Studies on the phase diagram of CaCl2-CaBr2 system

Article abstract of DOI:10.1016/j.tca.2010.03.027

A study on pseudo-binary CaCl₂-CaBr₂ system was carried out using differential thermal analysis (DTA) technique covering the complete composition range of the constituents between room temperature and 1073 K. From the DTA results the contour of solidus and liquidus temperatures with composition are plotted and the phase diagram of CaCl₂-CaBr ₂ system is constructed. The solidus and liquidus curves exhibit a minimum at 971K and at a composition of 56.5 mol% CaBr2. Below 971 K, the system exhibits complete solid solution. Long-term equilibration of samples of several compositions was carried out at 923 and 673K and the phases were characterized by XRD. The XRD results supported solid solution formation in the CaCl ₂-CaBr₂ system between 673 and 923 K.

Full text of DOI:10.1016/j.tca.2010.03.027

Thermochimica Acta 505 (2010) 69–72
Contents lists available at ScienceDirect
Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Studies on the phase diagram of CaCl –CaBr system
2
2
∗
Sajal Ghosh, R. Sridharan, T. Gnanasekaran
Liquid Metals and Structural Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A study on pseudo-binary CaCl –CaBr system was carried out using differential thermal analysis (DTA)
2
2
Received 17 November 2009
Received in revised form 25 March 2010
Accepted 31 March 2010
technique covering the complete composition range of the constituents between room temperature and
073 K. From the DTA results the contour of solidus and liquidus temperatures with composition are
1
plotted and the phase diagram of CaCl2–CaBr2 system is constructed. The solidus and liquidus curves
exhibit a minimum at 971 K and at a composition of 56.5 mol% CaBr2. Below 971 K, the system exhibits
complete solid solution. Long-term equilibration of samples of several compositions was carried out
at 923 and 673 K and the phases were characterized by XRD. The XRD results supported solid solution
formation in the CaCl2–CaBr2 system between 673 and 923 K.
Available online 8 April 2010
Keywords:
Binary phase diagrams
CaCl2–CaBr2 system
Differential thermal analysis
X-ray characterization
Alkaline earth halides
Solid solution
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
CaBr2 solution was concentrated over a hot plate and dried under
flowing argon gas. Dry CaBr2 obtained thus was further purified
by heating it up to its melting point in a quartz crucible placed in
a quartz vessel in a stream of dry argon gas containing HBr for 2 h
to remove any trace of volatile impurities. In a similar way CaCl2
(99.9%, M/s. Alfa Products, USA) was further purified by heating it
up to its melting point in a stream of dry argon gas carrying HCl
for 2 h. The purified salts were then cooled to room temperature
and transferred into a high purity argon atmosphere glove box
that is used for handling liquid sodium. Calculated amounts of
purified CaCl and CaBr were mixed to prepare samples of varying
compositions. Weighing, mixing and pelletising of these samples
were carried out in the argon atmosphere glove box. Before the
DTA run, sample was brought out of the glove box and loaded into
the DTA apparatus (SETSYS Evolution 16/18, M/s. Setaram, France).
While loading the samples (∼15 mg) in the DTA apparatus, the
sample got exposed to air for a very brief time (∼1 min.). In order
to remove the moisture picked up during this process, the samples
were first heated in the DTA apparatus under flowing argon gas at
Hydride ion conducting electrolytes are used in electrochemi-
cal sensors for detecting hydrogen in liquid sodium coolant of fast
reactors [1,2]. In this regard, several combinations of alkali/alkaline
earth halides mixed with alkali/alkaline earth hydrides such as
LiCl–KCl containing 0.2–2 mol% LiH [2], CaCl –CaH [3,4], a ternary
mixture of LiCl, CaCl2 and CaHCl [5] and binary CaHBr–CaBr2 [6]
systems have been investigated for using them as hydride ion con-
ducting electrolytes. Physical properties viz. thermal conductivity
and mechanical integrity of solid electrolyte systems containing the
hydride ion conducting phases like CaHCl and CaHBr depend upon
other phases present and the relevant phase diagrams. Binary phase
diagrams of LiCl–CaCl2 as well as that of LiBr–SrBr2 systems were
established by us earlier [7,8]. In this work, the binary phase dia-
gram of CaCl –CaBr system has been investigated by differential
2
2
2
2
2
2
thermal analysis (DTA) and by identifying equilibrium phases that
co-exist by X-ray diffraction (XRD) and the results are reported.
5
73 K for 1 h. Removal of moisture was confirmed from the thermo
2
. Experimental
gravimetric (TG) data and by measuring the melting points of the
pure components. Argon gas was used at a flow rate of 20 ml/min
during all the experiments. Because appreciable super-cooling
effects were observed during cooling runs with this system, tem-
peratures corresponding to various thermal events were obtained
from the analysis of the DTA curves of heating runs only. DTA
runs were recorded with a heating rate of 5 K/min from room
temperature to 923 K and at a rate of 1 K/min from 923 to 1073 K.
Several DTA runs were carried out with each sample so as to obtain
Calcium bromide was prepared by reacting CaCO3
Sigma–Aldrich, chelometric standard, A.C.S. reagent) and distilled
AR grade HBr (Merck, 47.8% azeotropic) solution. The resultant
(
∗ Corresponding author at: Liquid Metals & Structural Chemistry Division, Chem-
istry Group, Indira Gandhi Center for Atomic Research, Kalpakkam 603102, India.
Tel.: +91 44 27480302; fax: +91 44 27480065.
◦
E-mail address: gnani@igcar.gov.in (T. Gnanasekaran).
concordant data (reproducible to within ± 1 C) while first run was
0
040-6031/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2010.03.027

