Changes of vibrational modes upon melting solid Cs2NaScCl6, Cs3ScCl6, Cs3Sc2Cl9 and ScCl3

Article abstract of DOI:10.1016/S0009-2614(99)00583-7

Raman spectra of Cs₂NaScCl₆, Cs₃ScCl₆, Cs₃Sc₂Cl₉ and ScCl₃ compounds have been measured from 25°C to temperatures above their melting points. The vibrational modes of the discrete species ScCl₆^3- and Sc₂Cl₉^3- in the solids are transferred upon melting into the liquid and participate in an equilibrium involving ScCl₄^-. The spectra of solid ScCl₃ are dominated by the vibrational modes of a triple layer of ions consisting of edge-sharing ScCl₆^3- octahedra. Two main bands are seen in the melt spectra which infer a molecular liquid but a rather complex melt structure is suggested by the overall physicochemical data.

Full text of DOI:10.1016/S0009-2614(99)00583-7

1
6 July 1999
Chemical Physics Letters 308 Ž1999. 51–57
www.elsevier.nlrlocatercplett
Changes of vibrational modes upon melting solid Cs NaScCl ,
2
6
Cs ScCl , Cs Sc Cl and ScCl
3
6
3
2
9
3
a,b
a,b,)
G.D. Zissi , G.N. Papatheodorou
a
Institute of Chemical Engineering and High Temperature Chemical Processes – FORTH, P.O. Box 1414, GR-26500, Patras, Greece
b
Department of Chemical Engineering, UniÕersity of Patras, GR-26500, Patras, Greece
Received 15 March 1999; in final form 25 May 1999
Abstract
Raman spectra of Cs NaScCl , Cs ScCl , Cs Sc Cl and ScCl3 compounds have been measured from 258C to
2
6
3
6
3
2
9
3
y
3y
temperatures above their melting points. The vibrational modes of the discrete species ScCl and Sc Cl in the solids are
transferred upon melting into the liquid and participate in an equilibrium involving ScCl . The spectra of solid ScCl are
dominated by the vibrational modes of a triple layer of ions consisting of edge-sharing ScCl6 octahedra. Two main bands
are seen in the melt spectra which infer a molecular liquid but a rather complex melt structure is suggested by the overall
physicochemical data. q 1999 Elsevier Science B.V. All rights reserved.
6
2
9
y
4
3
3
y
1
. Introduction
theoretical w8x, computer simulation w9–11x and neu-
tron scattering w12,13x studies.
The melting behavior and the structural properties
Among the rare earths, scandiumŽIII. has the
of molten trivalent metal halides ŽLnX . and their
binary compounds with alkali halides ŽAX. depend
strongly on the physicochemical properties of the
trivalent salt w1,2x. Over the past twenty years, vibra-
tional Raman spectroscopy has been successfully
used for studying the changes of vibrational modes
on melting a series of elpasolite Cs NaLnX com-
pounds, binary compounds of the type A LnX and
LnX3 compounds in order to establish the melt
3
smallest ionic radius and forms the basis for the
systematic study of the structural and thermody-
namic properties of the end member of the series.
ScandiumŽIII. chloride can thus be used as a model
compound for predicting the behavior of rare-earth
and actinide halides in many processes such as the
recycling of spent nuclear fuel in which scandium
metal is itself among the uranium fission products.
The present work is concerned with the changes
of vibrational modes on melting Cs NaScCl , the
2
6
3
6
structure of the LnX –AX binaries and of the pure
component salts w1,3–7x. The structural properties of
LnX3 melts have been also the subject of recent
3
2
6
binary compounds Cs ScCl and Cs Sc Cl and
3
6
3
2
9
pure ScCl . Raman spectra of both the solids and
3
corresponding melts have been obtained at different
temperatures and the data are discussed in terms of
the liquid phase structure.
)
Corresponding author. E-mail: gpap@iceht.forth.gr; fax: q30
6
0
1 993255
009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S0009-2614Ž99.00583-7

