Local structure of molten LaCl3 analyzed by X-ray diffraction and La-LIII absorption-edge XAFS technique

Article abstract of DOI:10.1016/j.jallcom.2005.04.037

From the radial distribution analysis of X-ray diffraction, the nearest neighbor La-Cl distance and coordination number of Cl around La in molten LaCl₃ were estimated to be about 0.28 nm and nearly equal to 6, respectively. On the other hand, according to the XAFS measurements, the coordination number of Cl around La was evaluated at about 7. As a result, the local structure of molten LaCl₃ was thought to be described as an octahedral-like coordination scheme where La was surrounded by about six or seven Cl's. This finding led to the formation of octahedral complex ion, LaCl₆^3-, which was in agreement with results of Raman spectroscopy by other authors. The La-La distance was evaluated at about 0.5 nm from the position of a weak and broad second peak in the correlation function g(r), suggesting that a distorted corner-sharing connection of two octahedral complex species was predominant in the melt.

Full text of DOI:10.1016/j.jallcom.2005.04.037

Journal of Alloys and Compounds 408–412 (2006) 248–252
Local structure of molten LaCl3 analyzed by X-ray diffraction and La–LIII
absorption-edge XAFS technique
Y. Iwadatea,∗, K. Suzukia, N. Ondab, K. Fukushimab, S. Watanabec, H. Matsuurac,
A. Kajinamid, K. Takasee, N. Ohtorie, N. Umesakif, H. Kofujig, M. Myoching
a
Graduate School of Science and Technology, Chiba University, Inage-ku, Chiba 263-8522, Japan
b
Department of Applied Chemistry and Biotechnology, Chiba University, Inage-ku, Chiba 263-8522, Japan
c
Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan
d
Faculty of Engineering, Kobe University, Rokkodai, Nada-ku, Kobe 657-8501, Japan
Graduate School of Science and Technology, Niigata University, Igarashi-Ninocho, Niigata 950-2181, Japan
e
f
Japan Synchrotron Radiation Research Institute, Mikazuki, Hyogo 679-5198, Japan
g
Japan Nuclear Cycle Development Institute, Tokai-mura, Ibaraki 319-1194, Japan
Available online 2 June 2005
Abstract
From the radial distribution analysis of X-ray diffraction, the nearest neighbor La–Cl distance and coordination number of Cl around La in
molten LaCl3 were estimated to be about 0.28 nm and nearly equal to 6, respectively. On the other hand, according to the XAFS measurements,
the coordination number of Cl around La was evaluated at about 7. As a result, the local structure of molten LaCl3 was thought to be described
as an octahedral-like coordination scheme where La was surrounded by about six or seven Cl’s. This finding led to the formation of octahedral
complex ion, LaCl63−, which was in agreement with results of Raman spectroscopy by other authors. The La–La distance was evaluated at
about 0.5 nm from the position of a weak and broad second peak in the correlation function g(r), suggesting that a distorted corner-sharing
connection of two octahedral complex species was predominant in the melt.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Local structure; Molten salt; LaCl3; X-ray diffraction; XAFS
1. Introduction
The local structure of molten pure rare-earth trichlorides
MCl3 (M = Y, La–Ho) has been modeled after an octahedral
coordination (MCl6)3−, being supported by results of Raman
spectroscopy [3–5] and diffraction [6–9]. In the Raman stud-
ies, vibration modes corresponding to the octahedral coordi-
nation was observed for some rare-earth trichloride melts. In
the diffraction studies, the coordination number (CN) of the
first M–Cl correlation was evaluated to be about 6 for the pure
melts. However, another structural image has been recently
proposed in neutron diffraction (ND) and molecular dynam-
ics (MD) works. Wasse and Salmon [10,11] reported from
systematic ND studies of molten rare-earth trichlorides that
the CN of the first M–Cl correlation was not always fixed at
6; for example, that of molten LaCl3 was 8.2. Hutchinson et
al. [12,13] obtained MD simulation results of some rare-earth
trichloride melts by using a polarizable ionic model (PIM),
which reproduced well the ND results by Wasse and Salmon.
In order to develop a novel processing of spent nuclear
fuels, a lot of continuous efforts have been made from the
technological standpoints. Much attention has been recently
focused on a pyro-reprocessing of oxide fuels in molten salts
2+
with special composition and combination to make UO2
2+
and PuO2 ions coexist stably. However, many scientific
problems remain unsolved so as to establish this technique
[1,2]. As a preliminary part of the fundamental works to put
the processing into effect, the local structure of a LaCl3 melt
2+
instead of UO2 and/or PuO22+-containing melts was an-
alyzed by X-ray diffraction (XRD) and La–LIII absorption-
edge XAFS technique.
∗
Corresponding author. Tel.: +81 43 290 3433; fax: +81 43 290 3888.
E-mail address: iwadate@faculty.chiba-u.jp (Y. Iwadate).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.04.037

