Phase equilibria and some properties of solid solutions in the Tl 5Te3-Tl9BiTe6-Tl5Te 2Cl system

Article abstract of DOI:10.1134/S0036023611110040

Phase equilibria in the Tl₅Te₃-Tl₉BiTe6- Tl₅Te₂Cl system were studied by differential thermal analysis (DTA), X-ray powder diffraction, and measurements of microhardness and also emf of concentration circuits with reference to a thallium electrode. A number of polythermal sections, the isothermal sections of the phase diagram at 760 and 800 K, and projections of the liquidus and solidus surfaces were constructed. It was shown that the system is characterized by the formation of unlimited solid solutions with the Tl₅Te₃ structure. The concentration dependences of the crystal lattice parameters, microhardness, and emf in the solid solutions were described.

Full text of DOI:10.1134/S0036023611110040

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2011, Vol. 56, No. 11, pp. 1833–1838. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © D.M. Babanly, S.V. Askerova, Z.S. Aliev, M.B. Babanly, 2011, published in Zhurnal Neorganicheskoi Khimii, 2011, Vol. 56, No. 11, pp. 1917–1923.
PHYSICOCHEMICAL ANALYSIS
OF INORGANIC SYSTEMS
Phase Equilibria and Some Properties of Solid Solutions
in the Tl Te –Tl BiTe –Tl Te Cl System
5
3
9
6
5
2
D. M. Babanly, S. V. Askerova, Z. S. Aliev, and M. B. Babanly
Institute of Chemical Problems, National Academy of Sciences of Azerbaijan, Baku, Azerbaijan
Baku State University, Baku, Azerbaijan
eꢀmail: babanly_mb@rambler.ru
Received April 8, 2010
Abstract—Phase equilibria in the Tl Te –Tl BiTe –Tl Te Cl system were studied by differential thermal
5
3
9
6
5
2
analysis (DTA), Xꢀray powder diffraction, and measurements of microhardness and also emf of concentraꢀ
tion circuits with reference to a thallium electrode. A number of polythermal sections, the isothermal sections
of the phase diagram at 760 and 800 K, and projections of the liquidus and solidus surfaces were constructed.
It was shown that the system is characterized by the formation of unlimited solid solutions with the Tl Te
5
3
structure. The concentration dependences of the crystal lattice parameters, microhardness, and emf in the
solid solutions were described.
DOI: 10.1134/S0036023611110040
Tellurides of heavy metals, including thallium
The compound Tl BiTe6 melts congruently at 833 K
9
(
Tl BiTe , Tl SnTe , Ag TlTe5, etc.), are promising [16] and crystallizes in a tetragonal lattice with the
9
6
2
5
9
compounds for thermoelectric materials design [1, 2]. parameters
а = 8.855 Å, с = 13.048 Å, and Z = 2 [5].
The Tl–Bi–Te system was studied in the composition
Thallium telluride Tl Te has thermoelectric propꢀ
erties and, because of the specific of its crystal lattice
3], has a number of ternary analogues produced by
5
3
range Tl Te–Bi Te –Te [16]. Tl BiTe6 was found to
2
2
3
9
have an extensive homogeneity region (
δ
phase),
[
which almost completely occupies the composition
both cation and anion substitutions. Typical represenꢀ
tatives of its cationꢀsubstituted structural analogues
triangle Tl Te–Tl Te –Tl BiTe6.
2
5
3
9
IV
IV
V
V
The compound Tl Te Сl was detected while studyꢀ
5 2
are Tl B Te (B = Sn, Pb) [4] and Tl B Te (B = Sb,
4
3
9
6
ing the system Tl–Te–Cl along the section Tl Te–TlCl
Bi) [5, 6], and its typical anionꢀsubstituted structural
2
[
7]. The intermediate phase based on this compound
δ
analogues are Tl Te2Hal (Hal = Cl, Br, I) [7].