5
2
G.D. Zissi, G.N. PapatheodorourChemical Physics Letters 308 (1999) 51–57
5
2
. Experimental
active modes: GŽO .sA qE q2F . One mode
h
1g
g
2g
q
ŽF . involves the translatory motion of a Cs ion
while the other three correspond to the intermolecu-
lar vibrations of the ScCl
All the expected Raman modes are seen in the
spectra ŽFig. 1A. at 258C. With increasing tempera-
tures, the n ŽA . and n ŽE . modes shift slightly to
2
g
High-purity anhydrous scandiumŽIII. chloride was
3y
6
prepared by chlorination of Sc O ŽCerac Pure 99%.
octahedra.
2
3
using AlCl3 ŽFlukarPure)99%. while heating
slowly from 220 up to 2808C in sealed quartz tubes.
The AlCl excess was removed with sublimation at
3
1
1g
2
g
1
708C and further heating at 5008C. Subsequently,
lower energies while the frequency of the equatorial
the product obtained was sublimed in high dynamic
vacuum Ž;10 mbar. at ;6508C and was further
bending n ŽF . and the lattice n ŽF . modes re-
main constant. Upon melting, the lattice mode is
5
2g
L
2g
y7
purified by a final sublimation at ;7508C in sealed
fused silica tubes. The sources and methods of pu-
rification of CsCl and NaCl were the same as before
w5x. The binary compounds were prepared by mixing
replaced by the polarized liquid wing on the tale of
which the depolarized n5 ŽF . mode is superim-
2g
posed. The stretching n ŽA . mode is also trans-
1
1g
ferred into the melt, giving a non-symmetric band
the appropriate stoichiometries of the component
salts and melting them directly into the Raman cell.
The Cs NaScCl and Cs ScCl have congruent
which presumably covers additional bands Ž;310,
y1
340 cm .. Apart from a band broadening, the liquid
spectra did not change with increasing temperature.
The overall behavior of the spectra is similar to
that observed for a series of elpasolite compounds
w3–5x and is best interpreted to indicate that the
predominant scandiumŽIII. species in the melt are
2
6
3
6
melting points and thus a slow cooling of the melt
gave the crystalline compounds. The Cs Sc Cl melt
3
2
9
was initially quenched to room temperature and sub-
sequently annealed at temperatures below the peri-
tectic point Ž;6508C. for 24 h as described in Ref.
w14x. All purified anhydrous materials were handled
in fused silica tubes orrand in a nitrogen-filled glove
box with a water vapor content of -1 ppm.
3y
6
the ScCl
octahedra.
3.2. Cs ScCl
3
6
The Raman apparatus with the optical furnace and
the procedures for measuring high-temperature spec-
tra of solids and melts were the same as before w5x.
The crystal structure of Cs ScCl is not known,
3 6
although it is possible, like other compounds with
the same stoichiometry Že.g., Cs FeCl w16x., that it
3
6
3y
6
contains discrete ScCl
distorted octahedra. Fig.
1
B shows the spectra of the solid at different temper-
3
. Results and discussion
atures. On going from 25 to 3008C, the bands are
broadened and in certain cases overlap leading at
high temperatures to spectra that are ‘identical’ to
the high-temperature elpasolite spectra. It seems that
the space symmetry of the solid at 258C corresponds
to a structure having distortions that can originate
The measured Raman spectra are presented here
either as raw data in the conventional IVV Žv.,
IHV Žv. or IUV Žv. vs. Raman shift Žv. or in
the reduced isotropic RISOŽv. and anisotropic
RANISOŽv. vs. v representation. The reasons and
5
from the Fm3m ŽO . space group compound. With
h
advantages of using the latter form for presenting the
spectra of isotropic melts are given in Ref. w4x.
increasing temperature, the amplitudes of vibration
increase and the crystal is gradually transformed to
the higher-symmetry cubic structure. The three ob-
served bands at high temperature are assigned to the
main bands of the elpasolite like structure as marked
on Fig. 1B.
3
.1. Cs NaScCl
2 6
As for a large number of trivalent metal halides,
scandiumŽIII. chloride forms the Cs NaScCl elpa-
The spectra of molten Cs ScCl and Cs NaScCl
at ;8408C are almost identical, again supporting
2
6
3
6
2
6
5
solite crystals with a cubic Fm3m ŽO . space sym-
h
metry w15x. The scandiumŽIII. site symmetry is O ,
the view that the predominant species present are the
h
3y
6
3y
forming almost ideal discrete ScCl
octahedra in
ScCl6 octahedral units. It also appears that the
the solid. Factor group analysis w3x gives four Raman
high-frequency branch of the n ŽA . band covers
1
g

