Phase studies on the quasi-binary thallium(I) telluride-bismuth(III) selenide system

Article abstract of DOI:10.1016/j.tca.2012.03.016

The phase diagram for the quasi-binary Tl₂Te-Bi ₂Se₃ system was delineated from the results of phase studies by both common thermal analysis and X-ray diffraction. Numerical values of the phase transition temperatures at different alloy compositions within the whole concentration range, were obtained. They have been listed in a table that can be used by other authors in any computational treatment of the system. The diagram was compared with that published earlier by other authors employing differential thermal analysis as a main experimental method. As a result of the present study, three new chemical compounds have been found.

Full text of DOI:10.1016/j.tca.2012.03.016

Thermochimica Acta 538 (2012) 63–66
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Thermochimica Acta
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Phase studies on the quasi-binary thallium(I) telluride–bismuth(III)
selenide system
I. Mucha∗
Department of Analytical Chemistry, Faculty of Pharmacy, Wrocław Medical University, Szewska 38, 50-139 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The phase diagram for the quasi-binary Tl2Te–Bi2Se3 system was delineated from the results of phase
studies by both common thermal analysis and X-ray diffraction. Numerical values of the phase transition
temperatures at different alloy compositions within the whole concentration range, were obtained. They
have been listed in a table that can be used by other authors in any computational treatment of the
system. The diagram was compared with that published earlier by other authors employing differential
thermal analysis as a main experimental method. As a result of the present study, three new chemical
compounds have been found.
Received 28 December 2011
Received in revised form 15 March 2012
Accepted 19 March 2012
Available online 28 March 2012
Keywords:
Thallium telluride
Bismuth(III) selenide, Chalcogenide system
Quasi-binary phase diagram
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
low as compared to those of state-of-the-art thermoelectric
materials [5].
The thallium-containing chalcogenides (thallium-based ternary
chalcogenides) that are formed in the quasi-binary Tl2X–Bi2X3
(X = S, Se, Te) systems, belong to the materials with interesting
semiconductor properties. The investigation of the quasi-binary
chalcogenide sections Tl2Se–Bi2Se3 and Tl2Te–Bi2Te3 showed that
the formation of ternary compounds is quite common [1,2]. The
compounds involve promising set of physical properties, this giv-
ing a push to the search for new materials with interesting electrical
characteristics.
For instance, the newest result unambiguously established that
the compound TlBiSe2 was a strong topological insulator with
a single Dirac cone at the Brillouin-zone center. The large bulk
gap (∼0.20 eV) made TlBiSe2 a topological insulator with better
mechanical properties than the previous binary 3D topological
insulator family. For TlBiTe2, the observed negative bulk gap indi-
cated it as a semimetal, instead of a narrow-gap semiconductor as
commonly believed. This semimetality naturally explains its mys-
teriously small thermoelectric figure of merit comparing to other
compounds in the family. The unique band structures of TlBiTe2
and the band gap of 0.35 eV (the largest among known topological
insulators) also suggest it as a candidate for topological supercon-
ductors [3,4]. It was confirmed that Tl9BiTe6 may be of potential
for thermoelectric applications in the temperature range from
400 to 600 K. The thermal conductivity of Tl9BiTe6 is extremely
The system under consideration is of the type AIIIBVI–CVDVI
,
where A and C are metals and B, D are chalcogens neighbor-
ing in the same group of the Periodic Table. The thallium(I)
telluride–bismuth(III) selenide system was studied formerly once
[6]. From the phase diagram published in the paper [6] (Fig. 1), it
does not follow clearly whether any chemical compound is formed
in the system or not. However, comparison of two analogous sys-
tems Tl2Se–Bi2Se3 [1] and Tl2Te–Bi2Te3 [2] show existence at least
two compounds in each. Because of a very small difference in
the radius of selenium and tellurium, it may be supposed that in
Tl2Te–Bi2Se3 system also chemical compounds should be formed.
This is the reason why we decided to re-investigate the title system
by thermal analysis (TA) and XRD methods.
2. Experimental
2.1. Materials
The components of the system examined, i.e. thallium telluride
and bismuth selenide, were prepared from pure elements: thal-
lium 99.9 mass%, bismuth 99.9 mass%, selenium 99.9 mass% and
tellurium 99.99 mass% (all from Aldrich Chemical Co.). The metal
chalcogenides were synthesized by simple fusing stoichiometric
quantities of the elements, weighed with an accuracy of 0.0001 g,
in quartz tubes in a purified argon atmosphere (5 N pure, BOC Gazy,
Poznan) and then mixed for 15 min at temperature about 100 K
higher than the melting point of the respective metal chalcogenide.
∗
Tel.: +48 71 7840308; fax: +48 71 7840307.
E-mail address: igor@cheman.am.wroc.pl
0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2012.03.016

