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Publikacja / Publication Reinvestigation of phase equilibria in TbCl3-LiCl binary system, Dańczak A., Salamon Beata, Kapała Jan, Rycerz Leszek, Gaune-Escard Marcelle
DOI wersji wydawcy / Published version DOI http://dx.doi.org/10.1007/s10973-017-6315-3
Adres publikacji w Repozytorium URL / Publication address in Repository https://www.wir.ue.wroc.pl/info/article/WUTe580d740b0f44984837e4a4b6e4e8260/
Data opublikowania w Repozytorium / Deposited in Repository on Oct 10, 2017
Cytuj tę wersję / Cite this version Dańczak A., Salamon Beata, Kapała Jan, Rycerz Leszek, Gaune-Escard Marcelle:
Reinvestigation of phase equilibria in TbCl3-LiCl binary system, Journal of Thermal Analysis and Calorimetry, Springer Netherlands, vol. 130, no. 1, 2017, pp. 25-33, DOI:10.1007/s10973-017-6315-3
Reinvestigation of phase equilibria in TbCl3–LiCl binary system
A. Danczak1• B. Salamon2
• J. Kapała1• L. Rycerz1
• M. Gaune-Escard3
Received: 14 November 2016 / Accepted: 17 March 2017 / Published online: 31 March 2017
� Akademiai Kiado, Budapest, Hungary 2017
Abstract The phase equilibria in the terbium(III) chlo-
ride–lithium chloride pseudobinary system were estab-
lished by means of differential scanning calorimetry. It was
established that the pure terbium(III) chloride undergoes
solid–solid phase transition at 790 K and melts at 859 K.
The TbCl3–LiCl pseudobinary system is characterized by
the existence of two compounds. First one, namely Li3TbCl6, forms at 553 K and melts incongruently at 727 K.
Second compound, LiTbCl4, decomposes in the solid state
at 609 K. The composition of Li3TbCl6–TbCl3 eutectic
corresponding to terbium(III) chloride mole fraction
x = 0.521 (T = 665 K) was found from Tammann plot,
which predict, through application of the lever rule, the
variation of the enthalpy associated with eutectic melting
as a function of composition. The obtained results have
been compared with the literature data concerning for the
TbCl3–LiCl pseudobinary system. The phase diagram of
the TbCl3–LiCl pseudobinary system was also optimized
by CALPHAD method.
Keywords DSC � Terbium(III) chloride �Lithium chloride � Phase diagram � CALPHAD �Associate Solution Model
Introduction
Rare-earth halides have high practical impact in modern
technologies and are used in a number of applications like
reprocessing of nuclear wastes, recycling of spent nuclear
fuel, doses in high-intensity discharge lamps, lasers and new
highly efficient light sources with energy-saving features
[1–3]. Wide technological application of lanthanides halides
requires knowledge of their basic thermodynamic properties.
However, the thermodynamic data available in the literature
are still incomplete and very often contradictory.
The pioneering works in elucidating of phase equilibria
in lanthanide chloride–alkali metal chloride systems were
performed in the sixties of twentieth century mainly at the
Lomonosov University in Moscow [4]. The extensive
summaries about chloride systems up to 1977 can be found
in the Gmelin Handbook [5] and in the monograph of
Posypaiko and Alekseeva [6]. The Russian researchers
studied phase equilibria of almost all LnCl3–NaCl and
LnCl3–KCl (Ln = lanthanide) systems, most of the LnCl3–
CsCl systems and two systems with rubidium chloride
(NdCl3–RbCl, SmCl3–RbCl) [5, 6]. The visual-polythermal
method, which is not very accurate, was used in these
investigations. This technique can give quite good results
for liquidus lines and thus for the existence of congruently
melting compounds. However, the results for incongruently
melting compounds are sometimes doubtful. Furthermore,
solid–solid state reactions with small reaction enthalpies
can be overlooked. As a consequence, the investigations on
many systems were incomplete. This situation could be
& B. Salamon
1 Division of Analytical Chemistry and Analytical Metallurgy,
Faculty of Chemistry, Wrocław University of Science and
Technology, Wybrze _ze Wyspianskiego 27, 50-370 Wrocław,
Poland
2 Department of Inorganic Chemistry, Faculty of Engineering
and Economics, Wrocław University of Economics,
Komandorska 118/120, 53-345 Wrocław, Poland
3 CNRS UMR 7343-IUSTI, Ecole Polytechnique, 5 rue Enrico
Fermi, 13453 Marseille Cedex 13, France
123
J Therm Anal Calorim (2017) 130:25–33
DOI 10.1007/s10973-017-6315-3
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observed in the case of terbium(III) chloride–alkali metal
chlorides systems.