5
was shown to melt with decomposition by a syntectic
reaction at 708 K. At this temperature, the phase sepꢀ
Among structural analogues of Tl Te3, the comꢀ
5
pound Tl BiTe6 has recordꢀbreaking high thermoelecꢀ
9
aration region extends from ~7 to 83 mol % Tl Te. The
2
tric indexes [8, 9] and is considered a promising matrix
compound for design of new similar materials. One of
parameters of the tetragonal unit cell of Tl Те Cl
5
2
(
space group
I4/mcm) are a= 8.920 Å, с= 12.691 Å, and
way to improve the applied properties of Tl BiTe6 is to
9
Z = 4 [7].
obtain its base solid solutions. The following systems
were investigated for this purpose: Tl Te –Tl PbTe –
5
Tl BiTe [10], Tl Te –Tl SnTe –Tl BiTe [11], Tl Te –
3
4
3
9
6
5
3
4
3
9
6
5
3
EXPERIMENTAL
For preparing alloys in system A, we synthesized
Tl NdTe –Tl BiTe [12], and Tl Te –Tl BiTe –Tl Te I
9
6
9
6
5
3
9
6
5
2
[
13]. Unlimited solid solutions with the Tl Te3 strucꢀ
5
the initial compounds Tl Te , Tl BiTe6, and Tl Te Cl
.
ture were found to exist in these systems.
5
3
9
5
2
The first two compounds, which melt congruently,
were synthesized by alloying stoichiometric amounts
In this work, we studied phase equilibria in the
Tl Te –Tl BiTe –Tl Те Сl system (A) to obtain new
5
3
9
6
5
2
of highꢀpurity constituent elements in evacuated
cationꢀ and anionꢀsubstituted multicomponent solid
–2
(
~10 Pa) quartz ampoules at temperatures somewhat
solutions based on Tl BiTe6
.
9
(by 30–50 K) exceeding their melting points with subꢀ
sequent slow cooling of the melt.
The initial compounds of system A have been studꢀ
ied in detail. Tl Te melts congruently at 723 K [14, 15]
The compound Tl Te Cl was synthesized in two
5
3
5
2
and crystallizes in a tetragonal lattice (space group stages. Initially, TlCl was synthesized according to a
4/mcm) with the parameters = 8.929 = 12.620 Å, published procedure [17]. Metallic thallium was disꢀ
and = 4 [3]. Unlike other thallium tellurides, Tl Te3 solved at ~350 K in dilute (5%) H SO4 to obtain a
is a variableꢀcomposition phase with quite a wide Tl SO4 solution. To a boiling 2% Tl SO4 solution,
I
а
Å, с
Z
5
2
2
2
homogeneity range (~34.5–38 at % Te [14, 15]).
dilute HCl was added until the precipitation was comꢀ
1833

1834
BABANLY et al.
E, mV
300 К)
(
d)
(
4
4
4
50 446
35
20
424
H
, MPa
(c)
(b)
1
330
1300
1150
1000
a, Å
c, Å
8
.92
.88
13.00
12.80
12.60
8
8.84
T
, K
(а)
830
L
L + L1
8
00
50
00
L +
δ
7
L + L +
1
δ
δ
708
7
Tl BiTe6 80
9
60
40
20
2Tl Te Cl
5
2
mol %
Fig. 1. (a) Phase diagram of the Tl BiTe –Tl Te Cl system and (b) unit cell parameters, (c) microhardness, and (d) emf of conꢀ
9
6
5
2
centration circuits (1) versus composition.
pleted. After cooling the mother solution, TlCl was
The samples were investigated by DTA (NTRꢀ72
withdrawn, washed with distilled water, and dried for a pyrometer, Chromel/Alumel thermocouples, and x–y
long time in a drying cabinet at 380–390 K.
recorder), Xꢀray powder diffraction (Philips X’Pert
MPD diffractometer, Cu radiation), and measureꢀ
K
α
1
Then, Tl Te Сl was synthesized by alloying thalꢀ
5
2
ments of microhardness (PMTꢀ3 microhardness
meter, 20ꢀg load) and the emf of concentration cirꢀ
cuits of the type
lium monochloride with required amounts of Tl and
Te in an evacuated quartz ampoule. The alloying temꢀ
perature was ~850 K. This compound melts syntactiꢀ
cally [7]; the melt consists of two separating liquid
phases. For this reason, heterogeneous mixtures conꢀ
taining TiCl and the phases based on Tl Тe Cl and
(–)Tl (solid)/glycerol + KCl
(1)
+
TlCl/[Tl–Bi–Te–Cl] (solid) (+).