64
I. Mucha / Thermochimica Acta 538 (2012) 63–66
Fig. 1. Phase diagram for the system Tl2Te–Bi2Se3 (according to [6]).
2.2. Apparatus and measurements
The phase studies on the Tl2Te–Bi2Se3 system were performed
by making use of the TA method (cooling curve technique) in a
quartz apparatus designed for phase and cryometric investigations
at high temperatures [7]. The phase transition temperatures of the
examined samples were determined by using a thermopile Pt/Pt,Rh
calibrated at the freezing points of standards (lead, zinc, aluminium,
potassium chloride and silver). The molten samples (no less than
25 g) were vigorously stirred throughout the experiments with a
quartz stirrer to maintain equilibrium conditions of crystallization
of the melts. The cooling rate was 0.8–1.2 K/min, the accuracy of
measurements was 0.5 K. The thermopile was connected to a dual
display multimeter Fluke 45 which was joined to a computer for the
processing and display of the experimental data.
Fig. 2. Phase diagram for the system Tl2Te–Bi2Se3 (this work).
The composition of the eutectics was determined by a graphical
method, i.e. by simple extrapolation of the liquidus curves up to
their intersection with the solidus line, accordingly, the composi-
tions were given with an accuracy of 1 mol%.
To confirm the results of the TA, use was also made of the XRD
method. Alloy samples with compositions 5, 10, 20, 40, 50, 60, 68,
70, 80 and 90 mol% Bi2Se3 were prepared in the same way as metal
chalcogenides (Section 2.1). The solidified samples were powdered
in an Analysette 3 Spartan pulverisette 0 vibration mill (Fritsch) and
homogenized for 7 days under vacuum at 650 K, then quenched in
a cooling bath (ice + methanol). The XRD examinations of the alloys
as well as of pure components (Tl2Te and Bi2Se3) were performed
was evidenced by both the maximum (at 10.0 mol% Bi2Se3 and
794.7 K) on the liquidus line and the arrangement of the experimen-
tal points corresponding to the eutectic at 26.9 mol% Bi2Se3, melting
at 749.7 K. The points may be observed only at compositions higher
than 10.0 mol% Bi2Se3.
The other compound was formed at 70.0 mol% Bi2Se3, its
component molar ratio being Tl2Te:Bi2Se3 = 3:7 to which a for-
mula Tl6Bi14Te3Se21 may be ascribed. Rather sharp maximum
on the liquidus line at 946.7 K lies at 70.00 mol% Bi2Se3. More-
over, the experimental points corresponding to the next eutectic
(at 84.8 mol% Bi2Se3) melting at 913.9 K, appear just above the
compound composition. These facts unequivocally determine the
congruently melting compound composition.
At the component molar ratio Tl2Te:Bi2Se3 = 1:1, a compound
of composition Tl2Bi2TeSe3 is formed according to a peritectic
reaction L + Tl6Bi14Te3Se21 ↔ Tl2Bi2TeSe3 at 895.0 K. At 749.7 K this
compound with Tl18Bi2Te9Se3 (9:1) forms the eutectic containing
26.9 mol% Bi2Se3. The points corresponding to the eutectic temper-
ature may be observed only at compositions lower than 50.0 mol%
Bi2Se3.
Both the TA and the XRD data evidenced three new
chemical compounds 9Tl2Te·Bi2Se3 (Tl18Bi2Te9Se3), Tl2Te·Bi2Se3
(Tl2Bi2TeSe3) and 3Tl2Te·7Bi2Se3 (Tl6Bi14Te3Se21) (not found ear-
lier [6]) and confirmed the existence of terminal solid solutions ␣
and ␤ reported in [6].
using a Siemens D 5000 diffractometer, employing CuK␣
,
radi-
K␤
ation (20 mA, 35 kV). The measurements were made in 2Â angle
range of 15◦–60◦ with a 0.04◦ step and at least 2 s per step.
3. Results
The measurements of phase transformation temperatures in the
system thallium(I) telluride–bismuth(III) selenide were effected by
common thermal analysis within the whole concentration range.
Results of these measurements have been listed in Table 1 to enable
other authors to employ them in future computational treatment
of the system. From these data, the phase equilibrium diagram for
the system has been delineated – Fig. 2. The constructed phase dia-
gram was checked by the XRD examination aimed at identifying
phase compositions of different Tl2Te + Bi2Se3 alloys. It appeared
that in the system Tl2Te–Bi2Se3 three quaternary compounds were
formed.
4. Discussion
Results of X-ray examination of some Tl2Te + Bi2Se3 alloys are
presented in Fig. 3. As it will be shown later, the XRD patterns
entirely corroborate the results described in the previous section.
From comparison of the XRD pictures of pure Tl2Te (Fig. 3a) and
The first of the compounds found in this study, was formed
with a component molar ratio Tl2Te:Bi2Se3 = 9:1 to which the for-
mula Tl18Bi2Te9Se3 may be ascribed. This compound composition

