Phase equilibria in the system Tl2Te-SnTe-TlBiTe2

Article abstract of DOI:10.1134/S0020168508100063

The phase equilibria in the system Tl₂Te-SnTe-TiBiTe₂ (A) have been studied using differential thermal analysis, x-ray diffraction, and microhardness measurements. We have constructed the T-x phase diagrams along the SnTe-TlBiTe₂, SnTe-Tl₉BiTe₆, and Tl ₄SnTe₃-TlBiTe₂ joins, the 600-and 800-K sections of the phase diagram of system A, and its liquidus diagram. The results demonstrate that the system contains broad ranges of Tl₅Te ₃-structured and SnTe-based solid solutions (δ and γ₁ phases, respectively). There are also relatively small fields of the Tl₂Te-based phase (α) and low-and high-temperature TlBiTe₂-based solid solutions (γ₂ and γ′₂). The liquidus surface of system A comprises the primary crystallization fields of the δ, γ₁, and γ′₂ phases. The liquidus of the α phase is degenerate. The ternary eutectic between the δ, γ₁, and γ′₂ phases melts at 755 K.

Full text of DOI:10.1134/S0020168508100063

ISSN 0020-1685, Inorganic Materials, 2008, Vol. 44, No. 10, pp. 1060–1065. © Pleiades Publishing, Ltd., 2008.
Original Russian Text ©M.B. Babanly, G.B. Dashdieva, F.N. Guseinov, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 10, pp. 1187–1192.
*
Phase Equilibria in the System Tl Te–SnTe–TlBiTe2
2
M. B. Babanly, G. B. Dashdieva, and F. N. Guseinov
Baku State University, ul. Khalilova 23, Baku, AZ1148 Azerbaijan
e-mail: babanly_mb@rambler.ru
Received September 14, 2007
Abstract—The phase equilibria in the system Tl Te–SnTe–TlBiTe (A) have been studied using differential
2
2
thermal analysis, x-ray diffraction, and microhardness measurements. We have constructed the T–x phase dia-
grams along the SnTe–TlBiTe , SnTe–Tl BiTe , and Tl SnTe –TlBiTe joins, the 600- and 800-K sections of
2
9
6
4
3
2
the phase diagram of systemA, and its liquidus diagram. The results demonstrate that the system contains broad
ranges of Tl Te -structured and SnTe-based solid solutions (δ and γ phases, respectively). There are also rela-
5
3
1
tively small fields of the Tl Te-based phase (α) and low- and high-temperature TlBiTe -based solid solutions
2
2
(
γ and γ' ). The liquidus surface of system A comprises the primary crystallization fields of the δ, γ , and γ'
2
2
1
2
phases. The liquidus of the α phase is degenerate. The ternary eutectic between the δ, γ , and γ' phases melts
1
2
at 755 K.
DOI: 10.1134/S0020168508100063
I N* TRODUCTION
tions (δ phase), crystallizes in tetragonal symmetry
Tl Te structure, sp. gr. I4/mcm). The lattice parame-
(
5
3
Tellurides of heavy p-metals are thought to be
attractive hosts for the preparation of new thermoelec-
tric materials [1, 2]. In this family of compounds, tin,
bismuth, and thallium tellurides are of considerable
practical importance [1–5]. One way of enhancing the
ters of Tl SnTe are a = 8.82 Å and c = 13.01 Å (Z = 4)
4
3
[
15]. The Tl Te–TlBiTe system contains a compound of
2 2
composition Tl BiTe , which melts congruently at 830 K,
9
6
also has a tetragonal structure of the Tl Te type (a =
5
3
performance of thermoelectric materials is by utilizing 8.855 Å, c = 13.048 Å, Z = 2) [10, 16], and forms a con-
multicomponent compounds with complex structures tinuous series of solid solutions with Tl Te [10].
2
[
2, 6]. The key to the targeted synthesis of new multi-
As shown by Mazelsky and Lubell [17], the SnTe–
component tellurides of the above elements lies in
knowing the phase equilibria in the corresponding sys-
tems.
TlBiTe system contains limited solid solutions based
2
on SnTe (56 mol %) and TlBiTe (21 mol %).
2
This led us to study the phase equilibria in the com-
Dashdieva et al. [18] studied the phase equilibria in
position region Tl Te–SnTe–TlBiTe (A) of the quater- the composition region Tl Te–Tl SnTe –Tl BiTe of
2
2
2
4
3
9
6
nary system Tl–Sn–Bi–Te. The constituent binary tellu- system A. According to their results, the Tl SnTe -
4
3
rides Tl Te and SnTe melt congruently at 698 and 1080 K, Tl BiTe join is pseudobinary, with a continuous series
2
9
6
respectively [1, 7, 8]. Tl Te has a monoclinic structure of Tl Te -structure solid solutions (δ phase). Their lat-
2
5
3
[
9], and SnTe has a primitive cubic unit cell [7]. The ter- tice parameters follow Vegard’s law (a = 8.821–8.854 Å,
nary compound TlBiTe exists in two crystalline poly- c = 13.01–13.05 Å). The δ phase field extends over
2
morphs. At 780 K, the low-temperature, orthorhombic most of the Tl Te–Tl SnTe –Tl BiTe system [18].
2
4
3
9
6
phase transforms into the high-temperature, disordered
phase of variable composition, which melts congru-
ently at 830 K [10–12].
EXPERIMENTAL
The constituent binaries of system A have been
The constituent tellurides of system A were synthe-
sized by melting appropriate high-purity elemental
studied by several groups. The Tl Te–SnTe system was
2
reported to contain a ternary compound of composition
mixtures in silica tubes sealed off under a vacuum of
Tl SnTe , which has a broad homogeneity range
4
3
–2
~
10 Pa, followed by slow cooling. The synthesis tem-
(
[
12−40 mol % SnTe) and melts congruently at 825 K
13, 14]. This compound, as well as related solid solu-
perature was 750 (Tl Te), 1150 (SnTe), or 900 K
2
(
TlBiTe ), that is, slightly above the corresponding
2
*
melting point. The synthesized compounds were iden-
tified by differential thermal analysis (DTA) and x-ray
diffraction (XRD).
Presented in part at the XII Conference High-Purity Substances
and Materials: Preparation, Analysis, and Application, Nizhni
Novgorod, Russia, May 28–31, 2007.
1
060