In the work of Kutscher and Schneider [7], the phase
diagrams of lanthanide halide–alkali metal halide systems,
determined by means of DSC, could be found. Comparison
of Kutscher and Schneider data [7] with older literature
data [6] leads to conclusion that significant differences
exist between them. They include both temperatures of
phase transitions as well as existence of different
compounds.
Therefore, the systematic investigations of systems of
all lanthanides chlorides–alkali metal chlorides (alkali
metal = Na - Cs) were performed by the Seifert’s group
from the University of Kassel [4, 8]. The phase diagrams of
these systems were constructed on the basis of thermo-
chemical and structural measurements. According to this
data [9, 10], all TbCl3–MCl systems are characterized by
the existence of M3TbCl6 and MTb2Cl7 compounds which
melt incongruently in the TbCl3–NaCl system and con-
gruently in TbCl3–MCl (M = K - Cs) systems. Addi-
tionally the M2TbCl5 compounds are present in TbCl3–
MCl (M = Na, K, Cs) systems [9, 10]. The phase diagram
of TbCl3–NaCl shows also the existence of another com-
pound with stoichiometry NaTbCl4 [10].
The LiCl3–LnCl3 (Ln = lanthanide) were not investi-
gated by the Seifert’s group. Fragmentary information
concerning the phase equilibria in the systems of lan-
thanides chlorides with lithium chloride comes mainly
from Chinese articles [11–15]. According to them, the
LnCl3–LiCl (Ln = La - Nd) systems are eutectic systems,
the LnCl3–LiCl (Ln = Sm, Tb) systems are characterized
by the existence of LiLn2Cl7 compounds that decompose in
the solid state and the LnCl3–LiCl (Ln = Dy, Ho, Er, Yb)
systems are characterized by the existence of Li3LnCl6compounds that melt incongruently. The TbCl3–LiCl sys-
tem [15] (Fig. 1) is characterized by the existence of
LiTb2Cl7 compound that undergoes solid–solid phase
transition and decomposes in the solid state. The existence
of Li3TbCl6, is not shown on the phase diagram [15]. It
should be also pointed out that the authors did not inform
about solid–solid phase transition of terbium(III) chloride,
which is well known in the literature [16–19]. In our pre-
vious work [20], we reinvestigated the DyCl3–LiCl binary
system. The determined phase diagram of DyCl3–LiCl
binary system was found to be completely different from
the one presented in the literature [14]. Therefore, we
decided to reinvestigate also TbCl3–LiCl system. We used
differential scanning calorimetry (DSC) to study phase
equilibria in this binary system. This work is continuation
of multi-instrumental international program focused on
lanthanides halides and their systems with alkali metal
halides. The main purpose of this program is to contribute
new experimental data on unknown systems and, when
relevant, to assess the reliability of literature existing
information. The additional purpose of this work is
obtaining a consistent and reliable thermodynamic
description of investigated system, by the CALPHAD
method, which is semi-empirical technique for phase dia-
gram calculation.
Experimental
Chemicals and samples preparation
Terbium(III) chloride was synthesized from ter-
bium(III, IV) oxide (99.999%, Sigma Aldrich). The first
step of the synthesis was dissolution of Tb4O7 in hot
concentrated HCl. The obtained solution was evaporated,
and the terbium(III) chloride hexahydrate was crystallized.
The next step was the dehydration of chloride crystals. The
ammonium chloride was added in order to prevent the
oxychloride formation. The mixture of hydrated ter-
bium(III) chloride and ammonium chloride (45 mass%)
was slowly heated up to 1370 K in the quartz ampoule
under vacuum. During the heating process, water was
removed, and unreacted ammonium chloride was subli-
mated. The final step was purifying the crude terbium(III)
chloride by distillation under reduced pressure (*0.1 Pa).
The synthesized terbium(III) chloride was of high grade
(min. 99.9%). The purity was confirmed by chemical
analysis (complexometric method for terbium and mer-
curimetric method for chloride). The results were as fol-
lows: Tb: 60.11 ± 0.12% (59.91% theoretical); Cl:
40.18 ± 0.13% (40.09% theoretical).
