Electronic absorption spectra of lanthanides in a molten chloride: II. Absorption characteristics of neodymium(III) in various molten chlorides

Article abstract of DOI:10.1016/j.jallcom.2004.10.013

Electronic absorption spectra of trivalent neodymium in LiCl-KCl eutectic, NaCl-2CsCl eutectic, CaCl₂, LiCl, CsCl and mixtures of these at various temperatures were measured, and the variation of the absorption bands, ⁴G_5/2, ²G_7/2 ← ⁴I_9/2, were carefully analyzed. The dependence of their molar absorptivity, oscillator strength, and the degree of the energy splitting on temperature and melt composition were determined. As a result, it was suggested that the NdCl₆^3- complex keeps its octahedral symmetry in the NaCl-2CsCl eutectic, while this symmetry is more distorted in LiCl and CaCl₂. Based on these data, the chemical status of NdCl ₆^3- complex in molten chlorides was discussed.

Full text of DOI:10.1016/j.jallcom.2004.10.013

Journal of Alloys and Compounds 393 (2005) L1–L5
Letter
Electronic absorption spectra of lanthanides in a molten chloride
II. Absorption characteristics of neodymium(III) in various
molten chlorides
Toshiyuki Fujiia,∗, Takayuki Nagaib, Nobuaki Satoc, Osamu Shiraia, Hajimu Yamanaa
a
Research Reactor Institute, Kyoto University 2-1010, Asashiro Nishi, Kumatori, Sennan, Osaka 590-0494, Japan
b
Japan Nuclear Cycle Development Institute, 4-33, Muramatsu, Tokai, Ibaraki 319-1194, Japan
c
Institute for Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Sendai 980-8577, Japan
Received 21 July 2004; received in revised form 1 October 2004; accepted 4 October 2004
Available online 8 December 2004
Abstract
Electronic absorption spectra of trivalent neodymium in LiCl–KCl eutectic, NaCl–2CsCl eutectic, CaCl2, LiCl, CsCl and mixtures of these at
various temperatures were measured, and the variation of the absorption bands, 4G5/2, 2G7/2 ← I9/2, were carefully analyzed. The dependence
4
of their molar absorptivity, oscillator strength, and the degree of the energy splitting on temperature and melt composition were determined.
3−
As a result, it was suggested that the NdCl6 complex keeps its octahedral symmetry in the NaCl–2CsCl eutectic, while this symmetry is
3−
more distorted in LiCl and CaCl2. Based on these data, the chemical status of NdCl6 complex in molten chlorides was discussed.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Molten salt; Neodymium; Electronic absorption spectrometry; Oscillator strength; Molar absorptivity
1. Introduction
tion. A LiCl–KCl eutectic mixture (mole ratio of lithium to
potassium = 59/41) has been considered for metallic fuel re-
processing, and a NaCl–2CsCl eutectic mixture (mole ratio
of sodium to cesium = 1/2) for oxide fuel reprocessing. Re-
cently, CaCl2 has attracted a particular interest for the elec-
trolytic reduction of actinide dioxide fuels into metal. Since
the thermochemical stabilities of lanthanides and actinides
in melts greatly influence the performance of the reprocess-
ing process, and since their differences are the results of the
individual chemical status of their chloride complexes in dif-
ferent melts and other conditions, the analyses of the species
in the melts is an important subject for research of molten
salt system. Absorption spectrophotometry, which provides
information about the change of the chemical status of com-
plexes through the change in electronic absorption spectrum
is an adequate technique for this subject. In this paper, we
present the variation of the electronic absorption spectra of
Nd(III) in various molten chlorides at various temperatures.
The change of the coordination circumstance of Nd(III) will
also be discussed.
The pyrochemical reprocessing technique using molten
salts is considered to be one candidate for the future re-
processing technique of spent nuclear fuels. The chemical
characteristics of fission products, including lanthanides and
actinides, in molten salts are important information for un-
derstanding the pyrochemical reprocessing system. UV–vis
spectrophotometry is a strong analytical technique for char-
acterizing chemical species and their behavior in molten salts.
