New thallium neodymium tellurides

Article abstract of DOI:10.1134/S0020168508090070

The phase equilibria in the composition region Tl₂Te-Tl ₂Te₃-Nd₂Te₃-NdTe of the Tl-Nd-Te system have been studied by differential thermal analysis and x-ray diffraction, and the 500-K section of its phase diagram has been mapped out. The ternary compounds identified are Tl₉NdTe₆, Tl₄NdTe ₃, Tl₆Nd₄Te₉, Tl₄Nd ₆Te₁₁, Tl₂NdTe₃, TlNdTe₃, and TlNd₃Te₆. X-ray powder diffraction data indicate that the compounds Tl₉NdTe₆ and Tl₄NdTe₃ are structural analogs of Tl₅Te₃ and have tetragonal cell parameters a = 8.855 A, c = 13.010 A and a = 8.858 A, c = 12.998 A, respectively (sp. gr. I4/mcm, Z = 4). Tl₉NdTe ₆ and Tl₄NdTe₃ are phases of variable composition (δ-phase), and their fields cover most of the Tl ₂Te-Tl₅Te₃-Tl₄NdTe₃ composition triangle. The structure of Tl₅Te₃ and its ternary analogs is discussed, and the conclusion is drawn that, in spite of the broad field of the δ-phase, Tl₉NdTe₆ and Tl ₄NdTe₃ are daltonides, i.e., distinct chemical compounds.