Y. Iwadate et al. / Journal of Alloys and Compounds 408–412 (2006) 248–252
249
The CN was simulated to be 7.9 for molten LaCl3. These ND
and MD works concluded that the CN changes with cation
size in the pure melts. In any event, the present work was
carried out so as to study what structures were formed in
rare-earth trichloride melts according to two different exper-
imental techniques, XRD and XAFS.
work, the cumulant expansion technique [20] was used to
treat anharmonic vibration effect. Data analysis procedure
forhigh-temperaturemoltensaltswassummarizedin [21,22].
The analytical procedure is the same as the work by Okamoto
et al. [23].
3. Results and discussion
2. Experimental
3.1. X-ray diffraction
Hygroscopic LaCl3 was prepared at 650 K according to
the below reaction,
The analytical functions used in this work are described
briefly as follows. The interference function Q·i(Q) and pair-
correlation function g(r) are defined by Eqs. (1) and (2), re-
La2O3 + 6NH4Cl → 2LaCl3 + 6NH3 + 3H2O
spectively,
After chlorination, sublimation at 1300 K for 8 h was carried
out to remove impurities such as oxide and oxychlorides as
well as to purify the crude LaCl3. The purified sample was
sealed in a thin quartz tube cell to prevent spoilage.
ꢀ
[Iecuoh(Q) −
fi(Q)2]
i
Q · i(Q) = Q ·
ꢀ
(1)
(
fi(Q))2
High temperature XRD measurements were made at
1173 K. An X-ray diffractometer in θ–θ type reflection ge-
ometry was used with Mo K␣ radiation. The Q-range was
9.3 nm−1 ꢀ Q ꢀ 150 nm−1 in which 2θ is the scattering angle
and Q = 4π sin θ/λ, with λ equal to 0.071069 nm. Scattered X-
ray intensities were corrected for background, polarization,
absorption and Compton scattering, and normalized to the co-
herent scattering intensity. Atomic scattering and Compton
scattering factors were taken from references [14,15].
XAFS measurements were performed at the BL-7C beam
line in High Energy Accelerator Research Organization
(KEK) at Tsukuba, Japan. The operating energy and ring
current were 2.5 GeV and 300–450 mA, respectively. The
synchrotron radiation was monochromatized by double Si
(1 1 1) crystals. XAFS measurement of the La–LIII edge
for LaCl3 was impossible by using the quartz cell. The
edge energy E0 = 5.48 keV of the La–LIII edge is too low
to obtain sufficient intensity of transmission X-ray. Thus,
the procedure proposed by Matsuura et al. [16] was ap-
plied to this work, and a disk sample was used in which
the halide was scattered homogeneously in a boron ni-
tride (BN) matrix. Since the La–LIII edge is close to the
La–LIII edge for lighter lanthanides, the maximum k in the
La–LIII edge XAFS measurement is usually limited up to
about 80–90 nm−1. The multielectron excitation [17] due to
2p4d → 5d2 transition is not negligible for the La–LIII edge
of lighter lanthanides. The effect usually appeared as a pro-
jection around more than k = 56 nm−1 in the XAFS func-
tion. Thus, XAFS experiments were performed in the range
of 25.0 nm−1 ꢀ k ꢀ 54.6 nm−1. Step-scanning measureme-
nts for 1 s at a datum point were performed to obtain X-ray
absorption spectra.
i
ꢁ
Q
max
g(r) = 1 + (2π2rρ0)−1
×
Q · i(Q) sin(Qr) dQ (2)
0
where fi(Q) is the atomic scattering factor, ρ0 the average
number density, Iecuoh(Q) the total coherent intensity function,
and Qmax is the maximum value of Q for the scattering ex-
periment. The analytical procedures were almost identical to
those of Narten [24].
Fig. 1 shows three pair-correlation functions, g(r)’s. One
is calculated from the original intensity data and the other two
are corrected in the tail-ends of the coherent intensity i(Q) as
illustrated in Fig. 2, where the original i(Q) in the range of
Q ꢁ 130 nm−1 is multiplied by 1/3 (correction 1) and 1/10
(correction 2), respectively, in order not to affect the first
CN. The noise in g(r) is eliminated by these corrections. It
can be seen that the typical peaks appeared at about r = 0.28
and 0.5 nm, the slight shoulder around r = 0.4 nm. The first
The computer program code WinXAS ver. 2.3 developed
by Ressler [18] was used to analyze the XAFS data. The
FEFF8 code [19] was used to simulate the XAFS (includ-
ing phase shift and backscattering amplitude). Coordination
number Nj, interionic distance rj and Debye–Waller factor σj2
are obtained from the curve fitting in R-space. In the present
Fig. 1. Pair-correlation functions g(r) of molten LaCl3 at 1173 K.