The synthesized terbium(III) chloride and the commer-
cially available lithium chloride (99.9%, Alfa Aesar) were
used for samples preparation. The appropriate amounts of
both chlorides were melted in quartz ampoules and then
homogenized by shaking and solidified. The obtained melts
0 0.2 0.4 0.6 0.8 1
900
850
800
750
700
650
600
550
500
T/K
x(TbCl3)
β-LiTb2Cl7α-LiTb2Cl7
Fig. 1 TbCl3–LiCl phase diagram obtained by Chao-Gui et al. [15].
The experimental points were obtained by digitization directly from
published graphs [15]
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were ground to powder in agate mortar in a glove box.
Samples prepared in this way were used for differential
scanning calorimetry measurements (DSC).
Measurements
A Setaram DSC 121 scanning calorimeter (DSC) was used
to investigate the phase equilibria in the TbCl3–LiCl sys-
tem. The apparatus was calibrated by the Joule effect. Data
acquisitions and processing were computer operated. The
experimental quartz cells, 7 mm in diameter and 15 mm
long, were filled with the required amount of sample
(150–300 mg) and sealed under vacuum. DSC measure-
ments on samples with 18 compositions were taken with
heating and cooling rates 5 K min-1. Due to the super-
cooling effect, visible in Fig. 2, all experimental tempera-
ture and enthalpy data were determined from heating
curves. Solidus and liquidus temperatures were determined
as onset temperature and peak maximum temperature,
respectively.
Results and discussion
It was established that pure terbium(III) chloride undergoes
solid–solid phase transition at 790 K and melts at 859 K.
These results are in good agreement with the literature data
[16–19]. According to them, temperature of transition is
equal 790 K [16], 783 K [17], 783 ± 5 K [18] or 793 K
[19], and temperature of fusion is equal 854 K [16], 855 K
[17], 855 ± 3 K [18] and 857 K [19], respectively. Second
component of the system, namely lithium chloride, was
found to melt at 888 K. This temperature is in excellent
agreement in the literature data (883 K) [21].
Figure 3 shows the DSC curves obtained for the samples
with the mole fraction of terbium(III) chloride,
340
300
260
220
180
140
100
60
20
700 750 800 850 900 950
700 750 800 850 900 950
700 750 800 850 900 950
700 750 800 850 900 950
440
400
360
320
280
240
200
160
120
350
300
250
200
150
100
50
0
–50
25
0
–25
–50
–75
–100
–125
–150
–175
T/K T/K
T/K T/K
HF
/mW
HF
/mW
HF
/mW
HF
/mW
Tmelting = 888 K
Tmelting = 859 K
Tcrystallisation = 861 K Tcrystallisation = 833 K
Ttransition = 770 K
Ttransition = 790 KExo
Exo Exo
Exo
(a) (c)
(b) (d)
Fig. 2 DSC heating and cooling curves for pure system components, obtained at rate of 5 K min-1: a heating curve of LiCl, b cooling curve of
LiCl, c heating curve of TbCl3, d cooling curve of TbCl3
Reinvestigation of phase equilibria in TbCl3–LiCl binary system 27
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x = 0.098 (A), 0.300 (B) and 0.794 (C), obtained at heat-
ing rate of 5 K min-1. Depending on the composition of
the mixture, three of four endothermic effects were
observed on the heating curves. For all of the samples, the
effect at the highest temperature corresponds to the liq-
uidus temperature.
In the composition range 0\ x\ 0.250, where x is a
mole fraction of terbium(III) chloride, two endothermic
peaks were present in the all heating DSC curves in addi-
tion to liquidus effect (Fig. 3a).The thermal effect at 553 K
(a mean value from experiments) was observed on DSC
curves in the composition range up to x\ 0.500 (Fig. 3a).
Tammann diagram, i.e., dependence of the enthalpy related
to this effect (calculated per mole of mixture) on the
composition of the mixture was created (Fig. 4a). This
diagram is an important outcome from the theory of phase
diagrams [22–24]. It predicts, through application of the
lever rule, the variation of the enthalpy associated with a
first-order transformation as a function of concentration
[20]. From the intercept of the two liner parts on the dia-
gram, it was determined that the maximum value of
enthalpy related to the effect at 553 K corresponds to the
mole fraction of terbium(III) chloride x = 0.250. This
information suggests an existence of Li3TbCl6 compound
in investigated system.
