Thermodynamic re-optimization of the DyCl3-KCl system

Article abstract of DOI:10.1016/S0040-6031(01)00675-X

The intermediate compounds, K₃DyCl₆, K₂DyCl₅ and KDy₂Cl₇, in the DyCl₃-KCl system, are confirmed by X-ray powder diffraction method. With CALPHAD technology the phase diagram of the DyCl₃-KCl system is re-optimized and calculated using ChemSage software. A set of thermodynamic functions has been optimized based on an interactive computer-assisted analysis. The calculated phase diagram and optimized thermodynamic parameters are thermodynamically self-consistent.

Full text of DOI:10.1016/S0040-6031(01)00675-X

Thermochimica Acta 383 ;2002) 31±35
Review
Thermodynamic re-optimization of the DyCl3±KCl system
Xianran Xinga,*, Shuyu Daia, Zhenqi Zhua, Toshihiro Tanakab
aDepartment of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, PRChina
bDepartment of Materials Science and Processing, Osaka University, 2±1 Yamadaoka, Suita, Osaka 5650871, Japan
Received 6 May 2001; accepted 29 June 2001
Abstract
The intermediate compounds, K3DyCl6, K2DyCl5 and KDy2Cl7, in the DyCl3±KCl system, are con®rmed by X-ray powder
diffraction method. With CALPHAD technology the phase diagram of the DyCl3±KCl system is re-optimized and calculated
using ChemSage software. A set of thermodynamic functions has been optimized based on an interactive computer-assisted
analysis. The calculated phase diagram and optimized thermodynamic parameters are thermodynamically self-consistent.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Thermodynamic optimization; Intermediate compound; The DyCl3±KCl system
1. Introduction
;M ˆ alkali metals) by DTA, XRD and emf measure-
ments. They characterized the phase diagram by three
intermediate compounds, K3DyCl6, K2DyCl5 and
KDy2Cl7 in the DyCl3±KCl system. On the basis
of the phase diagram taken from [3], Hatem and
Gaune-Escard [6] assessed the DyCl3±KCl system
with surrounded statistical model ;SIM) and calcu-
lated the Gibbs free energy of formation of two
stoichiometries of K3DyCl6 and KDy3Cl10.
The purposes of present work are to identify the
intermediate compounds in the DyCl3±KCl system by
XRD method, and to self-consistently re-optimize the
phase diagram and thermodynamic properties with
ChemSage software. The thermodynamic properties
of mixed phase in liquid were described in Redlich±
Kister equation. The thermodynamic parameters of
pure salts DyCl3 and KCl were extracted from SGTE
databank [7].
The thermodynamic properties and phase diagrams
of the DyCl3±MCl systems ;M ˆ alkali metals) play
an important role to the production of metal dyspro-
sium and useful to the recycle of the spent fuel from
fast nuclear reactors.
To the DyCl3±KCl system, the phase diagram was
studied by Korshunov and Dorobot [1], Blachnik and
Selle [2], Mochinaga et al. [3] and Seifert and Kraemer
[4]. Blachnik and Selle [2] determined the part of the
phase diagram ;0:5 > xKCl > 0:0) and found the stoi-
chiometric compound KDy2Cl7 existed. The results
measured by Mochinaga et al. [3] demonstrate that
there exist only two stoichiometries of K3DyCl6 and
KDy3Cl10, and agree to the Korshunov's work [1]. But
the curve of phase transition at 375 8C in the range of
1:0 > xKCl > 0:4 could not be elucidated in the phase
rule. Seifert et al. [4,5] systematically investigated the
phase diagrams of the DyCl3±MCl systems
2. Experiments
* Corresponding author.
E-mail address: xing@ustb.edu.cn ;X. Xing).
The hexahydrate DyCl3Á6H2O was made from the
starting material Dy2O3 ;99.9%) dissolved in chlor-
0040-6031/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ; 0 1 ) 0 0 6 7 5 - X