250
Y. Iwadate et al. / Journal of Alloys and Compounds 408–412 (2006) 248–252
Fig. 2. Coherent scattering intensity i(Q) of molten LaCl3. The original i(Q)
in the range of Q ꢁ 130 nm−1 is multiplied by 1/3 (correction 1) and 1/10
(correction 2), respectively.
Fig. 3. Interference function Q·i(Q) of molten LaCl3.
peaks in g(r) are thought to be due to the nearest neighbor
La–Cl correlation, and then Cl–Cl and La–La with increasing
r. Since the intensities of the first peaks are not much greater
than those of the first minima just behind the peaks, the first
coordination shells might not be stabilized very much. This
implies the existence of some distinct or short-lived local
geometry.
For a better understanding of the melt structure, the struc-
tural parameters for each atomic pair are needed to be refined
by the correlation method in the (Q, Q·i(Q)) space, using a
non-linear least squares fitting of Eq. (3),
pairs were 0.282 and 0.412 nm for both melts, respectively.
The atomic distance ratio of rCl–Cl/rLa–Cl is thus calculated to
be 1.46, which is very close to that characterized by the octa-
√
hedral geometry, 2. This finding indicates that there exists
suchan ionicaggregation as LaCl63− complex ioninthe melt.
Further deduction of the La–La pair distance of 0.518 nm and
the existence of octahedra reaches a conclusion that two octa-
hedra are linked with each other through a common Cl atom
so as to form a probable associated species, [La2Cl11]5−. It
should be stressed that the estimated coordination number of
La–Cl pair, 6.20, is an averaged value, meaning that the CN
is not fixed at 6 and the distribution of CN should be admitted
to be different CN’s of 5, 6, 7 and so on, as described before
[27].
ꢀ ꢀ
[
nij fi(Q)fj(Q)
i
j
× exp(−bij Q2) sin(Qrij )/rij ]
Q · i(Q) =
ꢀ
(3)
(
fi(Q))2
3.2. XAFS
i
The extracted XAFS function k3χ(k) and radial distri-
bution function Φ(R) of molten LaCl3, where the latter is
sometimes called Fourier transform magnitude |FT|, are
shown in Figs. 4 and 5, respectively, together with those of
solid LaCl3. The maximum k value used in the Fourier trans-
formation was 54.6 nm−1 for the melt data. No corrections,
such as the phase shift and backscattering amplitude, were
made in the Φ(R) functions. The oscillation in the k3χ(k)
function and the peak height in the Φ(R) function decreases
by a rise of temperature or melting. A phase shift to lower k
direction by melting was also detected in the k3χ(k) function.
The first peaks in both Φ(R) functions are assigned to the
nearest La–Cl correlation. In the solid state, there are two
peaks over r = 0.38–0.50 nm. There is, however, no broad
peak around 0.47 nm in the molten state, which has been
reported to exist as the La–La correlation [23]. Structural
where nij, rij, and bij are the average coordination number, the
average interatomic distance, and the temperature factor for
the atomic pair i–j, respectively. Each atomic pair was pre-
sumed to be Gaussian-distributed, centered at rij with a mean
square displacement 2bij. The initial values of the structural
parameters on fitting were preset to be equal to the parame-
ters obtained in the g(r) analysis. Tentative assignments of the
atomic pairs were at first made by consulting the ionic radii by
Shannon [25], the crystallographic data [26] and the results
for molten LaCl3 [23]. The structural parameters within the
relatively short range, for example, for the La–Cl pair, were
calculated from the interference function Q·i(Q) data for the
large Q region. As for the other Cl–Cl and La–La correla-
tions, the Q·i(Q) data over the whole range of measurements
were available. As depicted in Fig. 3, the observed Q·i(Q)’s
were well reproduced by the least squares fitted parameters
for molten LaCl3 listed in Table 1. The statistical errors in
ꢂ
ꢃ
1/2
Table 1
nij, rij, bij, and ꢀri2j
were estimated to be about 0.2,
Least-squares fitted structural parameters for molten LaCl3
0.001 nm, 0.5 × 10−6 nm2, and 0.001 nm, respectively,
ꢄ
ꢅ
1/2
i–j
nij
rij (nm)
ꢀri2j
(nm)
bij (nm2)
ꢂ
ꢃ
1/2
where ꢀri2j
is defined by (2bij)1/2. The coordination
La–Cl
Cl–Cl
La–La
6.20
7.14
4.58
0.282
0.412
0.518
0.030
0.072
0.080
4.51E-4
2.58E-3
3.22E-3
number of the nearest neighbor La–Cl pair was estimated at
about 6.20 and the atomic distances of the La–Cl and Cl–Cl