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S. Ghosh et al. / Thermochimica Acta 505 (2010) 69–72
Fig. 1. DTA traces of pure CaBr2 and CaCl2.
Fig. 3. DTA traces obtained for samples of CaBr2–CaCl2 system (Pure CaBr2 to
0 mol% CaBr2).
neglected. Temperature calibration of the DTA unit was carried
out using high purity metal standards: gold, silver, aluminium
and zinc. For identifying the equilibrium phases that exist at high
temperatures, samples of different compositions, namely 90, 75,
5
0
522]. Examination of the DTA curve obtained for pure CaCl2,
shown in Fig. 1, indicates a single endothermic peak correspond-
ing to its melting point at 1043 K which is in agreement with the
value of 1046 K, reported in literature [9]. The observed melting
point of CaBr2 was 1011 K that was in close agreement to the val-
ues of 1015 and 1011 K reported in literature [10,11]. DTA traces of
these pure phases indicate that there are no other phase transitions
up to their melting points. DTA curves of all the other compositions
also showed a single endothermic peak between room temperature
and 1073 K. From the DTA curves of samples containing CaBr2 and
CaCl2 shown in Figs. 2 and 3, the liquidus temperatures at which
the DTA curves trace back to the base line were also recorded. It
can be seen from the results shown in Table 1, which the liquidus
temperatures decrease monotonically from the melting point of
pure CaCl2 up to ∼55 mol% CaBr2 and then increase monotonically
6
0, 45, 30 and 15 mol% taken in quartz tubes were melted followed
by equilibration at 923 K under flowing hydrogen gas for 190 h.
After the equilibration, the samples were quenched using liquid
nitrogen and then the equilibrium phases were loaded in capillary
tubes and characterized using XRD (Stoe Diffractometry, Bragg
Brandano geometry). Similar experiments were carried out with
samples equilibrated at 673 K for 400 h.
3
. Results and discussion
The DTA traces obtained for pure components CaBr , CaCl2 and
2
for samples of nineteen compositions starting from 5 to 95 mol%
CaBr2 are shown in Figs. 1–3. Solidus temperatures and liquidus
temperatures deduced from these DTA results for various compo-
sitions at a heating rate of 1 K/min are listed in Table 1. XRD results
of the samples equilibrated at 923 and 673 K, followed by quench-
ing in liquid N2 and those of pure CaBr2 and CaCl2 are shown in
Figs. 4 and 5, respectively.
further up to the melting point of pure CaBr . The results further
2
indicate that the onset of endothermic peak for each composition
also shows a similar trend, i.e., decrease in monotonically from the
melting point of pure CaCl up to ∼55 mol% CaBr and then increase
2
2
monotonically further up to the melting point of pure CaBr . In a
2
Both the XRD patterns of the pure CaBr2 and CaCl2 could be
assigned to orthorhombic structures [JCPDS file nos: 71-2044, 74-
Table 1
DTA results for CaCl2–CaBr2 system.
S. no.
Composition of
Onset temperature
Liquidus
CaCl2–CaBr2 (mol%)
(K) (± 1)
temperature (K) (± 1)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
100–0
95–05
90–10
85–15
80–20
75–25
70–30
65–35
60–40
55–45
50–50
45–55
40–60
35–65
30–70
25–75
20–80
15–85
10–90
05–95
0–100
1043
1030
1020
1006
1000
991
984
981
977
974
972
973
973
974
975
978
985
989
–
1038
1028
1018
1011
1000
994
990
985
981
978
978
979
980
981
985
990
995
1000
1007
–
1
1
1
1
1
1
1
1
1
1
2
2
992
1000
1011
Fig. 2. DTA traces obtained for samples of CaBr2–CaCl2 system (Pure CaCl2 to
5
5 mol% CaCl2).