The existence of Li3TbCl6 compound has been con-
firmed by thermal effect observed at 727 K for the samples
with mole fraction of terbium(III) chloride from
0\ x\ 0.250 (Fig. 3b). The enthalpy related to this effect
(per mole of the mixture) increases with the increasing
mole fraction of terbium(III) chloride up to x = 0.250
(Fig. 4b). This effect can be undoubtedly ascribed to
incongruently melting of Li3TbCl6 compound. Accordingly
the first effect (at 553 K) can be ascribed to formation or
solid–solid phase transition of Li3TbCl6 compound. Dis-
appearance of this effect in samples with mole fraction of
terbium(III) chloride starting from x = 0.500 suggests an
existence of a compound with stoichiometry of LiTbCl4.
550 600 650 700 750
550 600 650 700 750
800
600 650 700 750 800
850
T/K
T/K
T/K
50
0
–50
–100
–150
–200
0
–20
–40
–60
–80
–100
20
10
0
–10
–20
–30
–40
HF
/mW
HF
/mW
HF
/mW
Tonset(1) = 555 KTpeak = 833 K
Tpeak = 728 K
Tpeak = 778 K
Tonset(2) = 727 K
Tonset(2) = 611 K
Tonset(2) = 668 K
Tonset(3) = 662 K
Tonset(1) = 548 K
Tonset(1) = 601 K
(a)
(b)
(c)
Exo
Exo
Exo
Fig. 3 DSC heating curves for TbCl3–LiCl mixtures of different
compositions: a x(TbCl3) = 0.098, b x = 0.300 and c x = 0.794,
obtained at heating rate of 5 K min-1 x(TbCl3)0 0.2 0.4 0.6 0.8 1
A
B
C
D E14
12
10
8
6
4
2
0
H/k
J/m
ol–1
Δ
Fig. 4 Tammann diagrams, constructed on the basis of experimen-
tally determined enthalpy coming from the DSC heating curves, for
determination: a decomposition of Li3TbCl6 compound, b incongru-
ently melting of Li3TbCl6, c formation of LiTbCl4, d composition of
Li3TbCl6–TbCl3(s2) eutectic, e solid–solid phase transition of TbCl3
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Three or four thermal effects were observed on DSC
curves in the composition range 0.250\ x\1 (Fig. 3b, c).
Effects at the highest temperature correspond to the liquidus.
The thermal effect at 553 K visible on DSC curves of the
samples with mole fraction of terbium(III) chloride x\0.500
is connected with formation or phase transition of Li3TbCl6compound. Second thermal effect appears at 609 K. Tam-
mann diagram constructed for this effect is presented in
Fig. 4c. Intercepts of the two linear parts in this diagram give
the mole fraction of terbium(III) chloride x = 0.498 which is
in good agreement with composition of the LiTbCl4 com-
pound. Accordingly effect at 609 K can be ascribed to the
decomposition of the LiTbCl4 compound in the solid state.
The third endothermic effect observed at 665 K is
undoubtedly connected with Li3TbCl6–TbCl3 eutectic. The
eutectic composition, x = 0.521, was determined from the
Tammann plot displayed in Fig. 4d. The mixture with
eutectic composition melts with enthalpy, DfusHm of
11.7 ± 0.6 kJ mol-1 at temperature 665 K.
In the samples with mole fraction of terbium(III) chlo-
ride, x[ 0.9 effect related to the solid–solid phase transi-
tion appears at 785–790 K.
All the results of DSC investigations are presented in
Table 1, and the complete phase diagram is shown in
Fig. 5. It differs significantly from phase diagram given in
the literature [15]. Firstly, literature phase diagram does not
present phase transition of terbium(III) chloride, which is
well documented in the literature [19] and is present in our
phase diagram.
Secondly, the reinvestigated by us phase diagram is
characterized by the existence of two compounds with the
stoichiometry Li3TbCl6 and LiTbCl4, whereas according to
the literature data [15], only one compound with stoichiom-
etry LiTb2Cl7 exists in TbCl3–LiCl system. As a conse-
quence of existing of Li3TbCl6 compound, the Li3TbCl6–
TbCl3 eutectic was found by us in the system (x = 0.521 and
T = 665 K). The literature data [15] shows existence of
LiCl3–TbCl3 eutectic (x = 0.501 and T = 718 K).