5
2
TlCl crystallize from the melt even on slow cooling.
Therefore, to ensure the completeness of the interaction,
the ampoules were slowly (for ~5 h) cooled to 700 K
while being continuously shaken and kept at this temꢀ
perature for ~300 h.
The assembly of the electrochemical cell and emf
measurements were described in detail previously
[
18]. The emf of circuits (1) was measured by a comꢀ
pensation method with a V7ꢀ43A digital voltmeter
within the temperature range 300–400 K.
The synthesized compounds were identified by difꢀ
ferential thermal analysis (DTA) and Xꢀray powder
diffraction.
RESULTS AND DISCUSSION
Samples of system A with compositions lying along
three mutually perpendicular radial sections were preꢀ Tl BiTe6 and Tl Te –Tl Te Cl in the Tl Te –Tl BiTe –
The boundary quasiꢀbinary constituents Tl Te –
5
3
9
5
3
5
2
5
3
9
6
pared by vacuum alloying of the preliminarily syntheꢀ Tl Те Cl system are characterized by unlimited mutual
5
2
sized initial compounds with subsequent homogenizꢀ solubility of components and are described by
ing annealing at ~700 K for 800–1000 h. diagrams without extreme points [16, 19].
Т–х
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 11 2011

PHASE EQUILIBRIA AND SOME PROPERTIES
1835
mV
Some properties of the initial compounds and solid solutions in the Tl Te –Tl BiTe –Tl Te Cl system
5
3
9
6
5
2
Unit cell parameters, Å
Section
Phase
Tmelt, K
M
,
MPa
980
E,
a
c
Tl BiTe –Tl Te Cl
Tl BiTe
6
830
8.855(5)
8.870(6)
8.883(7)
8.900(7)
8.916(6)
8.920(7)
13.048(8)
12.975(7)
12.900(8)
12.840(7)
12.758(7)
12.691(8)
446
438
9
6
10
4
2
9
Tl Bi Te Cl
755–815
730–805
720–790
715–740–768
708
1280
1320
1340
1335
1340
9
.2 0.8 5.6 0.4
Tl Bi Te Cl
9
.4 0.6 5.2 0.8
Tl Bi Te Cl
9
.6 0.4 4.8 1.2
Tl Bi Te Cl
427
424
9
.8 0.2 4.4 1.6
Tl Te Cl
2
10
4
Tl BiTe –[Tl Te Cl]
Tl Bi Te Cl
775–825
750–813
730–795
715–758
710–718
8.868(5)
8.884(6)
8.896(7)
8.910(6)
8.924(7)
12.969(8)
12.893(8)
12.814(8)
12.735(8)
12.655(7)
1180
1280
1300
1340
1350
440
435
9
6
10
5
9.2 0.8 5.8 0.2
Tl Bi Te Cl
9.4 0.6 5.6 0.4
Tl Bi Te Cl
9.6 0.4 5.4 0.6
Tl Bi Te Cl
425
420
9.8 0.2 5.2 0.8
Tl Te Cl
10
5
[
Tl Bi Te ]–Tl Te Cl Tl Bi Te
760–808
8.892(6)
8.898(7)
8.904(6)
8.909(7)
8.915(7)
12.840(9)
12.809(8)
12.783(8)
12.754(8)
12.722(7)
1170
1270
1330
1380
1390
432
430
9.5 0.5
6
10
4
2
9.5 0.5
6
Tl Bi Te Cl
735–800
9.6 0.4 5.6 0.4
Tl Bi Te Cl
722–783
9.7 0.3 5.2 0.8
Tl Bi Te Cl
715–765
427
9.8 0.2 4.8 1.2
Tl Bi Te Cl
710–735–745
9.9 0.1 4.4 1.6
The Tl BiTe –Tl Te Cl system (Fig. 1), which forms
We also found that the microhardness and emf in
phase), is, as a alloys in the Tl BiTe –Tl Te Cl system are continuous
whole, nonꢀquasiꢀbinary because of the incongruence functions of composition: their values vary continuꢀ