I. Mucha / Thermochimica Acta 538 (2012) 63–66
65
Table 1
The phase transition temperatures at different alloy compositions within the whole concentration range of the system Tl2Te–Bi2Se3.
No.
Comp. (mol% Bi2Se3)
Temp. (K)
No.
Comp. (mol% Bi2Se3)
Temp. (K)
1
2
3
4
5
6
7
8
0.00
2.03
2.03
7.86
7.86
691.9
698.0
721.0
760.0
790.0
794.7
743.8
793.1
787.0
740.9
779.1
750.0
763.8
751.9
792.0
747.6
808.6
749.7
822.1
748.7
845.9
749.3
864.8
745.5
749.3
873.9
887.5
877.0
746.8
900.0
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
47.99
48.86
48.86
48.86
50.51
51.80
53.06
53.31
53.31
55.41
55.41
56.12
58.74
58.74
59.98
62.52
66.60
66.60
70.00
70.79
70.79
74.43
74.43
78.44
78.44
84.84
90.38
90.38
97.00
97.00
100.00
882.0
900.7
888.0
744.6
906.2
909.3
913.9
913.7
895.0
913.9
895.8
922.5
927.3
894.9
932.8
893.0
943.4
895.0
946.7
946.4
909.1
943.0
913.0
936.3
911.0
913.9
932.2
911.0
956.1
910.7
966.8
10.26
13.21
13.21
15.79
18.80
18.80
21.53
21.53
26.89
30.56
30.56
33.78
33.78
36.56
36.56
39.11
39.11
41.89
41.89
43.93
43.93
45.70
45.70
47.32
47.99
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
the alloy containing 5 mol% Bi2Se3 (Fig. 3b) it can be seen that
the peaks in the latter figure are shifted compared to the former
which provides an evidence of the terminal solid solution (␣) on the
Tl2Te matrix. The existence range of the solution ␣ did not surpass
10 mol% Bi2Se3, i.e. up to the compound 9:1 (Fig. 3c).
Although the maximum on the liquidus line (794.7 K) at
10.0 mol% Bi2Se3 (Fig. 2) provides a strong and sufficient evidence
for the compound 9Tl2Te·Bi2Se3, an additional confirmation fol-
lows from comparison of XRD diffraction patterns on either side
of the compound, i.e. Fig. 3b, d and e. The patterns of the sample
5 mol% Bi2Se3 (Fig. 3b) shows superimposed peaks characteristic of
the compound 9:1 and pure Tl2Te, while those in Fig. 3d (20 mol%
Bi2Se3) and in Fig. 3e (40 mol% Bi2Se3) – the peaks characteristic of
the compound 9:1 and 1:1.
The X-ray diffraction patterns of the alloy samples of composi-
tions of 20 mol% Bi2Se3 (Fig. 3d), and 40 mol% Bi2Se3 (Fig. 3e) show
superimposed peaks characteristic of the compound Tl18Bi2Te9Se3
(9:1) together with strong peaks corresponding to the next com-
pound Tl2Bi2TeSe3 (1:1), which is clearly proved by the XRD pattern
in Fig. 3f (50 mol% Bi2Se3).
The XRD picture of the compound Tl6Bi14Te3Se21 (70.0 mol%
Bi2Se3) is shown in Fig. 3i. The alloy samples of compositions
of 60 mol% Bi2Se3 (Fig. 3g), and 68 mol% Bi2Se3 (Fig. 3h) show
superimposed peaks characteristic of the compound Tl2Bi2TeSe3
(1:1) together with the peaks corresponding to the compound
Tl6Bi14Te3Se21 (3:7) while those in Fig. 3j (80 mol% Bi2Se3) and in
Fig. 3k (90 mol% Bi2Se3) present the peaks characteristic of the com-
pound 3:7 and pure Bi2Se3 (all of them correspond to two phased
regions).
From comparison of the XRD pictures of pure Bi2Se3 (Fig. 3l) with
the alloys containing 90 mol% Bi2Se3 (Fig. 3k) and 80 mol% Bi2Se3
(Fig. 3j), it can be seen that the peaks in the figures are shifted com-
pared to the first one, which provides a good evidence for a region
of the terminal solid solution (␤) on the Bi2Se3 matrix. The last
statement appears to be quite consistent with the arrangement of
Fig. 3. The X-ray diffraction patterns of some Tl2Te + Bi2Se3 alloys.