Y. Iwadate et al. / Journal of Alloys and Compounds 408–412 (2006) 248–252
251
Table 2
Structural parameters of La–Cl pair obtained from the XAFS curve fitting in R-space
Sample
i–j
nij
rij (nm)
σj2 (nm2)
C3 (nm)
C4 (nm)
LaCl3 at RT
La–Cl
La–Cl
6.00
3.21
0.278
0.305
0.0000349
0.0000895
–
–
–
–
LaCl3 at 673 K
LaCl3 at 1173 K
La–Cl
La–Cl
8.03
7.12
0.279
0.282
0.0001534
0.0002181
0.0000220
0.0006892
−0.00019305
0.00068978
of solid LaCl3 at RT can be described by two types of short
La–Cl correlation at 0.278 nm and long one at 0.305 nm.
From detailed investigation of the peak heights and half
widths in Fig. 5, the local structure of solid LaCl3 is found
to be different from that at RT, giving decreases of coordina-
tion number (6.00 + 3.21 → 8.03) and interatomic distance
(0.278 nm, 0.305 nm → 0.279 nm), implying the existence of
another stable state in solid LaCl3 before melting at 1150 K
[28]. The decrease in coordination number for La–Cl pair
from RT to 1173 K is thought to be due to the increase in mo-
lar volume and to be well interpreted by that on melting, for
instance, 19.1% [29]. As pointed out in introduction section,
Wasse and Salmon [10,11] reported that the CN of the first
La–Cl correlation in molten LaCl3 was 8.2. In contrast, the
corresponding CN in solid LaCl3 was 9, as described in [26].
This small difference in CN from 9 (solid) to 8.2 (melt) would
be rather difficult to be explained from the large expansion in
molar volume on melting, 19.1% [29] since the change of CN
from 6 (solid) to 4.0–4.5 (melt) on melting of several alkali
halides corresponded to the volume change of about 20%.
There was also found a discrepancy between the results
of XRD and XAFS concerning the coordination number of
La–Cl pair. However, this would be almost within the per-
missible range by taking into account the statistical errors
estimated in XRD and XAFS experiments.
Fig. 4. Extracted XAFS function k3χ(k) of molten LaCl3.
parameters of La–Cl pair obtained from the XAFS curve
fitting in R-space are listed in Table 2. In the curve fitting for
solid LaCl3, no third nor fourth cumulant [20] was contained
for evaluating anharmonic vibration effects. As can be seen
from Table 2, some structural changes seem to occur in LaCl3
from RT to 1173 K since the CN and the interatomic distance
of La–Cl pair varied certainly. The first coordination shell
4. Conclusions
The results of molten LaCl3 indicate that there is such an
3−
ionic aggregation as LaCl6 complex ion in the melt, and
two octahedra are linked with each other through a common
Cl atom to form a probable associated species [La2Cl11]5−
.
It is also found from this work that some structural changes
seem to occur in LaCl3 from RT to 1173 K, but XAFS
experiments were performed in the very limited range of k
due to the restriction on machine time and so on. Further
accumulation of the data by diffraction and XAFS is desired.
Acknowledgements
The synchrotron radiation XAFS experiments were per-
formed at BL-7C beam line in High Energy Accelerator Re-
search Organization (KEK) at Tsukuba with the approval of
KEK-PF (Prop. Nos. 2002G281 and 2003G084). The XAFS
facilities and computer time made available to us from KEK
are gratefully acknowledged.
Fig. 5. Radial distribution function Φ(R) of molten LaCl3.

252
Y. Iwadate et al. / Journal of Alloys and Compounds 408–412 (2006) 248–252
[14] J.A. lbers, W.C. Hamilton (Eds.), International Tables for X-ray Crys-
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