Table 1 Results of the DSC experiments performed on the TbCl3–LiCl binary system
x(TbCl3) T1/K T2/K T3/K T4/K T5/K Tliq/K
Li3TbCl6 formation LiTbCl4 decomposition Li3TbCl6–TbCl3(s2) eutectic Li3TbCl6 decomposition TbCl3 transition Liquidus
0.000 – – – – – 883
0.050 553 – – 726 – 865
0.098 555 – – 727 – 833
0.150 554 – – – – –
0.195 556 – – 727 – 756
0.245 556 – – 728 – 739
0.300 548 611 662 – – 728
0.326 547 616 665 – – 720
0.352 558 611 661 – – 703
0.375 550 610 664 – – 691
0.425 555 613 664 – – 677
0.486 548 613 665 – – 680
0.616 – 610 664 – – 716
0.703 – – 672 – – 751
0.720 – 606 671 – – 758
0.794 – 601 668 – – 778
0.914 – 596 660 – 785 823
1.000 – – – – 790 859
The standard uncertainties of measured temperature and mole fraction equal to 1 K and 0.002, respectively
0 0.2 0.4 0.6 0.8 1
x(TbCl3)
900
850
800
750
700
650
600
550
500
T/K
888
859
790(790)
(859)
(888)
(727)728
554
609
663 0.450(664 0.521)
(609)
(553)
LiCl + L
LiCl + Li3TbCl6
LiCl + LiTbCl4
LiTbCl4 + TbCl3(s1)
Li3TbCl6 + TbCl3(s1)
Li3TbCl6 + LiTbCl4
Li3TbCl6 + LL + TbCl3(s1)
L + TbCl3(s2)
Li3T
bCl 6
L
LiT
bCl 4
Fig. 5 TbCl3–LiCl phase diagram: circles—experimental points,
lines—calculated values. Values given in parenthesis coming from
experiment, while without parenthesis coming from our calculation
Reinvestigation of phase equilibria in TbCl3–LiCl binary system 29
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CALPHAD calculation
The thermodynamic modelling of phase diagrams by the
CALPHAD method takes into account the critical
assessment of all available thermochemical properties of
the investigated system. This technique is based on
modelling the Gibbs energies of all phases present in a
system. A detailed description of the CALPHAD method
was published by Lukas et al. [25] and by Saunders and
Miodownik [26]. Thermodynamic properties of a system
are presented by the phase diagram, but often, the
available data do not provide a complete description of
all phases formed in a system. Optimization of phase
diagram by CALPHAD involves the calculation of
unmeasured properties of phases in the investigated sys-
tem. The ability to control the final properties of a system
is one of the most important goals in thermodynamics;
therefore, the calculation of phase diagrams using the
CALPHAD method has become a necessary complement
to material research.
The calculation by the CALPHAD method was made
using the literature data for pure system components
given by Kubashewski et al. [21]. Thermodynamic
functions of the pure system components are presented in
Table 2. For compounds present in the system under
investigation, namely Li3TbCl6 and LiTbCl4, the Neu-
mann–Kopp rule was applied. This means that the
enthalpy of formation and entropy of formation of these
compounds are not dependent on temperature. The data
on mixing enthalpy of the system under investiga-
tion used in the optimization were measured by
Rycerz et al. [27].
All calculations presented in this work were made by
BINGSS and BINFKT programs of Lukas. The special
management program PHDMAN [28] was used for fast
optimization. A detailed description of the optimization
procedure by BINGSS and BINFKT programs was given
by Lukas and Fries [29]. The PHDMAN program was
expanded for fast data change in trial-and-error approach
and Bayes regression using BINFKT program.