As the first step of our spectrophotometric study of trivalent
lanthanides with molten salts, we have measured electronic
absorption spectra of neodymium in a LiCl–KCl eutectic
mixture at 873 K, and have determined molar absorptivities
of its detectable absorption bands in the visible region [1].
From a technological viewpoint of pyrochemical repro-
cessing, three different chloride melts have attracted atten-
∗
Corresponding author.
E-mail address: tosiyuki@hl.rri.kyoto-u.ac.jp (T. Fujii).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.10.013

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T. Fujii et al. / Journal of Alloys and Compounds 393 (2005) L1–L5
2. Experimental details
Anhydrous neodymium trichloride was synthesized from
the reaction of neodymium sesquioxide, Nd2O3, with carbon
tetrachloride under an inert atmosphere. Nd2O3 was obtained
from Nihon Yttrium Co. Ltd. (99.99% purity), and CCl4 from
Wako Pure Chem. Ind. Ltd. (99% purity). Nitrogen gas of
99.99% was purchased from Nippon Sanso Co. Ltd. Nd2O3
powder in a quartz boat was set in a quartz reaction tube
of a horizontal electric furnace. Under N2 atmosphere, the
CCl4/N2 mixed gas was introduced to the reaction tube for
6 h at 773 K. Subsequently, for the growth of granules, the
temperature of the furnace was raised to 973 K and kept for
9 h with a N2 flow. The product identified as NdCl3 by XRD
analysis was supplied to the present study.
Anhydrous alkaline chloride samples used, LiCl–KCl eu-
tectic mixture (mole ratio of lithium to cesium = 59/41),
NaCl–2CsCl eutectic mixture (mole ratio of sodium to ce-
sium = 1/2), CaCl2, LiCl, and CsCl were products of Ander-
son Physics Laboratory Engineered Materials.
All the experiments were carried out in a glove box sys-
tem filled with dry Ar continuously purified to remove oxy-
gen and humidity. The content of impurity O2 and H2O
in the inside atmosphere was continuously maintained less
than 1 ppm. Absorbance measurements were performed with
various molten chlorides containing ca. 0.07 mol dm−3 (M)
NdCl3. The details of our experimental apparatus and the pro-
cedure for absorbance measurements can be seen elsewhere
[1,2]. The temperature of the sample during the experiment
was controlled to the desired value within 3◦.
Fig. 1. Electronic absorption spectra of Nd(III) in molten chlorides: (a)
CaCl2 at 1086 K, (b) mixture of 72 mol% LiCl–KCl and 28 mol% CaCl2 at
1084 K, (c) LiCl–KCl eutectic at 1047 K, (d) mixture of 50 mol% LiCl–KCl
and 50 mol% NaCl–2CsCl at 1023 K, and (e) NaCl–2CsCl eutectic at
1023 K.
3. Results and discussion
The electric dipole transition between f-orbitals that show
abnormal variations in intensity are classified as hypersensi-
tive transitions. These variations are attributed to the action of
an inhomogeneous electromagnetic field from the surround-
ing medium. In the case of neodymium, transitions from
4
2
4
the ground level I9/2 to excited levels of G7/2 and G5/2
,
4
4
which are abbreviated as G5/2,2G7/2 ← I9/2, are sensitive
to the coordination circumstances, and they are commonly
used to examine the coordination of Nd3+ [3]. The transi-
tion 4G5/2 ← I9/2 is known as the hypersensitive transition,
4
which is accompanied by and overlapped with a less intensive
non-hypersensitive transition to the level 2G7/2. Figs. 1 and 2,
which are given in units of molar absorptivity, show the ob-
served absorption bands over the wavelength region from
562 to 625 nm, which corresponds to these two transitions
of neodymium in various molten chlorides. These absorption
bands are composed of several transitions between degen-
erate terms of the ground and excited levels. These transi-
tions have been explained in the literature [4], as schemati-
cally shown in Fig. 3, and the observed absorption peaks in
Figs. 1 and 2 are attributable to transitions (I)–(V) of Fig. 3.
Fig. 2. Electronic absorption spectra of Nd(III) in LiCl, CsCl, and these
mixtures at 973 K. Dotted spectrum is obtained for Nd(III) in 14 M LiCl at
room temperature (∼298 K).