PHASE EQUILIBRIA IN THE SYSTEM Tl Te–SnTe–TlBiTe
1061
2
2
Alloys of system A were prepared by vacuum-melt-
a, Å
ing telluride mixtures. We prepared SnTe–TlBiTe2,
Tl SnTe -TlBiTe , and SnTe–Tl BiTe alloys and a
6.5
(c)
4
3
2
9
6
number of alloys beyond these joins. Using DTA results
6
6
.4
.3
for a number of unhomogenized alloys and earlier data
[
10, 13, 18], we selected heat-treatment temperatures at
which the alloys were equilibrated for 500 h. The alloys
annealed at 600 K were furnace-cooled, and those
annealed at 800 K were quenched in cold water.
H, åP‡
(
(
b)
a)
1500
1
000
500
The alloys were characterized by DTA (NTR-74
pyrometer, Chromel–Alumel thermocouples), XRD
(
DRON-2 powder diffractometer, CuK radiation), and
α
T, K
microhardness measurements (PMT-3 tester, 0.2-N
indentation load).
1
100
1080
RESULTS AND DISCUSSION
1000
L
The present experimental data and earlier results
[
10, 13, 14, 18] for the constituent binaries and the
2
9
8
00
00
Tl Te–Tl SnTe –TlBi Te system are summarized in
4
3
6
2
L + γ1
Figs. 1–6.
8
7
30
80
The present T–x phase diagram of the SnTe–
e1 γ'2
TlBiTe system (Fig. 1a) differs somewhat from that
2
γ +γ'
1
2
reported by Mazelsky and Lubell [17]. According to
γ1
our results, this system involves both eutectic and
e
1
γ2
eutectoid phase relations. The eutectic (Â ) is located at
700
Sn Te
1
γ1+γ2
ꢀ85 mol % TlBiTe , with a melting point of 815 K.
2
The homogeneity range of the SnTe-based phase (γ1
phase) extends to 80 mol % TlBiTe at 800 K and to
20
40
mol %
60
80 TlBiTe2
2
7
2 mol % TlBiTe at 600 K. The formation of low- and
2
Fig. 1. T–x phase diagram of the SnTe–TlBiTe system (a);
2
high-temperature TlBiTe -based solid solutions (γ -
and γ' phases) is accompanied by a reduction in the
temperature of the polymorphic transformation from
2
2
composition dependences of the microhardness (b) and lat-
tice parameter (c). The crosses in panels b and c represent
samples quenched from 800 K.
2
7
65 to 750 K and a transition to eutectoid phase rela-
tions. The eutectoid point (Â ) is located at ꢀ90 mol %
2
Fig. 2, the 600-K section comprises single- (δ, γ , and
1
TlBiTe and 750 K (Fig. 1a). The width of the homoge-
neity range of the γ' phase is 10 mol % (800 K), and
that of the γ phase is ꢀ5 mol % (600 K).
2
γ ), two-, and three-phase fields. The γ and γ phase
2
1
2
2
fields extend along the constituent binary SnTe–
2
TlBiTe and are 2–3 mol % in width. The δ phase field
2
XRD and microhardness data are consistent with the extends over most of the Tl Te–Tl SnTe –Tl BiTe
2 4 3 9 6
T–x phase diagram of the SnTe–TlBiTe system. Pow- region. In addition, there are two fields (α and ï) near
2
der XRD patterns of the alloys containing 0–70 mol % Tl Te. The alloys in the α field are isostructural with
2
TlBiTe (annealing at 600 K) or 0–80 mol % TlBiTe2 Tl Te. We failed to detect an α + δ two-phase field,
2
2
(
quenching after annealing at 800 K) show only reflec- which leads us to assume that the α
δ phase trans-
tions from an SnTe-based cubic phase, and its lattice formation is morphotropic. In the ï field, the Tl Te–
2
parameter varies almost linearly with composition,
from a = 6.327 (SnTe) to 6.465 Å (80 mol % TlBiTe2)
SnTe–TlBiTe plane is unstable: the equilibrium alloys
2
contain, in addition to the α and δ phases, a Tl-based
(Fig. 1c). The microhardness ç of the γ phase rises
1
metallic phase [18]. This rare effect, also encountered
steadily from ꢀ850 (SnTe) to ꢀ1400 MPa (80 mol %
TlBiTe ), and that of the γ phase rises from ꢀ500
in the constituent system Tl Te–SnTe, was analyzed in
2
2
2
detail elsewhere [13, 14].
(
TlBiTe ) to 600 MPa (95 mol % TlBiTe ). The micro-
2
2
The phase diagram of the SnTe–Tl BiTe system
hardness of the samples quenched from 800 K slightly
9
6
exceeds that of the corresponding samples annealed at (Fig. 3) is similar in appearance to that of a pseudobin-
6
00 K (Fig. 1b).
The XRD, DTA, and microhardness data for the
ary eutectic, but this system is not pseudobinary. The
alloys of this system consist of two phases, γ + δ, but,
1
alloys annealed at 600 K were used to map out the sub- as seen in Fig. 2, the compositions of the equilibrium γ
solidus phase diagram of system A (Fig. 2). As seen in and δ phases lie beyond this join. Comparison of Figs. 3
1
INORGANIC MATERIALS Vol. 44 No. 10 2008