32
X. Xing et al. / Thermochimica Acta 383 82002) 31±35
Fig. 1. XRD patterns of the sample at xDyCl ˆ 0:4 in the DyCl3±KCl system. 1: K2DyCl5; 2: KDy2Cl7; 3: DyOCl.
3
hydric acid and recrystallized by vaporizing. The pure
DyCl3 was prepared by dehydrating the hexahydrate
in an HCl stream. Precautions must be taken into
account to avoid the formation of oxychlorides. KCl
;AR grade) was dried at 400 8C for 24 h.
The samples ;about 2.0 g) were weighed according
to the appropriate amounts and fully mixed by melting
in vacuum sealed quartz ampoules with a gas ¯ame.
The melts were homogenized by shaking and solidi-
®ed by rapid cooling. The ampoules could be used
after annealing for 1 week at 300 8C.
Fig. 1 clearly shows the XRD patterns of the sample
at xDyCl ˆ 0:4. The compounds K2DyCl5 and
3
KDy2Cl7 were identi®ed and indexed as orthorhombic
and monoclinic systems, respectively. However, the
weak traces of oxychloride DyOCl also was detected.
Our results con®rmed the phase diagrams determined
by Seifert and Kraemer [4].
3. Optimization
The thermodynamic properties of two pure compo-
nents DyCl3 and KCl were extracted from SGTE data-
bank provided by the Scienti®c Group Thermodata [7]
;listed inTable 1). Theenthalpies of mixinginthe melts
were taken from the works of Gaune-Escard et al. [8].
Phase identi®cation and structure characterization
were carried out using a Rigaku diffractometer with
Cu Ka radiation. For measurements of the lattice
parameters of compounds, pure Si powder was added
to the samples as an internal standard.
Table 1
The thermodynamic data of pure salts DyCl3 and KCl [7]
Compounds
DHfꢀꢁ298†
Sꢀꢁ298†
Cp ˆ a ‡ bT ‡ cT2 ‡ d=T2 ꢁJmolÀ1KÀ1
†
T ;K)
DfusH
;kJ molÀ1
)
;J molÀ1
)
;kJ molÀ1
)
a
b
104Âc
10À5Âd
94.5584
0.01799
0.00000
À1.42256
298±924
DyCl3
KCl
À995.792
À436.684
157.737
82.555
25.5200
26.2800
144.7660
50.4776
0.00000
0.00592
0.00000
0.00000
0.00000
924±1300
298±1044
À1.44174
143.5700
À0.16804
0.99657
À8.21784
1044±1710

X. Xing et al. / Thermochimica Acta 383 82002) 31±35
33
Table 2
Comparisons of relative reactions in the DyCl3±KCl system
Using the thermochemistry calculation software
ChemSage [9], the thermodynamic data in the
DyCl3±KCl system was critically assessed and ana-
lyzed together with the available phase diagram data.
For the modeling of all liquid phases, the Redlich±
Kister formula [10] was employed as given in follow-
ing equation:
Reaction
Present work
[4]
xDyCl T ;K) xDyCl
T ;K)
3
3
L ˆ DyCl3;s) ‡ K3DyCl6;s)
0.140 938
0.110 930
0.470 728
0.480 718
0.785 805
K3DyCl6;s) ˆ L ‡ K2DyCl5;s) 0.420 723
L ˆ K2DyCl5;s) ‡ KDy2Cl7;s) 0.450 709
n
X
L ˆ KDy2Cl7;s) ‡ KCl ;s)
0.855 811
iÀ1
Gex ˆ xAxB LiꢁxA À xB† ꢁJ molÀ1
†
;1)
iˆ1
where Li ˆ ꢁai ‡ biT†. When n ˆ 2, the thermody-
pounds K3DyCl6, K2DyCl5 and KDy2Cl7 were esti-
mated and listed in Table 3.
namic parameters can be optimized as
L0 ˆ À74599:32 À 34:6961T;
;2)
DyCl3ꢁs† ‡ 3KClꢁs† ˆ K3DyCl6ꢁs†
DyCl3ꢁs† ‡ 2KClꢁs† ˆ K2DyCl5ꢁs†
2DyCl3ꢁs† ‡ KClꢁs† ˆ KDy2Cl7ꢁs†
;3)
;4)
;5)
L1 ˆ ‡73155:05 À 36:6412T
The calculated phase diagram is shown in Fig. 2 and
basically agrees to the experimental information [4]
except in the melting points of the end DyCl3. The
comparisons of relative reactions between [4] and
present work were tabulated here ;see Table 2).
Fig. 3 demonstrates that the calculated enthalpies of
mixing in the liquid melts at 1070 K are good in the
agreement with the experimental work [8].
Table 3
Estimated thermodynamic properties of intermediate compounds
Compounds
ÀDHfꢀꢁ298†
Sꢀꢁ298†
Cp ;J molÀ1 KÀ1
)
;kJ molÀ1
)
;J molÀ1
)
K3DyCl6
K2DyCl5
KDy2Cl7
2209.018
1901.700
2485.493
669.204
348.753
405.230
162.703
234.509
255.792
With the optimized thermodynamic properties of
the DyCl3±KCl system, the enthalpies of formation,
entropies and heat capacities of intermediate com-
Fig. 2. Calculated phase diagram of the DyCl3±KCl system. ; ) Experimental data selected from [4].

