Thermal behaviour of titanium seleno-telluride Ti5Ch8 in Ar and N2 atmospheres

Article abstract of DOI:10.1016/j.tca.2004.10.004

A series of TiSe_1.60-xTe_x, where 0 ≤ x ≤ 1.60, was prepared. The samples were characterized by powder X-ray diffraction and thermogravimetric analysis in Ar and N₂ atmospheres. The XRD measurements showed that the titanium seleno-tellurides form a continuous solid solution of a hexagonal structure. Thermal behaviour of the compounds was studied by thermogravimetric analysis for samples x = 0, 0.16, 0.80, 1.44, 1.60 in the temperature interval from 25 to 1273 K. After heating in Ar, no mass loss was observed in selenide (x = 0). In contrast, for telluride (x = 1.60) mass loss took place due to a formation of ditelluride. For seleno-tellurides in nitrogen atmosphere both disproportion reaction and a reaction of ditelluride with N₂ occurred simultaneously.

Full text of DOI:10.1016/j.tca.2004.10.004

Thermochimica Acta 428 (2005) 91–94
Thermal behaviour of titanium seleno-telluride
Ti5Ch8 in Ar and N2 atmospheres
O.Yu. Pankratovaa,∗, J.S. Stepanovaa, R.A. Zvinchuka, A.V. Suvorova,
T. Hatanpaab, V. Kozlovb, M. Leskelab
a
Department of Chemistry, St. Petersburg State University, Universitetskiy pr. 26, Petergoff, St. Petersburg 198504, Russia
Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FIN 00014, Finland
b
Received 6 August 2004; received in revised form 12 October 2004; accepted 15 October 2004
Available online 26 November 2004
Abstract
A series of TiSe1.60−xTex, where 0 ≤ x ≤ 1.60, was prepared. The samples were characterized by powder X-ray diffraction and thermo-
gravimetric analysis in Ar and N2 atmospheres. The XRD measurements showed that the titanium seleno-tellurides form a continuous solid
solution of a hexagonal structure. Thermal behaviour of the compounds was studied by thermogravimetric analysis for samples x = 0, 0.16,
0.80, 1.44, 1.60 in the temperature interval from 25 to 1273 K. After heating in Ar, no mass loss was observed in selenide (x = 0). In contrast,
for telluride (x = 1.60) mass loss took place due to a formation of ditelluride. For seleno-tellurides in nitrogen atmosphere both disproportion
reaction and a reaction of ditelluride with N2 occurred simultaneously.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Titanium selenide; Titanium telluride; Solid solution; Synthesis; XRD; TGA; Titanium tellurides decomposition
Series of nonstoichiometric compounds Ti2Ch3, Ti5Ch8
and TiCh2 with a hexagonal structure of the CdI2-type have
been observed in binary systems of titanium–selenium and
titanium–tellurium from sesqui- to dichalcogenide. Broad-
ness of homogeneity ranges in solid solutions, discreteness of
different phases, and structures of the compounds determined
by powder X-ray diffraction vary in different chalcogenides.
The literature on these topics is contradictory. Therefore it
is expedient to obtain single crystal of the phases in order to
establish reliably their symmetries and structures. Chemical
vapor transport (CVT) is a unique technique for preparing
single crystal chalcogenides of specific stoichiometry. Rim-
mington and Balchin [1] were the first to succeed in growing
single crystal series of composition Ti(Se,Te)2 by the iodine
vapor transport technique.
telluride as an individual compound but we found [3] that
it was the last member of hexagonal structures in series
Ti2Te3–Ti5Te8. In the titanium–selenium system, Gronvold
and Langmyhr [4] and Bernusset [5] observed formation
of the monoclinic Ti5Se8 (TiSe1.60). In our previous study
[6], we revealed a continuous solid solution of selenides
Ti2Se3–Ti5Se8 of a hexagonal structure. Among the hexago-
nal selenides, Ti5Se8 has the shortest lattice parameters. We
synthesized TiSe1.60 single crystal by two variants of CVT
that differ by the choice of selenium source, namely Se or
TiSe2. Unlike other selenides studied, TiSe1.60 has the Laue
¯
group 3m with a superstructure 2a, 2c [7].
The goal of this study was to determine the homogeneity
range of the solid solution Ti5Ch8 (TiSe1.60−xTex) and exam-
ine the thermal behaviour of the titanium seleno-tellurides in
Ar and N2 atmospheres. These mixed chalcogenides have
not been investigated earlier. In addition, thermogravimetric
analysis (TGA) is very important for choosing optimal con-
ditions for CVT to obtain single crystal of nonstoichiometric
chalcogenides [8].
In the titanium–tellurium system, the Ti5Te8 (TiTe1.60
)
phase is formed. Brunie and Chevreton [2] considered this
∗
Corresponding author. Tel.: +7 812 4284071; fax: +7 812 4286939.
E-mail address: mtoop@mail.ru (O.Yu. Pankratova).
0040-6031/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2004.10.004