of melting of the compound Tl Te Cl [7]. Within the ously between the and values of the initial comꢀ
9
6
5
2
a continuous solid solution series (
δ
9
6
5
2
Н
Е
5
2
pounds (Figs. 1c, 1d). The fact the each of the samples
comprises a single phase was confirmed by microhardꢀ
ness measurements (which gave a single value). The
continuous change in the emf of circuits (1) is indicaꢀ
tive [18] of a continuous change in the activity of the
potentialꢀforming component (thallium) in alloys,
which indirectly proves the formation of unlimited
composition range ~53–100 mol % Tl BiTe6, the
phase primarily crystallizes from a homogeneous melt;
δ
9
near the composition Tl Te Cl (0–53 mol % Tl BiTe6),
5
2
9
the
liquid1
phases leave the
δ
phase crystallizes by the syntectic reaction liquid +
; and the compositions of the separating
plane of this system.
δ
Т–х
The results of Xꢀray powder diffraction analysis and solid solutions in the system.
microhardness and emf measurements (table; Figs. 1b–
The results obtained from analyzing some polyꢀ
thermal sections of the phase diagram of system A
1
d) confirm the DTA data.
(
table; Figs. 2, 3) indicate the formation of unlimited
Xꢀray powder diffraction analysis showed that all
samples are comprised of a single phase and have a tetꢀ
solid solutions with the Tl Te3 structure in the system.
5
ragonal structure of the Tl Te type. The unit cell
Along Tl BiTe –[Tl Te Cl] and [Tl Bi Te ]–
5
3
9
6
10
5
9.5 0.5
6
parameters of the
δ
phase vary virtually linearly from Tl Te Cl sections (hereafter, within the square brackꢀ
10 4 2
those for pure Tl BiTe6 to those for Tl Te Cl ets are the initial “components” of the sections that
9
5
2
(а
= 8.859–8.920
Å,
с
= 13.051–12.691 Å; Fig. 1b).
are not chemical compounds), there are continuous
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 11 2011

1836
BABANLY et al.
E
, mV
300 К)
E, mV
(300 К)
430
(c)
(b)
(
c)
(
4
46
4
50
30
4
4
20
4
20
H
, MPa
410
1400
1300
1200
H
, MPa
1340
(b)
1400
1200
1000
1350
T
8
, K
00
L
(а)
(
а)
L + L1
T, K
8
30
L +
δ
L
750
L + L +
1
δ
L + L1
8
00
50
00
L +
δ
708
Tl Te Cl
2
δ
7
00
7
L + L +
1
δ
[
Tl Bi Te ] 20
9.5 0.5
40
mol %
60
80
7
7
18
10
6
10
4
δ
7
Tl BiTe6 80
60
40
mol %
20 [Tl Te Cl]
10 5
9
Fig. 3. (a) [Tl9.5Bi0.5Te ]–Tl Te Cl polythermal section
of system A and (b) microhardness and (c) emf of concenꢀ
tration circuits (1) versus composition.
Fig. 2. (a) Tl BiTe –[Tl Te Cl] polythermal section of
system A and (b) microhardness and (c) emf of concentraꢀ
tion circuits (1) versus composition.
6
10
4
2
9
6
10
5
2Tl Te3
5
changes in onset and end temperatures of crystallizaꢀ
tion (Figs. 2a, 3a) and in the microhardness, emf
(
Figs. 2b, 2c, 3b, 3c), and unit cell parameters (table).
Such composition–property diagrams are characterisꢀ
tic of systems with unlimited solid solutions.
