Reinvestigation of phase equilibria in TbCl3–LiCl binary ...€¦ · Reinvestigation of phase...

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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

[email protected]

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]

26 A. Danczak et al.

123


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

123


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


123


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


123


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

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her

mo

dy

nam

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so

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ðÞ¼

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91

03

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LiC

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59

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(s1)

28

9–

78

31

47

.79

98

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30

,24

0.1

64

91

.96

-9

4.1

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1.5

5

Tb

Cl 3

(s2)

78

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123


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


123


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


123


References

1. Naumov VS, Bychkov AV, Lebedev A. Advenctes in molten

salts: from structural aspects to waste processing. In: Gaude-Es-

card M, editor. Properties of liquid-salt nuclear fuel and its

reprocessing technology. Danbury: Begell House Inc; 1999.

p. 432–53.

2. Junming T, Bath NY. Quarty Metal Halide Lamps with Improved

Lumen Maintenance. U.S. Patent Application Publication. 2008;

US2008/0093993 Ai.

3. Junming T, Bath NY. Quarty Metal Halide Lamps with Improved

Lumen Maintenance. U.S. Patent Application Publication. 2010;

US 7, 786,674 B2.

4. Seifert HJ. Ternary chlorides of the trivalent late lanthanides:

Phase diagrams, crystal structures and thermodynamic properties.

J Therm Anal Calorim. 2006;83:479–505 and references therein.

5. Gmelin L. Handbook of inorganic chemistry, vol. C5. 8th ed.

Berlin: Springer; 1977.

6. Prosypajko VI, Alekseeva EA. In: Bell HB, editor. Phase equi-

libria in binary halides. New York: IFI/Plenum; 1987.

7. Kutscher J, Schneider A. Zur Systematik der Zustandsdiagramme

von Lanthaniden(III)-halogenid-Alkalihalogenid-System. Z Anorg

Allg Chem. 1974;408:135–45.

8. Seifert HJ. Ternary chlorides of the trivalent early lanthanides:

Phase diagrams, crystal structures and thermodynamic properties.

J Therm Anal Calorim. 2002;67:789–826 and references therein.

9. Mitra S, Uebach J, Seifert HJ. Ternary chlorides in the systems

ACl/TbCl3 (A = K, Rb, Cs). J Solid State Chem.

1995;115:484–9.

10. Seifert HJ, Sandrock J, Uebach J. Thermochemical and structural

investigations on the systems NaCl/TbCl3 and NaCl/DyCl3. Acta

Chem Scand. 1995;49:653–7.

11. Chao-Gui Z, Shou-Lin H, Si-Qiang W. Phase diagram of binary

systems ErCl3–MCln (M = Li, Ca, Pb; n = 1 or 2). Chem J Chin

Univ. 1993;14:992–5.


systems YbCl3–MCln (M = Li, Mg, Ca, Pb; n = 1 or 2). Acta

Phys Chim Sin. 1994;10:342–7.


systems HoCl3–MCln (M = Li, Mg, Ca, Pb; n = 1 or 2). Acta

Chim Sin. 1994;52:735–9.

14. Chao-Gui Z, Zhong-Dong Z, Si-Qiang W. Phase diagram of

binary systems DyCl3–MCln (M = Li, Mg, Ca, Pb; n = 1 or

n = 2). Inst Sci Tech Inf China. 1994;18:263.

15. Chao-Gui Z, Shou-Lin H, Si-Qiang W. Phase diagrams of the

binary systems TbCl3-MCln (M = Li, Mg, Ca, Pb; n = 1 or 2).

Trans Nonferrous Met Soc China. 1993;3(2):33.

16. Rycerz L, Gaune-Escard M. Enthalpies of phase transitions and

heat capacity of TbCl3 and compounds formed in TbCl3–MCl

systems (M = K, Rb, Cs). J Therm Anal Calorim.

2002;68:973–81.

17. Dworkin AS, Bredig MA. Enthalpy of lanthanide chloride, bro-

mides and iodides from 298–1300 degrees k-enthalpies of fusion

and transition. High Temp Sci. 1971;3(1):81.

18. Konings RJM, Kovacs A. Thermodynamic properties of the

lanthanide(III) halides. Handb Phys Chem Rare Earths.

2003;33(213):147–247.

19. Goryushkin VF, Zalymova SA, Poshevneva AI. Thermal con-

stants of the conversion of yttrium subgroup lanthanide trichlo-

ride. Zh Neorg Khim. 1990;35:3081.

20. Danczak A, Rycerz L. Reinvestigation of the DyCl3–LiCl binary

system phase diagram. J Therm Anal Calorim.

2016;126:299–305.

21. Kubaschewski O, Alcock CB, Spencer PJ. Materials thermo-

chemistry. 6th ed. Oxford: Pergamon Press; 1998.

22. Findlay A. The phase rule and its applications. New York:

Longmans, Green and Co.; 1911.

23. Guenet JM. Contributions of phase diagram to the understanding

of organized polymer-solvent systems. Thermochim Acta.

1996;284:67–83.

24. Rycerz L. Practical remarks concerning phase diagram determi-

nation on the base of DSC measurements. J Therm Anal Calorim.

2013;113:231–8.

25. Lukas HL, Fries SG, Sundman B. Computational thermody-

namics, the Calphad method. Cambridge: Cambridge University

Press; 2007.

26. Saunders N, Miodownik AP. CALPHAD (calculation of phase

diagrams): a comprehensive guide. 1st ed. Oxford: Pergamon;

1998.

27. Rycerz L, Gaune-Escard M. Mixing enthalpy of TbCl3–MCl

liquid mixtures (M = Li, Na, K, Rb, Cs). High Temp Mater Proc.

1998;2(4):483–6.

28. Kapała J. Management program for BINGSS phase diagram

optimizer. In: An international conference on phase diagram

calculation and computational thermochemistry CALPHAD

XXXIII, Krakow, 30.05–04.06.2004, 2004.

29. Lukas HL, Fries SG. Demonstration of the use of ‘‘BINGSS’’

with the Mg–Zn system as example. J Phase Equilib.

1992;13(5):532–41.

30. Szczygieł I, Salamon B, Kapała J. Modelling of thermodynamic

properties of NdBr 3-MBr (M = Li–Cs) pseudobinary systems.

CALPHAD. 2016;53:130–5.

31. Salamon B, Rycerz L, Kapała J, Gaune-Escard M. Phase diagram

of NdI3–RbI pseudo-binary system. Thermodynamic properties

of solid compounds. J Phase Equilib. 2015;404:9–16.

32. Kapała J, Salamon B. Thermodynamic assessment of TbCl3–MCl

(M = Na, K, Rb, Cs) binary systems. CALPHAD. 2013;43:139–42.

33. Sommer F. Associacion model for the description of thermody-

namic functions of liquid alloys part 1. Z Metallkd.

1982;73:72–6, and part 2. Z Metallkd. 1982;73:77–86.

34. Krull HG, Singh RN, Sommer F. Generalised association model.

Z Metallkd. 2000;91:356–65.

35. Photiadis GM, Brresen B, Papatheodorou GN. Vibrational modes

and structures of lanthanide halide-alkali halide binary melts

LnBr 3-KBr (Ln = La, Nd, Gd) and NdCl3–ACl (A = Li, Na, K,

Cs). J Chem Soc Farad Trans. 1998;94:2605–13.


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