S. Ghosh et al. / Thermochimica Acta 505 (2010) 69–72
71
Fig. 6. Phase diagram of CaCl2–CaBr2 system.
Fig. 4. XRD pattern of samples equilibrated at 923 K.
The phase diagram consists of the following four regions:
Complete solid solution of CaCl2 and CaBr2
Solid solution rich in CaCl2 + liquid
Solid solution rich in CaBr2 + liquid
Liquid
binary system with no significant mutual solubility between the
pure components, one expects a sharp endothermic peak for the
eutectic reaction at eutectic temperature (an isothermal event) for a
range of compositions. Further, this isothermal endothermic peak is
expected to be followed by a broad tailing that retraces the base line
at liquidus temperature for each composition. The absence of such
a DTA pattern with isothermal peak indicates the CaBr –CaCl sys-
The minimum in the liquidus curve shown in Fig. 6 is at
56.5 mol% CaBr2 and at 971 K where it would meet the solidus
curve. It is noted here that the DTA curves of samples of this
system showed a width of 6 and 4 K even for the endothermic
2
2
tem does not exhibit a simple eutectic reaction. On the other hand
the observation of a monotonic variation of onset temperature of
endothermic peaks with change in composition indicates that the
components of the system exhibit mutual and complete solid solu-
bility at high temperatures if not from room temperature onwards.
The solid solution region is essentially a single-phase region and
the degree of freedom in this region is two. This means that both
temperature and composition can be changed in this region with-
out appearance of another phase. On heating a solid solution of a
known composition, liquid phase appears at a temperature which
is characteristic of that composition and falls on the solidus curve
as the degree of freedom is one. On further heating, more of liquid is
formed due to preferential dissolution of either CaCl or CaBr solid
peaks of the melting of pure CaCl2 and CaBr , respectively. Hence
2
at compositions near the minimum of liquidus temperatures, the
experimentally measured liquidus points are found to be outside
the liquidus curve shown as dotted line. Further, on decreasing
sample size to ∼4 mg the width did not come down both for the
pure compounds and compositions. The fact that the liquidus curve
shows a minimum at 56.5 mol% CaBr2 instead of a continuous
monotonic increase from pure CaBr2 to pure CaCl , indicates that
2
this system may exhibit a region where two limited solid solu-
tions co-exist, probably at lower temperatures. Evidence of this
possibility could not be obtained from the DTA results as DTA
curves of all the compositions showed a single endothermic peak
between 298 and 1073 K. Also, XRD results obtained for 100% CaBr2,
100% CaCl2 and for the phases obtained after long-term equilibra-
tion of compositions 90, 75, 60, 45, 30 and 15 mol% CaBr2 at 923
and 673 K, qualitatively agreed with the DTA results. The samples
are moisture sensitive in nature and the equilibrated phases were
taken in Lindemann capillary tubes and sealed gas tight with wax
inside argon glove box. Due to the small sample size, the XRD
pattern obtained could be used only to characterize the phases
as the signal to noise ratio of many XRD peaks are moderate. It
could be seen from Figs. 4 and 5 that the XRD pattern is nearly
the same for different compositions and only shifting of major
XRD peaks are observed with compositional change. From the X-
ray data of the most intense peak obtained for 1 1 1 plane, which
showed high signal to noise ratio, we calculated the d111 values
and are given in Table 2. These values are plotted against com-
position of CaBr2 in Fig. 7 and the fitted curve is found to vary
2
2
into the liquid phase. The two-phase region would end ultimately
at the liquidus temperature when the solid phase completely dis-
appears. From all these observations made on DTA results, phase
diagram ofCaBr –CaCl system is constructedandis shownin Fig. 6.
2
2
linearly from pure CaCl2 to pure CaBr . The fact that both phases
2
CaCl2 and CaBr2 belong to the same crystal structure namely,
−
−
orthorhombic and the ionic radii of Cl and Br are very close
1.81 and 1.96 Å, respectively [12]), satisfy the condition needed
(
for complete solid solution formation of these two phases. Fig. 7
indicates that when the bulkier bromide ions substitute for the
smaller chloride ions with increasing composition of CaBr , the
2
lattice probably experiences isotropic expansion. More detailed
analysis of X-ray data is not possible with these X-ray data and
Fig. 5. XRD pattern of samples equilibrated at 673 K.

7
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S. Ghosh et al. / Thermochimica Acta 505 (2010) 69–72
Table 2
the formation of complete solid solution. All these results corrob-
orate the fact that there are no terminal solid solution regions
and gives discernible evidence for complete solid solution between
923 K > T > 673 K.
Variation of lattice constant, d111, with composition of samples equilibrated at 923
and 673 K.
Compositions
mol%CaBr2)
d111 (Å) (equilibrated at
923 K)
d111 (Å) (equilibrated at
673 K)
(
1
00
3.2009
3.1956
3.1680
3.1460
3.1186
3.1029
3.0818
3.0559
3.2009
3.1951
3.1730
3.1510
3.1294
3.0974
3.0765
3.0559
4. Conclusion
9
7
6
4
3
1
0
5
0
5
0
5
0
The phase diagram of CaCl –CaBr2 system is constructed based
2
on DTA analysis of samples with compositions in the range of 100%
CaCl2 to 100% CaBr2 in steps of 5 mol% and the XRD results of sam-
ples of several compositions equilibrated at 923 and 673 K. The
solidus and liquidus curve exhibit a minimum at 971 K and at a
composition of 56.5 mol% CaBr2.
Acknowledgements
The authors gratefully acknowledge Dr. H.N. Jena and Dr S.
Kalavathi for their help in characterization of materials by XRD
and Shri R. Aswathraman for his help in data treatment of XRD
patterns.
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Fig. 7. Linear variation of lattice constant with composition of CaBr2.
the linear increase of d111 values observed for equilibrated sam-
ples with increase in composition of CaBr , qualitatively support
2