T. Fujii et al. / Journal of Alloys and Compounds 393 (2005) L1–L5
L3
Nd3+ has a similar highly symmetric octahedral coordination
as in molten elpasolite.
The result for the LiCl–KCl eutectic (spectrum (c)) shows
a sharper spectrum than that of the NaCl–2CsCl eutectic.
This suggests that the degree of the symmetric ligand field
effect is lower in LiCl–KCl, while also suggesting a lower
symmetry of the NdCl63− complex in the LiCl–KCl eutectic
than in the NaCl–2CsCl eutectic. The results obtained for the
mixture of LiCl–KCl and NaCl–2CsCl (spectrum (d)) show
intermediate properties between spectra (c) and (e). It is rea-
3−
sonably understood that the octahedral symmetry of NdCl6
in mixture (d) is distorted more than that in the NaCl–2CsCl
eutectic, but the degree of distortion is smaller than in the case
of the LiCl–KCl eutectic. The same trend can be seen in the
comparison of spectra (a)–(c). Spectrum (a) for pure CaCl2 is
3−
the sharpest, suggesting that, the NdCl6 complex has the
highest distortion in the various melts tested. The mixture
of CaCl2 and LiCl–KCl seems to have intermediate property
between the systems (a) and (c).
3−
Fig. 3. Diagram of hypersensitive transitions of NdCl6
.
Fig. 2 shows the electronic absorption spectra of Nd(III) in
LiCl, CsCl, andtheirmixturesat973 K. Spectra(f)through(i)
indicate the effect of the addition of CsCl to pure LiCl. Peak
splitting in (h)–(j) is clear, but it is not clear in (f) and (g). This
suggests that there is possibly a sharp change in the coordi-
nation circumstance during the process from (f) through (h),
and this changing point corresponds to a CsCl content around
40%. The spectrum of the CsCl system is similar to that of
the NaCl–2CsCl system shown in Fig. 1, spectrum (e). This
convinces us that Nd(III) in pure CsCl may have a coordi-
nation similar to that in the NaCl–2CsCl. Consequently, we
suggest that Nd(III) in systems containing more CsCl than
40% has quite a highly symmetric coordination, similar to
that in molten elpasolite. In other words, the addition of LiCl
over 60% makes it lose the symmetry to a certain distorted
octahedral symmetry.
A neodymium compound Cs2NaNdCl6, which is one kind
of elpasolite, is known to provide highly symmetric octahe-
dral coordination to Nd(III) in its crystal matrix [4], and the
absorption spectrum of Nd(III) in elpasolite is regarded as
a reference for spectral identification of octahedral symme-
try of f-elements. In Fig. 1, spectrum (e) shows four peaks,
which include transitions (I*), (I, II*), (II), and (IV, V), and
this is quite similar to the spectrum of Nd(III) in molten el-
pasolite [4]. Molten elpasolite is considered to maintain the
high symmetry of octahedral coordination of Nd as in solid
elpasolite, and the four peaks are thus considered to indicate
the result of the degeneracy of energy levels 4G5/2 and 4I9/2
,
under the ligand field by the highly symmetric octahedral co-
ordination. The similarity of the four peaks observed in the
NaCl–2CsCl melt to that in molten elpasolite suggests that
Table 1
Sum of splittings of electronic energy levels, molar absorptivity at 589 nm, and oscillator strength of hypersensitive transitions
ꢀ
Spectrum
Medium
Temperature (K)
␧
(M−1 cm−1
)
f (×106)
(ꢀE)a (cm−1
)
589
(a)
(b)
(c)
(d)
(e)
CaCl2
1086
1084
1047
1023
1023
15.3
13.6
11.9
11.7
10.9
25.2
27.1
25.5
24.9
24.4
419
440
457
465
470
b
(Li, K, Ca)Clx
LiCl–KCl
(Li, K, Na, Cs)Clc
NaCl–2CsCl
a
ꢀE(4I9/2) + ꢀE(4G5/2). Peak positions were analyzed by dividing the raw spectrum into four simple Gaussian functions.