The first step of optimization of the phase diagram is
preparation of the appropriate data in format used by
BINGSS and BINFKT programs. The next step is focused
on fitting curves, which represent the dependence of
interaction parameter (k) and mixing enthalpy (DHMIX) on
mole fraction of terbium(III) chloride. The experimental
mixing enthalpy [27] data were used for calculation of the
liquid phase properties in the investigated system by
associate solution model (ASM) with the constant ratio
p(LiCl)/q(TbCl3) equals 3. The ASM is applicable to
binary mixture consisting of the system constituent com-
ponents that exhibit a compound-forming tendency caused
Ta
ble
2T
her
mo
dy
nam
icfu
nct
ion
so
ffo
rmat
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of
the
pu
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on
ents
use
dfo
rca
lcu
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on
s
Co
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ou
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Tem
per
atu
rera
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e/K
S0(2
98
.15
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-DH
SE
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G0 mTðÞ�
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ER
29
8:1
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ðÞ¼
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�1=
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LiC
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9–
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31
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.79
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.7-
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0.1
64
91
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-9
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Tb
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78
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––
30 A. Danczak et al.
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by short range ordered regions of molecules. Then the
liquid phase is formed by associates remaining in equilib-
rium with free molecules that are outside of the chemically
short range ordered regions. The ASM assumes the fol-
lowing formation of associates:
pLiCl þ qTbCl3 ¼ LiClð Þp TbCl3ð Þq; ð1Þ
where p and q are the number of molecules taking part in
the formation of associates. This model was successfully
applied in our previous works [30–32]. The detailed
description of ASM is given by Sommer [33] and
Krull et al. [34]. The assumed value of the p(LiCl)/
q(TbCl3) ratio corresponds to the main associate
{3Li? ? TbCl63-} existence in the liquid phase in the
investigated system. The spectroscopic studies on molten
lanthanide halides and their mixture with alkali halides
showed a presence of predominant LnX63- octahedra spe-
cies in melts (Ln = lanthanide, X = halide) [35].
In the binary TbCl3–LiCl solution, the mole fractions of
lithium chloride and terbium(III) chloride were described
as x(LiCl) and x(TbCl3), respectively. After the association
process, the solution behave as LiCl–TbCl3–(LiCl)p
(TbCl3)q pseudoternary liquid and the mole balance is
denoted as:
n ¼ nLiCl þ nTbCl3 þ na; ð2Þ
where n is the total number of moles, nLiCl, nTbCl3 and na
are the number of mole of lithium chloride terbium(III)
chloride and associate, respectively.
The Gibbs energies of mixing were described by the
formula:
GMIX Tð Þ¼R �T � yLiCl � lnyLiClþyTbCl3 � lnyTbCl3 þya � lnyað Þ�nþ �18500:0þ0:5 �Tð Þ�ya �nþf �2000:0�8:7 �Tð Þ�yLiCl �yTbCl3
þ �9567:0þ16:0 �Tð Þ�yLiCl �ya
þ �14000:0þ8:0 �Tð Þ�yTbCl3�yag�n
ð3Þ
where ya is a mole fraction of associate in the mentioned
pseudoternary system, and mole fractions of lithium chlo-
ride (yLiCl) and terbium(III) chloride (yTbCl3 ) are repre-
sented by the formulas:
yLiCl ¼ xLiCl � 1 þ pþ q� 1ð Þ � ya½ � � p � ya ð4ÞyTbCl3 ¼ xTbCl3 � 1 þ pþ q� 1ð Þ � ya½ � � q � ya ð5Þ
The next step concerns the finding a good fit of liquidus
and eutectic points with iteration method by changing the
parameters of formation Gibbs energy of solid and liquid
phases. The thermodynamic properties (Gibbs energies of
formation) of Li3TbCl6 are described by the following
formula (SGTE description):
DfG Tð Þ ¼ 4504:1�10:2 � T ð6Þ
The calculated dependence of formation Gibbs energy
on temperature of Li3TbCl6 takes positive values in the
298–442 K temperature range (marked as dashed line in
Fig. 6). This fact settles questions concerning the nature of
thermal effects observed at 553 K (paragraph about phase
diagram construction). They are related to the formation of
this compound from terbium(III) chloride and LiTbCl4.
Due to narrow temperature range of existence, small
number of experimental points in equilibria with liquid
phase and lack of heat capacity, only values of Gibbs
formation energy of LiTbCl4 at the highest temperature of
existence (609 K) are estimated (-2.2 kJ mol-1).
The quality and compliance with experimental data of
the fitting were controlled by the deviation applied to
enthalpy of mixing and liquidus points, simultaneously.
The calculated data are in good agreement with the
measured one. However, there is a slight difference
between estimated by the Tammann plot and calculated by
CALPHAD eutectic composition. In addition, there is
small discrepancy between measured dependence of mix-
ing enthalpy on mole fraction and optimized course of this
dependence. These facts may result from the experimental
data obtained by various calorimetric techniques. All these
data could contain unknown systematic errors. It is very
likely that reason for this discrepancy is also the accuracy
of experimental data. Nevertheless, the interpretation of the
phase diagram made in this work leads to consistent and
real thermodynamic description of all phases in the system
investigated. The comparison of calculated and experi-
mental invariant points is shown in Table 3.