Full text of DOI:10.1134/S0020168508090070

ISSN 0020-1685, Inorganic Materials, 2008, Vol. 44, No. 9, pp. 935–938. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © S.Z. Imamalieva, F.M. Sadygov, M.B. Babanly, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 9, pp. 1054–1057.
New Thallium Neodymium Tellurides1
S. Z. Imamalieva, F. M. Sadygov, and M. B. Babanly
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 composition region Tl Te–Tl Te –Nd Te –NdTe of the Tl–Nd–Te sys-
2
2
3
2
3
tem have been studied by differential thermal analysis and x-ray diffraction, and the 500-K section of its phase
diagram has been mapped out. The ternary compounds identified are Tl NdTe , Tl NdTe , Tl Nd Te ,
9
6
4
3
6
4
9
Tl Nd Te , Tl NdTe , TlNdTe , and TlNd Te . X-ray powder diffraction data indicate that the compounds
4
6
11
2
3
3
3
6
Tl NdTe and Tl NdTe are structural analogs of Tl Te and have tetragonal cell parameters a = 8.855 Å, c =
9
6
4
3
5
3
1
3.010 Å and a = 8.858 Å, c = 12.998 Å, respectively (sp. gr. I4/mcm, Z = 4). Tl NdTe and Tl NdTe are phases
9 6 4 3
of variable composition (δ-phase), and their fields cover most of the Tl Te–Tl Te –Tl NdTe composition tri-
2
5
3
4
3
angle. The structure of Tl Te and its ternary analogs is discussed, and the conclusion is drawn that, in spite of
5
3
the broad field of the δ-phase, Tl NdTe and Tl NdTe are daltonides, i.e., distinct chemical compounds.
9
6
4
3
DOI: 10.1134/S0020168508090070
1
INTRODUCTION
three compounds have cubic structures (NdTe: a =
.262 Å, sp. gr. Fm3m), Nd Te (a = 9.456 Å, sp. gr.
6
3
4
Ternary rare-earth chalcogenides are among the
most promising electronic materials. In addition to high
heat resistance and stability to sharp changes in exter-
nal conditions, many of such compounds offer unique
magnetic, optical, and thermoelectric properties [1–3].
I43d), Nd Te (a = 8.427 Å, sp. gr. I43d); the others crys-
tallize in lower symmetries [1, 5].
2
3
Earlier [12], we studied the pseudobinary join
Tl Te–Nd Te of the Tl–Nd–Te system and identified
2
2
3
This paper addresses the solid-state equilibria in the
Tl–Nd–Te system in the composition region
Tl Te−Tl Te –Nd Te –NdTe.
three ternary compounds: Tl NdTe (congruent melting
9
6
at 820 K), Tl Nd Te (solid-state decomposition at 700 K),
6
4
9
2
2
3
2
3
and Tl Nd Te (peritectic melting at 1080 K).
4
6
11
The constituent binary systems Tl–Te and Nd–Te
Tl NdTe and Tl Te were found to form a continu-
9
6
5
3
have been studied by several groups. The compounds
ous series of solid solutions with an α
δ morpho-
identified in the Tl–Te system are Tl Te, Tl Te , TlTe,
2
5
3
tropic transition near Tl Te (the α and δ-phases are,
2
and Tl Te [4–7]. The former two compounds melt con-
2
3
respectively, Tl Te- and Tl NdTe -based solid solu-
2
9
6
gruently at 698 and 723 K, respectively, and the latter
tions) [12].
two melt peritectically at 573 and 511 K, respectively.
The existence of Tl Te was called into question in a
2
number of reports [8, 9]. A recent structural study by
EXPERIMENTAL
Cerny et al. [10] has, however, confirmed the existence
In our preparations, we used extrapure elements:
TV-3 tellurium, TL-000 thallium, and Nm-0 neody-
mium. Binary compounds and alloys were synthesized
by reacting appropriate elemental mixtures at 750–800
of Tl Te, which has been shown to crystallize in mono-
2
clinic symmetry with lattice parameters a = 15.662 Å,
b = 8.987 Å, and c = 31.196 Å (Z = 44, sp. gr. C /c).
2
Tl Te has a tetragonal structure with lattice param- (thallium tellurides) or 1350–1400 K (neodymium tel-
5
3
–2
eters a = 8.929 Å and c = 12.620 Å (Z = 4, sp. gr. lurides) in silica tubes pumped down to ~10 Pa, fol-
I4/mcm) [8, 11]. TlTe and Tl Te crystallize in tetrago- lowed by furnace-cooling. Using differential thermal
2
3
analysis (DTA) results for unhomogenized cast sam-
ples, we selected heat-treatment temperatures at which
the alloys were equilibrated for 500–1000 h. Next the
alloys were characterized by DTA and x-ray diffraction
nal and monoclinic structures, respectively [4, 5].
The tellurides identified in the Nd–Te system [1, 5]
are NdTe, Nd Te , Nd Te , Nd Te , NdTe , Nd Te , and
3
4
2
3
4
7
2
2
5
NdTe . NdTe melts congruently at 2300 K; the other
3
(XRD).
neodymium tellurides form peritectically. The first
DTA was performed at temperatures from 300 to
1
1400 K. We failed to construct the í–ı–Û phase dia-
gram or any of its T–x sections because, in this temper-
ature range, complete melting was only achieved for
Presented in part at the XII Conference High-Purity Substances
and Materials: Preparation, Analysis, and Application, Nizhni
Novgorod, Russia, May 28--31, 2007.)
9
35