Mixture of 72 mol% LiCl–KCl and 28 mol% CaCl2.
Mixture of 50 mol% LiCl–KCl and 50 mol% NaCl–2CsCl.
b
c
Table 2
Sum of splittings of electronic energy levels, molar absorptivity at 589 nm, and oscillator strength of hypersensitive transitions
ꢀ
Spectrum
Medium
Temperature (K)
␧
(M−1 cm−1
)
f (×106)
(ꢀE) (cm−1
)
589
(f)
(g)
(h)
(i)
LiCl
973
973
973
973
973
17.4
16.7
14.0
11.9
9.9
30.6
31.6
28.9
25.2
22.4
424
LiCl–CsCl, [Li]/[Cs] = 84/16
LiCl–CsCl, [Li]/[Cs] = 60/40
LiCl–CsCl, [Li]/[Cs] = 40/60
CsCl
441
466
463
478
(j)

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T. Fujii et al. / Journal of Alloys and Compounds 393 (2005) L1–L5
In order to know the coordination circumstance change of
molten chlorides in terms of the absorption transition prob-
ability, we evaluated the oscillator strength for our results
shown in Figs. 1 and 2. The oscillator strength can be defined
as,
ꢁ
9n
(n2 + 2)2
f = 4.319 × 10−9
ε(ν)dν
(1)
where ε(ν) is the molar absorptivity at energy ν (cm−1) and n
is the refractive index of the solvent medium. Data for n are
available for molten LiCl [5], KCl [5], NaCl [5], CsCl [5],
and CaCl2 [6]. For the mixed molten chlorides, the refractive
indexes were estimated by a linear interpolation between the
values of pure component melts [7]. Tables 1 and 2 show
the calculated oscillator strengths. The molar absorptivities
at 589 nm are shown together.
As shown in Fig. 3, the energy difference between transi-
tions (I) and (I*), as well as that between transitions (II) and
(II*), is equal to the energy difference between two levels as a
result of the splitting of the ground level, ꢀE(4I9/2). In a sim-
ilar manner, the energy difference between transitions (I) and
(II), as well as that between transitions (I*) and (II*), is equal
to the splitting of the excited level, ꢀE(4G5/2). The electric
field given by the octahedral coordination of Cl− promotes
the splitting of these terms, and thus the change on ꢀE can
be used as a probe for the change of the coordination sym-
metry of NdCl63−. Although some have studied ꢀE values
Fig. 4. Temperature dependence of molar absorptivity of Nd(III) in LiCl,
LiCl–KCl eutectic, or NaCl–2CsCl eutectic.
of the coordination circumstance of Nd(III) in LiCl–CsCl
mixture possibly occurs in the [Li]/[Cs] region from 60/40
to 100/0. Structure of the molten LiCl–CsCl, which possibly
influences the coordination status of dissolved Nd(III), have
been studied by neutron diffraction analysis [9] and Raman
spectroscopic analysis [10]. A difference in the melt structure
has been pointed out between the Cs+-rich mixture and the
Li+-rich mixture [10]. This structure change of the melt may
influence the symmetry of the Nd(III) complex, causing the
sharp change of Nd(III) spectrum, but further investigation is
required for clarifying this correlation.
3−
of NdCl6 for some molten chlorides [4,8], it was difficult
in this study to isolate the peaks of transitions (I) and (II*)
due to limited resolution of the spectrum. However, since the
peak positions of the transition (I*) and (II) were detectable,
we discuss the trend of the sum of ꢀE(4I9/2) and ꢀE(4G5/2),
ꢀ
whichisdefinedby (ꢀE) = ꢀE(4I9/2) + ꢀE(4G5/2). This
value can be obtained as the difference between transitions
(I*) and (II) listed in Tables 1 and 2.