20
740
780
The [Tl Bi Te ]–Tl Te Сl polythermal section
760
b
9
.5 0.5
6
10
4
2
40
(
(
Fig. 3a) passes through the phase separation region
δ
liquid + liquid ) caused by the syntectic nature of
1
7
60
800
60
80
Tl Te Cl melting. Within the corresponding composiꢀ
5
2
7
tion range (~40–100 mol % Tl Te Cl2), the
δ
phase
monovariant
syntectic reaction, and the phase diagram contains the
liquid + liquid1 threeꢀphase region. Notably, the
10
4
8
00
L + L1
crystallizes by the liquid + liquid1
δ
80
720
820
740
+
δ
tie lines in the liquid + liquid twoꢀphase regions in
Figs. 2a and 3a do not lie insie the T–x plane and vary
continuously with varying temperature.
1
a
Tl BiTe
9
80
60
40
20 2Tl Te Cl
5 2
6
Tl BiTe , mol %
9 6
Figure 4 presents liquidus and solidus surface proꢀ
Fig. 4. Projections of liquidus (solid lines) and solidus
(dashed lines) surfaces in the Tl Te –Tl BiTe –Tl Te Cl
system (A).
jections of the Tl Te –Tl BiTe –Tl Te Cl system, where
5
3
9
6
5
2
5
3
9
6
5
2
the isotherms on these surfaces are indicated by solid
and dashed lines, respectively. The liquidus consists of
two fields that describe the primary crystallization of
8
00ꢀK isothermal section of the phase diagram of
system A (Fig. 5a) comprises four fields. The liquid +
twoꢀphase region is located between the correspondꢀ
) and allows one to
the phase from a singleꢀphase melt and from two sepꢀ
δ
arating liquid phases. The curve ab separating these
fields represents monovariant syntectic equilibrium:
δ
L +
surface (shown in dashed lines) that characterizes the determine their coupled mutually saturated composiꢀ
end of crystallization of solid solutions. tions. By comparing Fig. 5a and Figs. 2a, 3a, it is easy
δ
. The solidus of the system consists of only one ing singleꢀphase fields (liquid and
δ
δ
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 11 2011

PHASE EQUILIBRIA AND SOME PROPERTIES
1837
2Tl Te3
5
(a)
T
= 800 K
20
L
40
6
0
δ
+ L
L + L1
80
δ
Tl BiTe6
9
80
60
Tl BiTe , mol %
40
20
2Tl Te Cl
5 2
9
6
2Tl Te3
5
(b)
L'
T
= 760 K
20
L
4
S'
0
δ
+ L
L''
6
0
L + L1
+
8
0
L
S''
L + L1
60
+
δ
L
Tl BiTe6
9
80
40
20
2Tl Te Cl
5 2
Tl BiTe , mol %
9
6
Fig. 5. Isothermal sections of the phase diagram of system A at (a) 800 and (b) 760 K.
to show that the tie lines in the liquid +
δ
twoꢀphase phases in the liquid + liquid +
δ
threeꢀphase region
1
regions of the above polythermal sections lie outside are also represented by points located outside this
their T–x planes. Figure 5a also demonstrates the liqꢀ plane and, in Fig. 5a, they are separated conditionally
uid + liquid phase separation region, which occupies according to the phase rule requirements.
1
a considerable part of the concentration triangle from
the side of Tl Te Br. Obviously, the tie lines liquid +
The data obtained on phase equilibria in the
5
2
Tl Te –Tl BiTe –Tl Te Cl system can be used for
5
3
9
6
5
2
liquid in this region are also outside the plane of this
1
choosing the compositions of melts and their crystalliꢀ
isothermal section.
zation temperature in growing single crystals of
solutions of given compositions.
δ
ꢀsolid
In 760 K isothermal section (Fig. 5b), phase equiꢀ
libria are somewhat different. In the region liquid +
δ
(
field S'L'L''S''), the tie lines connect the conjugated
curves of liquidus (L'L'') and solidus (S'S''); and in the
other two twoꢀphase regions (liquid + and liquid +
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