9
36
IMAMALIEVA et al.
6
0
VI
VII
Nd Te
3
Tl Te
2
2
3
Nd Te
3
4
V
TlTe
NdTe
IV
5
0
III
δ + III
δ + TlTe
4
3
0
I
Tl Te
5
α + δ
δ
δ
II
Tl Te
2
3
0
α
Fig. 1. 500-K section of the Tl Te–Tl í –Nd Te –NdTe phase diagram: (I) Tl NdTe , (II) Tl NdTe , (III) Tl Nd Te , (IV)
2
2
3
2
3
9
6
4
3
6
4
9
Tl Nd Te , (V) Tl NdTe , (VI) TlNdTe , (VII) TlNd Te .
4
6
11
2
3
3
3
6
alloys containing at least 70 mol % thallium tellurides. per unit cell. The basic structural component of Tl Te
5
3
At the same time, using joint analysis of DTA and pow- is a tellurium octahedron with a thallium atom, Tl(2), in
der XRD data for more than 30 equilibrated samples in its center (Fig. 2). The octahedra share corners to form
the composition region Tl Te–Tl í –Nd Te –NdTe, a Tl Te , or (TlTe ) , framework. The other 16 thallium
2
2
3
2
3
4
12
3 4
we identified at least seven new thallium neodymium atoms per unit cell, Tl(1), reside between the octahedra
tellurides: Tl NdTe , Tl NdTe , Tl Nd Te , Tl Nd Te , and link them along the ë axis, forming the Tl (TlTe )
3 4
9
6
4
3
6
4
9
4
6
11
16
Tl NdTe , TlNdTe , and TlNd Te .
unit cell.
2
3
3
3
6
3+
B substitution for half of the Tl(2) atoms, located
in the center position of the octahedra, produces
RESULTS AND DISCUSSION
Figure 1 shows the 500-K section of the Tl Te–
+
3+
2+
2
Tl BTe , or Tl [(Tl B )Te ] , compounds, and B
0.5 0.5
9 6 16 3 4
Tl í –Nd Te –NdTe phase diagram, which clearly
2
3
2
3
substitution for all of the Tl(2) atoms produces Tl BTe ,
4 3
illustrates the subsolidus phase relations in the system
under consideration.
or Tl (BTe ) compounds.
16
3 4
It is well known that Nd can be present in com-
The powder XRD patterns of Tl NdTe and
9
6
3+
2+
pounds in two oxidation states: Nd and Nd . The radii
Tl NdTe were found to be very similar to that of
4
3
of these ions are 0.995 and 1.111 Å, respectively, which
Tl í . Using the Powder-2 program, they were
5
3
3+
+
is close to the radii of Tl (1.050Å) and Tl (1.360Å)
17]. Therefore, all of the Tl(2) atoms in the structure of
Tl Te can, in principle, be replaced with Nd atoms. The
indexed in the Tl í structure (Tables 1, 2) with the
5
3
[
following tetragonal cell parameters:
5
3
a = 8.855 Å, c = 13.010 Å (Z = 4) in Tl NdTe ;
9
6
electroneutrality of the system is then maintained
a = 8.858 Å, c = 12.998 Å (Z = 4) in Tl NdTe .
4
3
through changes in the oxidation states of the Tl and Nd
ions. It seems likely that, upon TlTe3
Tl Nd Te
0.5 0.5 3
The results for thoroughly homogenized Tl Te–
2
substitutions, the intermediate compositions Tl1 – xNdx
Tl í – Tl NdTe alloys indicate that almost all of the
5
3
4
3
(
0 < x < 0.5) are typical substitutional solid solutions, in
alloys in this composition region consist of
which some of the Tl ions in the center position of the
Tl í -structured solid solutions (δ-phase) [13] (Fig. 1).
5
3
3+
octahedra are replaced by Nd . The composition
Tl Nd Te can be considered a distinct compound
There are two types of cation-substituted ternary
IV
0.5
0.5
3
structural analogs of the compound Tl Te : Tl B Te
5
3
4
3
because the Tl and Nd occupy specific positions ion the
V
(
B = Sn, Pb) [14] and Tl B Te (B = Sb, Bi) [15, 16].
9 6
center of the octahedra and are in particular oxidation
Both types of the ternary analogs of Tl Te were identi-
+
3+
5
3
states: Tl and Nd . Upon further substitution,
Tl Nd Te NdTe , all of the intermediate states
fied for the first time in the Tl–Nd–Te system: Tl NdTe
9
6
0
.5
0.5
3
3
and Tl NdTe . The existence of these compounds and
4
3
are also substitutional solid solutions, with a gradual
variation in the oxidation state of Nd from 3+ to 2+. The
boundary composition Tl (NdTe ) corresponds to a
the broad range of δ solid solutions can be interpreted
in terms of the structural features of Tl Te .
5
3
16
3 4
Tl Te crystallizes in tetragonal symmetry and distinct compound because all of the tellurium octahe-
5
3
2+
forms its own structure type [4] with four formula units dra are occupied by Nd ions.
INORGANIC MATERIALS Vol. 44 No. 9 2008