The minimum on dependence of mixing enthalpy on
mole fraction, which is indicative of associate formation, in
this system is observed at 0.300 x(TbCl3), which does not
coincide exactly with the assumed p/q ratio equal to 3. This
phenomenon can be explained by the nature of mixing
300 400 500 600 700
T/K
8
4
0
–4
–8
–12
fG/k
J/m
ol–1
Δ
Fig. 6 Dependence of formation Gibbs energy of Li3TbCl6 on
temperature
Reinvestigation of phase equilibria in TbCl3–LiCl binary system 31
123
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enthalpy. It is the sum of two various and independent
processes. The first one is an effect of associate formation
with sharp minimum at 0.25 mol fraction of terbium(III)
chloride. The second process is related to the regular
solution and appearance of dispersed minimum at 0.5 mol
fraction of terbium(III) chloride. It shifts the minimum of
mixing enthalpy from {3Li? ? TbCl63-} associate com-
position (Fig. 7) to the compositions richer in terbium(III)
chloride.
Figure 6 shows the mixing entropy dependence on the
associate ratio na/nmax, where nmax is the maximal amount
of {3Li? ? TbCl63-} associate in the liquid phase at
x(TbCl3) = 0.25. For 4.7\ SMIX\ 5.3, this dependence is
multivalued, which is shown by dashed line in Fig. 8.
Taking into account this dependence, we assume that the
amount of {3Li? ? TbCl63-} associate is at least 25%.
Conclusions
1. Reinvestigated TbCl3–LiCl binary system shows
completely different behaviour than reported in the
literature.
2. It includes one eutectic and two stoichiometric com-
pounds, namely Li3TbCl6 and LiTbCl4. The composi-
tion and temperature of eutectic were determined as
x(TbCl3) = 0.521; Teut = 665 K. The first compound,
Li3TbCl6 forms at 553 K, and decomposes peritecti-
cally at 727 K, whereas the second one, LiTbCl4,
decomposes in the solid state at 609 K.
3. Experimentally determined phase diagram together
with the result of the previous work (mixing enthalpy)
and literature data on pure components were used in
procedure of phase diagram optimization by CAL-
PHAD method.
4. The calculated phase diagram is in a good agreement
with the measured one, which confirms the compati-
bility of all the results used in optimization procedure.
5. CALPHAD method proved assumption about Li3-
TbCl6 formation at higher temperatures from ter-
bium(III) chloride and LiTbCl4.
6. The structure of the liquid phase of the investigated
system can be described on the basis of calculations.
Acknowledgements The work was co-financed by a statutory
activity subsidy from Polish Ministry of Science and Higher Educa-
tion for the Faculty of Chemistry of Wrocław University of Science
and Technology and for the Faculty of Engineering and Economics of
Wrocław University of Economics. We would like to thank Dr. Hans
Leo Lukas from Max-Planck-Institut fur Metallforschung, Stuttgart,
Germany, for his set of programs for optimization and calculation of
the phase diagrams.
Table 3 Measured and calculated by CALPHAD method invariant points in TbCl3–LiCl system
Equlibria Invariant points CALPHAD invariant points
T/K x (TbCl3) T/K x (TbCl3)
Li3TbCl6 = LiCl ? L 727a – 728 0.264
LiCl ? LiTbCl4 = Li3TbCl6 553a – 554 –
L = Li3TbCl6 ? TbCl3(s2) 665a 0.521 ± 0.016b 665 0.451
LiTbCl4 = Li3TbCl6 ? TbCl3(s2) 609a – 609 –
a Experimental valueb Estimated value from Tammann diagram
x(TbCl3)0 0.2 0.4 0.6 0.8 1
0
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–3.5
H/k
J/m
ol–1
Δ
Fig. 7 Mixing enthalpy [27] dependence on composition: open
circles—experimental results, solid lines—calculated by CALPAHD
method
0 20 40 60 80 100
6
5
4
3
2
1
0
SM
IX/J
/mol
/K
na nmax–1/%
Fig. 8 Dependence of configurational mixing entropy on ratio
describing amount of associate{3Li? ? TbCl63-} in its maximal
amount at mole fraction of TbCl3 equal to 0.25
32 A. Danczak et al.
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