1
062
BABANLY et al.
Tl Te
2
α
X
δ
Tl BiTe
6
8
0
9
Tl SnTe
4
3
6
0
γ2 +δ
4
0
γ1 + δ
2
0
γ1 + γ
2
γ1
γ2
TlBiTe2
SnTe
20
40
60
80
mol % TlBiTe2
Fig. 2. 600-K section of the T–x–y phase diagram of the Tl Te–SnTe–TlBiTe system.
2
2
and 5 demonstrates that the SnTe–Tl BiTe join passes
Because of the very small slope of the Â Ö curve
2
9
6
through primary crystallization fields and intersects the over the SnTe–Tl BiTe join, this eutectic transforma-
9
6
eutectic curve  Ö, representing the univariant equilib- tion occurs in a very narrow temperature range, and is
2
rium
represented by a single, sharp DTA peak. For this rea-
son, the L + γ + δ field in Fig. 3 is marked by a dashed
1
L
γ1+ δ.
(1)
line. Along this join, the homogeneity ranges of the γ1
and δ phases are ꢀ3–4 mol % in width (600 K).
More complex phase relations were found along the
T, K
Tl SnTe -TlBiTe join (Fig. 4), which passes through
4
3
2
1
080
five phase fields below the solidus (Fig. 2). Its liquidus
comprises three branches, corresponding to the primary
crystallization of the δ-, γ -, and γ' phases, which are
1
2
1
000
L
separated by the eutectic curves Â Ö and Â Ö (Fig. 5).
1
2
The 755-K horizontal represents the four-phase equi-
librium
9
8
00
00
LE
γ + γ' + δ,
(2)
1
2
L + γ1
830
65
and the 748-K horizontal represents the eutectoid equi-
librium
γ1
L + δ
7
δ
γ2'
γ + γ + δ.
(3)
1
2
7
00
Below this horizontal, the alloys containing 50–
γ1 + δ
8
7 mol % TlBiTe consist of three phases: γ + γ + δ
2
1
2
(
Fig. 2). In the composition ranges ꢀ7–50 and 80–90
8
mol % TlBiTe , the solidus line represents the univari-
2
2
0
40
60
mol %
80 Tl BiTe
9 6
SnTe
ant eutectic processes (1) and (4), respectively:
Fig. 3. T–x phase diagram of the SnTe–Tl BiTe system.
LE
γ2' + δ.
(4)
9
6
INORGANIC MATERIALS Vol. 44 No. 10 2008