66
I. Mucha / Thermochimica Acta 538 (2012) 63–66
experimental points corresponding to eutectic at 84.8 mol% Bi2Se3
melting at 913.9 K. The points reach as far as to 97.00 mol% Bi2Se3
(Table 1, No. 59) which corroborates the solid solution existence
above this alloy composition.
The melting points of compounds containing 50.0 mol%
Bi2X3 increase in the direction of higher stability of respec-
tive thallium chalcogenobismuthates (793.2 K TlBiTe2 → 895.0 K
Tl2Bi2TeSe3 → 951.1 K TlBiSe2), resulted from more ionic character
of chemical bonding in Bi2Se3 than in Bi2Te3.The same tendency
can be observed for the compounds formed above 50.0 mol%
Bi2X3 (817.9 K Tl0.83Bi1.06Te2 → 946.7 K Tl6Bi14Te3Se21 → 992.4 K
Tl0.83Bi1.06Se2).
In case of compounds formed at 10.0 mol% Bi2X3, an opposite
effect may be observed. The melting point of Tl9BiTe6 contain-
ing as much as 90.0 mol% Tl2Te is by 19.4 K higher than that of
Tl18Bi2Te9Se3 and 28 K higher than that of Tl9BiSe6. This is undoubt-
edly effect of quite substantial ionic contribution to the chemical
bonding in Tl2Te. The stability of thallium telluride is higher than
that of thallium selenide which results in higher melting point of
the first.
5. Conclusions
The results of the present study differ considerably from those of
[6] in some details. The authors of [6] considered the Tl2Te–Bi2Se3
system to be non-quasibinary while it appeared a quasi-binary
one. They reported the presence of ␣, ␣ꢀ, ␤ and ␥ phases together
with two- and three-phase regions. It should especially be noted
that according to the phase diagram published in [6] (Fig. 1.) no
quaternary compounds were formed, however we observed two
maximums on the liquidus line at 10.0 mol% Bi2Se3 and at 50.0 mol%
Bi2Se3.
There are two possible reasons for these discrepancies. The first
is the limitations of the differential thermal analysis employed by
the former authors. In the DTA method the samples are not stirred
and therefore the phase transformations occur under nonequilib-
rium conditions. Unlike DTA, the TA method used in this study
enabled the solid and liquid phases to be in equilibrium on cool-
ing due to efficient stirring, which resulted in precise temperature
measurements of the phase transitions in the examined alloys.
The other reason is that the metal chalcogenide alloys may form
glasses on solidifying, which makes exact temperature measure-
ment impossible. The glass formation appears very easily when the
alloy is not stirred.
The phase diagram for the system Tl2Te–Bi2Se3 resembled that
for the Tl2Se–Bi2Se3 [1] and Tl2Te–Bi2Te3 [2] systems. The similar-
ity of the phase diagrams might be expected because the systems
are analogous. In all of them at least three compounds are formed,
in the system Tl2Te–Bi2Te3 one additional (fourth) compound
TlBi7Se11 is formed.
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