ꢀ
As shown in Table 1, (ꢀE) decreases from spectrum (e)
to (a). This means that the octahedral symmetry of NdCl6
3−
The temperature dependence of the electronic absorption
spectrum of Nd(III) was studied for LiCl, the LiCl–KCl eu-
gradually gets distorted in this order. The f-values increased
in this order (except (a)), meaning that the transition prob-
ability increased along with the increasing distortion of the
symmetry, in compliance with the Judd–Ofelt Theory. These
two observations agree in supporting the increasing distor-
tions of the octahedral symmetry along with the variation of
the melt from NaCl–2CsCl to CaCl2. The exception of (a)
may be due to the strong polarizing power of Ca2+ surround-
ing NdCl63−, which is likely to make the electron-donating
ability of Cl− weaker, resulting in a decrease of oscillator
tectic, and the NaCl–2CsCl eutectic. The spectra obtained
ꢀ
are shown in Fig. 4. The f values and the (ꢀE) values
are determined and are shown in Fig. 5. In all systems, os-
cillator strengths increase with temperature. This indicates
3−
that the distortion of the NdCl6 complex from octahedral
symmetry grows with increasing temperature. Thermal mo-
tion of ions may have generated asymmetric vibrations of
the NdCl63− complex. The electron-donating ability of Cl−,
which is another cause of the increase of the f-value and is
likely to be influenced by coexisting alkaline cations, may
strength.
ꢀ
As shown in Table 2, for spectra (h)–(j), (ꢀE) values
are similar, but f increases by adding LiCl. This suggests
that the octahedral symmetry of NdCl63− is retained, but the
have been influenced by the temperature increase.
ꢀ
FortheLiCl–KClandtheNaCl–2CsCleutectics, (ꢀE)s
decrease with increasing temperature. This suggests the re-
duction of the ligand field of octahedral symmetry of the
NdCl63− complex, and thus it means an increased distortion
of octahedral symmetry with increasing temperature. The in-
crease of oscillator strength, which is also the result of the
electron-donating ability of Cl− may increase with the in-
ꢀ
creasing LiCl content. On the other hand, (ꢀE)-value of
spectrum (f) through (h) shows a sharp increase. This sup-
ports the above-mentioned suggestion of a significant change

T. Fujii et al. / Journal of Alloys and Compounds 393 (2005) L1–L5
L5
3−
of the octahedral symmetry of NdCl6 complex in various
molten chlorides was discussed. The observed characteris-
tics of the oscillator strength and splitting of the hypersensi-
tive transitions, 4G5/2,2G7/2 ← I9/2, suggested that NdCl6
4
3−
complex in the NaCl–2CsCl eutectic and in molten CsCl have
quite high octahedral symmetry, but that they are distorted in
molten LiCl and CaCl2. The observations for the mixture of
LiCl, KCl, and CsCl were reasonably intermediate between
the LiCl and CsCl observations. With increase of tempera-
ture, the oscillator strength increased and the peak splitting
decreased. This indicates that the distortion of NdCl63− com-
plex from octahedral symmetry proceeds with the increasing
temperature.
Acknowledgments
The authors are grateful to Dr. Victor V. Kolin of Khlopin
Radium Institute for his useful comments on absorption spec-
trophotometry with molten salts. The authors are grateful to
Professor Dr. George N. Papatheodorou of FORTH for his
valuable comments and discussions on this study. The authors
wish to thank Mr. Roy Jacobus for his help in improving the
English expressions of this paper.
Fig. 5. Temperature dependence of oscillator strength and energy of split-
tings for Nd(III) hypersensitive transitions. f-values (solid marks), (ꢀE)
values (open marks).
ꢀ
distortion of the octahedral symmetry, agrees with this result
ꢀ
ꢀ
of for (ꢀE). In contrast, for the LiCl system, (ꢀE)s did
not show appreciable change with increasing temperature,
while its oscillator strength showed an increase with increas-
ing temperature similar to that of other melts. It is considered
that the octahedral symmetry of the NdCl63− complex in LiCl
is quite distorted, which is strongly supported by its higher
oscillatorstrengththaninothermeltscontainingCsCl. There-
fore, the ligand field effect by octahedral symmetry to Nd3+
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ꢀ
are not resolved, and the low (ꢀE) observed for LiCl can
be attributable to this effect. Increased temperature, which
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4. Conclusions
The characteristics of the hypersensitive absorption band
of Nd3+ in various chloride melts were observed, and the state