G.D. Zissi, G.N. PapatheodorourChemical Physics Letters 308 (1999) 51–57
53
Fig. 1. Raman spectra of polycrystalline and molten Cs NaScCl ŽA. and Cs ScCl ŽB.. Conditions: l s488 nm, powers200 mW,
2
6
3
6
0
y1
y1 y1
slitss4 cm , time constants0.3 s, scan rates0.8 cm
s
.
two weak polarized bands at ;340 and ;310
cm . The origin of these bands will be discussed
later in the Letter.
metry. The overall structure is related to the
y1
4
Cs Cr Cl type wP6 rmmcŽD .x adopted by a series
3
2
9
3
6h
of compounds including the Cs Sc I w6x.
3
2 9
3
9
y
For the Sc Cl
molecular ion, the expected w17x
thirteen Raman active vibrations span the representa-
2
3
.3. Cs Sc Cl
3 2 9
X
X
Y
tion: GŽD .s4A q5E q4E . Fig. 2A shows the
3
h
1
The rhombohedral Cs Tl Cl type structure is
temperature dependence of the polycrystalline solid
3
2
9
6
3d
adopted by Cs Sc Cl having an R3cŽD . space
spectra and the assignments of the main bands at 258
3
2
9
group w14x. In the crystal, confacial bioctahedra
are marked on the basis of the normal coordinate
3y
9
3y
Sc Cl
occur with approximately D3h point sym-
analysis of the isostructural Cr Cl
ion. The two
2
2
9

5
4
G.D. Zissi, G.N. PapatheodorourChemical Physics Letters 308 (1999) 51–57
Fig. 2. Raman spectra of polycrystalline and molten Cs Sc Cl ŽA. and ScCl ŽB.. Conditions as in Fig. 1.
3
2
9
3
unmarked low-frequency bands are presumably lat-
tice modes. Due to the temperature broadening of the
bands, the spectra at 6408C show only the six
transferred into the melt, while the lattice modes are
X X
replaced by the liquid wing. The n ŽA . and n ŽA .
1
1
2
1
solid modes shift slightly to the ‘blue’ and appear as
3
y
X
strongest bands assigned to Sc Cl . The two A
polarized bands in the melt. In addition, a new
2
9
1
y1
X
X
modes at 336 and 260 cm
correspond to the
polarized band between n ŽA . and n ŽA . is pre-
1
1
2
1
scandiumŽIII.-chloride terminal n and bending n
sent.
t
b
3
y
modes, respectively, having a ratio n rn f0.77
It is noteworthy that the n ŽSc Cl . band is in
the same frequency range as the n ŽScCl . vibra-
b
t
2
2
9
3
y
which is close to the same ratio as was found for
1 6
3
9
y
Cr Cl
.
tion. This observation along with the correlation of
2
3
y
Upon changing phase, most of the strongest bands
the solid–liquid bands for the Sc Cl
and the
2
9
3y
9
y1
of the Sc Cl
in the high-temperature solid are
presence of the new band at 310 cm suggest that
2