NEW THALLIUM NEODYMIUM TELLURIDES
Table 1. Indexing scheme for the XRD pattern of Tl NdTe6
937
9
2
θ, deg
d, Å
Qobs
Qcalc
∆Q
I, %
h k l
1
1
1
2
2
3
3
3
3
3
3
4
4
4
4
4
5
5
6
6
6
4.200
9.660
9.700
3.500
8.500
0.540
1.700
4.170
4.880
7.200
9.960
1.600
2.240
3.300
5.800
7.200
2.060
4.200
0.140
1.620
3.500
6.2321
4.5119
4.5029
3.7826
3.1294
2.9248
2.8204
2.6219
2.5702
2.4150
2.2544
2.1692
2.1378
2.0879
1.9796
1.9241
1.7553
1.6909
1.5373
1.5039
1.4639
257.47
491.23
255.07
491.39
2.41
–0.16
1.80
10
10
10
8
1 1 0
1 1 2
1 1 2
2 1 1
2 2 0
2 1 3
2 2 2
2 0 4
3 1 2
3 2 1
2 2 4
0 0 6
3 2 3
3 3 0
4 2 0
4 1 3
4 3 1
4 2 4
5 2 3
3 3 6
4 3 5
493.19
491.39
698.91
696.75
2.16
1021.12
1168.98
1257.13
1454.68
1513.79
1714.61
1967.61
2125.21
2188.09
2293.93
2551.79
2701.13
3245.62
3497.55
4231.39
4421.42
4666.35
1020.26
1169.39
1256.59
1455.42
1511.65
1717.01
1965.56
2126.90
2189.66
2295.60
2550.66
2699.79
3247.41
3495.95
4230.19
4422.50
4665.34
0.86
13
100
50
9
–0.41
0.54
–0.74
2.14
8
–2.40
2.05
8
8
–1.70
–1.56
–1.66
1.13
13
12
9
9
1.34
13
10
8
–1.79
1.60
1.20
17
10
5
–1.08
1.00
Table 2. Indexing scheme for the XRD pattern of Tl NdTe3
4
2
θ, deg
d, Å
Qobs
Qcalc
∆Q
I, %
h k l
1
1
1
1
2
2
2
2
3
3
3
4
4
5
5
5
6
6
6
2.140
4.100
9.660
9.700
3.460
7.440
8.460
8.500
0.540
1.700
4.840
2.280
7.200
4.180
4.200
8.700
0.100
0.360
4.340
7.2846
6.2761
4.5119
4.5029
3.7890
3.2478
3.1337
3.1294
2.9248
2.8204
2.5730
2.1359
1.9241
1.6915
1.6909
1.5716
1.5383
1.5323
1.4468
188.45
253.88
186.64
254.89
1.81
–1.02
–0.43
1.54
10
5
1 0 1
1 1 0
1 1 2
1 1 2
2 1 1
0 0 4
2 2 0
2 2 0
2 1 3
2 2 2
3 1 2
3 2 3
4 1 3
4 2 4
4 2 4
3 0 7
5 2 3
5 1 4
6 1 1
491.23
491.65
5
493.19
491.65
5
696.55
696.42
0.12
7
948.03
947.04
0.99
5
1018.32
1021.12
1168.98
1257.13
1510.50
2191.99
2701.13
3495.07
3497.55
4048.70
4225.89
4259.05
4777.31
1019.57
1019.57
1169.94
1256.33
1511.23
2189.52
2699.30
3495.97
3495.97
4047.32
4228.66
4260.65
4774.72
–1.25
1.55
10
12
100
79
7
–0.96
0.79
–0.73
2.47
11
12
10
6
1.82
–0.90
1.58
1.38
5
–2.78
–1.60
2.59
14
10
3
INORGANIC MATERIALS Vol. 44 No. 9 2008

9
38
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2
3
4
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. Diagrammy sostoyaniya dvoinykh metallicheskikh
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. Asadov, M.M., Babanly, M.B., and Kuliev, A.A., Phase
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1
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4
3
Allg. Chem., 1993, vol. 619, pp. 1155–1160.
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9
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INORGANIC MATERIALS Vol. 44 No. 9 2008