F50 is related to
the structure of CeF4,14 with the intercalation of lithium
cations allowed by opening a channel network. One part of
Liþ is locked, and the other part is relatively free to move
along the channel network. The insertion of Liþ into the
channel leads to the reduction of one equivalent part of Ce4þ
to Ce3þ. This is the first mixed-valence cerium fluoride.
A long-range ordering of trivalent and tetravalent cerium
cations has been observed. Structural details deduced from
the crystal chemistry have been proven by spectroscopic
analyses. The two kinds of Liþ environments have been
quantitative probe of the amount of Ce .
4þ 36,38,39 Indeed, it
has been demonstrated that the integral area of the peak
labeled 10 with respect to the total Ce3d area could be trans-
lated into the percentage of Ce4þ. The mean percentage of
Ce4þ can be then calculated through the following relation:
ð10Þ%
Ce4þ% ¼
ꢀ 100
14
7
where (10)% is the percentage of the peak area correspond-
ing to the component 10 with respect to the total Ce3d area.
By using this formula, the experimental ratio Ce4þ/Ce3þ is
equal to 1.75. By taking into account the refined weight
amounts (13% for CeF3 and 4% for CeF4), we can reach the
clearly identified by Li NMR spectroscopy. And the pre-
sence of both Ce3þ and Ce4þ valences has been clearly
established by XPS. The observed Ce4þ/Ce3þ ratio is in
perfect agreement with the previously refined Li5.5Ce12F50
composition. The structure description indicates that Li1þx-
Ce12F50 is a good candidate to realize the electro-
chemical intercalation-disintercalation of Liþ in order to
change and to control the Ce3þ/Ce4þ ratio. Investigations are
in progress.
4þ
calculated formula Li5.2Ce3.23þCe8.8 F50, very close to the
refined one (Li5.5Ce3.53þCe8.5 F50).
4þ
5. Conclusion
The structure of Li5.5Ce12F50 has been successfully solved,
refined, and described by combining single-crystal X-ray
Supporting Information Available: (Table SI1) Atomic posi-
tional and displacement parameters for Li5.5Ce12F50. The
equivalent isotropic displacement parameter Ueq, defined as
one-third of the trace of the orthogonalized Uij tensor, is given
for cerium positions. (Table SI2) Anisotropic displacement
parameters (pm2) for cerium atoms in Li5.5Ce12F50. This ma-
terial is available free of charge via the Internet at http://
pubs.acs.org.
(36) Holgado, J. P.; Alvarez, R.; Munuera, G. Appl. Surf. Sci. 2000, 161,
301–315.
(37) Pfau, A.; Schierbaum, K. D. Surf. Sci. 1994, 321, 71–80.
(38) Yu, X.; Li, G. J. Alloys Compd. 2004, 364, 193–198.
(39) Hughes, A. E.; Gorman, J. D.; Paterson, P. J. K. Corros. Sci. 1996,
38, 1957–1976.