PHASE EQUILIBRIA IN THE SYSTEM Tl Te–SnTe–TlBiTe
1063
2
2
T, K
8
30
8
8
25
00
L + γ + γ'
1
2
L + δ
L + γ'2
γ'2
L + γ1
780
δ
7
7
50
00
L + γ + δ
1
γ + γ' + δ
1
2
γ1 + δ
γ + γ + δ
1
2
γ2
0
.33 Tl SnTe 20
40
60
80
TlBiTe2
4
3
mol %
Fig. 4. T–x phase diagram of the Tl SnTe –TlBiTe system.
4
3
2
Tl Te
2
D (830 K)
8
0
2
D
1
3
(
825 K)
6
0
8
00
e2
780 K)
e3(763 K)
(
(755K)
E
4
0
1
2
0
2
SnTe
20
40
60
80
e
TlBiTe2
1
(
815 K)
mol %
Fig. 5. Liquidus diagram of the Tl Te–SnTe–TlBiTe system. Primary crystallization fields: (1) γ , (2) γ' , (3) δ. The dashed line
2
2
1
2
represents the pseudobinary join Tl SnTe –Tl BiTe .
4
3
9
6
INORGANIC MATERIALS Vol. 44 No. 10 2008

1
064
BABANLY et al.
Tl Te
2
L
L + δ
8
0
Tl BiTe
9 6
δ
Tl SnTe
4
3
L + δ
6
0
L
4
0
2
0
L + γ1
γ'2
SnTe
20
γ1
40
60
80 γ + γ TlBiTe2
1
2
mol %
Fig. 6. 800-K section of the Tl Te–SnTe–TlBiTe phase diagram.
2
2
The homogeneity ranges of the δ and γ phases along
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1
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INORGANIC MATERIALS Vol. 44 No. 10 2008