G.D. Zissi, G.N. PapatheodorourChemical Physics Letters 308 (1999) 51–57
55
the melt structure is composed of more than one
predominant species. In support of this are the
changes observed in the liquid spectra with the tem-
perature variation ŽFig. 3.. The reduced Raman in-
tensity representation w4x used here corrects the rela-
tive intensities Žand slightly the frequencies. of the
bands with the Boltzmann thermal population factor
w1,4x and thus, the changes seen reflect mainly the
relative concentrations of the species present. On
The latter assignment is based on the proximity of
this band to other tetrahedral species in melts w1x.
Furthermore, from the known stretching frequencies
of ironŽIII. in octahedral and tetrahedral species w7x
3
y
y
we can calculate the ratio n ŽFeCl .rn ŽFeCl . s
1
6
1
4
3
y
0.88 which can be combined with the n ŽScCl .s
1
6
y1
y
275 cm
to give a scaled frequency n ŽScCl . s
1
4
312 cmy1.
The overall temperature-induced changes seen in
going from 710 to 8308C, the intensity of the 312
the spectra ŽFig. 3. suggest that three predominant
species in the melt participate in an equilibrium of
the type:
y1
cm band increases relative to that of the other two,
y1
while the intensities of the 344 and 278 cm bands
do not change in a parallel way. Thus, as suggested
y1
3y
3y
y
above, we assign the 344 cm
band to the
band to both the
2Sc
2
Cl
9
| ScCl
6
q3ScCl
4
.
3
y
y1
n ŽSc Cl . and the 278 cm
2
2
9
The shoulder bands appearing on the n ŽScCl y.
branch in the spectra of Fig. 1A and 1B at ;340
and 310 cm indicate that the Sc Cl
are also present in melts rich in alkali chloride.
3
3
y
3y
n ŽSc Cl . and the n ŽScCl .. The band at 312
1 6
1
2
9
1
6
y1
cm is assigned to the stretching vibration of a new
y1
3y
y
scandiumŽIII. species in a four-fold Žtetrahedral. co-
2
9
and ScCl4
y
ordination, n ŽScCl ..
1
4
3
.4. ScCl3
2
The rhombohedral wC ŽR3.x structure of scan-
3
i
diumŽIII. chloride consists of chlorine–metal–chlo-
rine ‘triple layers’ stacked along the hexagonal axis
of the centrosymmetric unit cell w18x. Distorted octa-
3y
6
hedral ScCl
units share edges with three other
neighboring octahedra forming an infinite two-di-
1
3
mensional structure with D space symmetry. The
d
vibrational spectra of the solid are dominated by the
triple layer vibrations w19x and the expected Raman
1
3
active modes are: GŽD .s2A q4E . All six
d
1g
g
modes were seen and are marked with their assign-
ments in Fig. 2B. The relative band intensities and
their temperature dependence give a spectral pattern
similar to those obtained for the isostructural solids
YBr w4x and GdBr w5x. The spectra show that the
3
3
triple layers do not exhibit a solid phase transition at
temperatures up to 8008C.
Upon melting, the spectra change drastically; two
main bands are present and the spectra extend up to
y1
5
00 cm . The low-frequency bands are substituted
y1
by the liquid wing, while the A1g mode at 266 cm
is probably transferred into the melt giving the band
marked as P at ;278 cm . A new band P at 430
cm , of approximately equal intensity with the P1
band, is also present in the spectra.
y1
1
2
y1
Fig. 3. Reduced Raman spectra of molten Cs Sc Cl at 710 and
3
2
9
8
308C. Conditions as in Fig. 1.