34
X. Xing et al. / Thermochimica Acta 383 82002) 31±35
Fig. 3. Enthalpies of mixing in the liquid melts at 1070 K. ;*) Experimental data selected from [8].
4. Discussion
are one of the essential factors to in¯uence the shape of
phase diagrams and to determine the type of reaction
in the system. Our experimental results found there
were three stoichiometric compounds and con®rmed
the phase diagram proposed by Seifert and Kraemer
[4]. The Seifert and Kraemer's work probably is more
reliable in such case. However, the comparisons listed
in Table 2 show that there still exists considerable
difference between present results and those in [4].
From the thermodynamic point of view, the relation
between enthalpy of fusion of pure component i, and
the limiting slope of liquids ;given by the left-hand
side of Eq. ;6)) can be described by the following
formula:
Hatem and Gaune-Escard [6] used the phase
diagram proposed by Mochinaga et al. [3] and
assessed the phase diagram of the DyCl3±KCl system.
They selected the thermodynamic data relative to
KCl from JANAF thermodynamic tables [11] and
relative to DyCl3 from [12]. They pointed out that
the results presented by Seifert and Kraemer [4] were
probably not correct because of the melting points
of two pure salts are much different from others ;see
Table 4).
Although the melting points of two pure salts are
very important to the phase diagram, the difference
between the values of melting points of different
sources is not enough to change the type of the phase
diagram. We think that the intermediate compounds
dxli
dT
D
fusHꢁꢀi†
nRTf2usꢁi†
lim
ˆ
;6)
x !1
i
where n ˆ 1, Tfus8i) is the melting point of the pure
component. At the DyCl3 end, the tangent from the
theoretically calculated limiting slope is in good
agreement with the one from the experimental phase
diagram. But at the KCl end, unreasonable difference
could be found between theoretically calculated
results and the phase diagram measured by Seifert
and Kraemer [4], probably due to their abnormal value
of the melting point ;TKCl ˆ 1073 K).
Table 4
The melting points ;K) of two pure salts from different sources
Salts
Hatem and
Gaune-Escard [6]
Seifert and
Kraemer[4]
SGTE [7]
DyCl3
KCl
928
919
924
1044
1073
1044

X. Xing et al. / Thermochimica Acta 383 82002) 31±35
35
5. Conclusions
References
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DyCl3±KCl system have been con®rmed by the
XRD method. Using the software ChemSage and
employing the Ridelich±Kister equation in liquid
phases, the phase diagram and thermodynamic data
available in the DyCl3±KCl system were critically
reassessed. The thermodynamic properties of three
stoichiometric compounds K3DyCl6, K2DyCl5 and
KDy2Cl7, were estimated. The calculated phase dia-
gram and optimized thermodynamic parameters are
thermodynamically self-consistent.
[2] R. Blachnik, D. Selle, Z. Anorg. Allg. Chem. 454 ;1979) 90.
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Acknowledgements
The project is ®nancially supported by National
Natural Science Foundation of China ;No. 29971004).
AuthorsaregratefultoDr. G. Eriksson;GTTCompany)
for the providing the calculation software, ChemSage.