5
6
G.D. Zissi, G.N. PapatheodorourChemical Physics Letters 308 (1999) 51–57
From our previous studies we have found that the
the phase transition and the molar volume changes
are small. For the more compact solid structures
having the lanthanide in eight- or nine-fold coordina-
tion, molar volume changes up to ;25% may occur
w5x. In contrast, the melting of the rhombohedral
Raman spectra of a series of molten LnX ŽLnsY,
3
La, Nd, Gd; XsCl, Br. rare-earth halides, consist of
a simple pattern having a strong polarized band ŽP .
1
3
y
in the frequency range of the LnX6 octahedra plus
a weak polarized band ŽP . at higher frequencies
ScCl3 is followed by a rather high molar volume
2
w3–5x. The liquid spectra are practically independent
change Ž;43%., indicating that there is probably a
lower than six-fold coordination in the melt. Such
of the solid structure and the P and P bands are
1
2
more likely related to the triple layers of the rhombo-
hedral like solid w5x which contribute to the loose
network structure of the melt w3–5x. The relative
frequencies of these two bands Dn sŽn yn .r
large changes have also been measured for the AlCl3
and FeCl melts ŽTable 1. which are known to form
3
neutral ŽAl Cl . or partially ionized ŽFe Cl . dimers
2
6
2
6
having the cation in four-fold coordination w7x. The
relative differences between the terminal and bridg-
ing frequencies of molecular Al Cl are close to the
P
P
P
1
2
n
varies from ;0.25 to ;0.58. Table 1 lists
P1
these values along with other physicochemical prop-
erties of selected rare-earth halides. For all the melts
studied so far, the P and P bands overlap and the
2
6
DnP of the ScCl melt ŽTable 1. and this supports
3
structural similarities for the two salts.
1
2
relative integrated intensity of P is at least three
In other words, the spectral behavior ŽFig. 2B. in
1
times higher than that of P while for the ScCl melt
combination with the physicochemical properties of
2
3
the P and P bands are well separated and their
molten ScCl listed in Table 1 do not converge for
1
2
3
intensities approximately equal. However, the DnP
supporting a common structure. A part of the data
perceives an ionic liquid with a loose network-like
structure of edge bridged octahedra Ži.e., an YCl3
like liquid. while another part insinuates coordina-
tion changes on melting and a molecular liquid Ži.e.,
an Al Cl like liquid.. The latter structure may also
of ScCl is close to that measured for the YBr melt
3
3
and both have, like the other lanthanide halides, an
ionic electrical conductivity ŽTable 1..
The molar volume changes DVm on melting the
rare-earth halides depend on the structure and the
metal coordination in the solid. In all these melts, a
six-fold coordination has been proposed for the Ln
cation w2–5,9,22x. When the solid is rhombohedral or
2
6
account for the ionic conductivity of liquid ScCl if,
3
like in the case of the FeCl w7x, a partial ionization
3
takes place in the melt. Furthermore, degradation of
monoclinic, the lanthanide is in six-fold coordination
and thus there are no coordination changes during
the network like structure to form dimers ŽSc Cl .,
2
6
3y
6
3y
octahedral anions ŽScCl , Sc Cl . and cationic
2
9
Table 1
Melting characteristics of rhombohedral ŽRh.rmonoclinic ŽMo. trivalent metal halides
Salt Žstructure. a
m.p. b
CN solid c
CN liquid d
e
P
DVmf
Ž%.
s g
Dn
Ž10 S my1.
2
ŽK.
ScCl3 ŽRh.
YCl3 ŽMo.
1233
987
1186
1043
466
6
6
6
6
6
6
;4.8
;6
;6
;6
4
0.55
0.37
0.58
0.33
Ž0.50.
Ž0.32.
43
0.5
–
–
88
63
0.51
0.39
–
0.44
5.10
YBr ŽRh.
3
GdBr ŽRh.
3
h
e
y7
AlCl3 ŽMo.
FeCl3 ŽRh.
577
;3.8
0.04
a
Ref. w20x.
b
Ref. w21x.
c
Coordination number in solid ŽRef. w20x..
d
e
f
Coordination number in melt ŽRefs. w2–5,7,12,22x..
Relative shift of the P , P bands: Dn sŽn yn .rn ŽRef. w5x..
1
2
P
P
2
P
1
P
1
Molar volume change on melting: DV s100ŽV yV .rV ŽRefs. w23,24x..
m
l
s
s
g
h
sselectrical conductivity ŽRefs. w7,23,24x..
Corresponds to the ratio Žn yn .rn of the terminalrbridging frequencies of the dimers ŽM Cl . in the liquid phase.
t
b
b
2
6

G.D. Zissi, G.N. PapatheodorourChemical Physics Letters 308 (1999) 51–57
57
q
species w7x of the type Sc Cl would give an aver-
w7x G.N. Papatheodorou, G.A. Voyiatzis, Chem. Phys. Lett. 303
Ž1999. 151.
2
5
3
q
age coordination number for Sc of between 4 and
, which agrees with the neutron diffraction value
w8x Z. Akdeniz, G. Pastore, M.P. Tosi, Nuovo Cimento 20 D
6
Ž1998. 595, and references therein.
w12x ŽTable 1..
w9x R. Takagi, F. Hutchinson, P.A. Madden, A.K. Adya, M.
Gaune-Escard, J. Phys.: Condens. Matter 11 Ž1999. 645.
w10x E.A. Pavlatou, P.A. Madden, M. Wilson, J. Chem. Phys. 107
Ž1997. 10446.
However, at the present time no definite conclu-
sions can be made. Additional Raman studies of
ScCl vapors and of ScCl melts diluted with alkali
chlorides are needed in order to deduce the origin of
the P and P bands w25x. Computer simulation stud-
ies involving calculations of vibrational frequencies
3
3
w11x F. Hutchinson, M.K. Walters, A.J. Rowley, P.A. Madden, J.
Chem. Phys. Žin press..
1
2
w12x J.C. Wasse, P.S. Salmon, Physica B 241–243 Ž1998. 967.
w13x J.C. Wasse, P.S. Salmon, J. Phys.: Condens. Matter 11
Ž1999. 1381.
w14x K.R. Poeppelmeier, J.D. Corbett, T.P. McMullen, D.R.
Torgeson, R.G. Barnes, Inorg. Chem. 19 Ž1980. 129.
w15x L.R. Morss, M. Siegal, L. Stenger, N. Edelstein, Inorg.
Chem. 9 Ž1970. 1771.
analogous to those reported for LaCl3 melts w10x
would also be elucidating.
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