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O.Yu. Pankratova et al. / Thermochimica Acta 428 (2005) 91–94
1. Experimental
In this work, a continuous series of TiSe1.60−xTex, where
0 ≤ x ≤ 1.60 with ꢀx = 0.16 was prepared via the direct syn-
thesis in an ampoule. The starting materials were selenium
(99.9%), tellurium (99.99%) and metallic titanium obtained
from the titanium hydride TiH2 (with a purity of more than
99%) by dehydrogenation in vacuum at 1153 K. The stoi-
chiometric amounts of the initial elements were put in quartz
ampoules, which were evacuated to a residual pressure of
0.13 Pa and sealed. To obtain homogenous chalcogenides [6],
the synthesis was carried out in a furnace using two heating
steps: 170 h at 823 K for primary reaction and 720 h at 1123 K
in the final step. After that, the ampoules were quenched in
cold water at 273 K. The fast cooling was performed in order
to retain the structure adopted at a given annealing tempera-
ture.
Fig. 1. Powder XRD patterns for TiSe1.60−xTex (x = 0.16, 0.80, 1.44). As-
terisks denote a reflection of the superstructure 2a, 2c (according to single
crystal data [7]).
The phase purity and composition, homogeneity and lat-
tice parameters were determined by powder X-ray diffrac-
tion at room temperature using DRON-3 diffractometer (Cu
K␣, external standard ␣-Al2O3). Lattice parameters were ob-
tained by least squares calculation for an interval of reflection
angles, 40◦ < 2θ < 104◦. Theuncertainityinlatticeparameters
Table 1
Powder XRD pattern of TiSe1.60
N
I/I0 (%)
Θ (◦)
7.45
h k l
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
57
2
0 0 1
1 0 0
0 0 2
1 0 1
1 0 2
0 0 3
1 1 0
1 0 3
1 1 2
2 0 1
0 0 4
2 0 2
1 0 4
1 1 3
2 0 3
2 1 1
1 1 4
1 0 5
2 1 2
3 0 0
2 1 3
0 0 6
3 0 2
14.42
14.95
16.25
21.00
22.72
25.58
27.35
30.15
31.00
31.00
34.10
34.90
35.40
39.05
42.12
42.25
43.60
44.95
48.30
49.70
50.55
52.15
86
77
66
27
17
33
6
100
100
5
<1
<1
5
19
<1
13
4
˚
value was 0.005 A.
Thermal behaviour of titanium seleno-telluride Ti5Ch8
was studied by thermogravimetric analysis. A Mettler Toledo
TA8000 system equipped with TGA850 thermobalance was
used. Sample amounts were about 10 mg. Dynamic exper-
iments (5 K/min) from 298 to 1273 K and back to 373 K
were performed under flowing argon (99.99%) and nitrogen
(99.999%) atmospheres. The phases of samples decomposed
after TGA measurements were determined using Philips An-
alytical PW1710/00 (Cu K␣, 1.5◦ < 2θ < 154◦, internal stan-
dardCaF2)andDRON-3diffractometers. Allreflectionswere
indexed using Param and TREOR softwares.
2
6
5
1
2. Results and discussion
2.1. Solid solution TiSe1.60−xTex
X-ray diffraction patterns showed that all samples are sin-
gle phases. The titanium seleno-tellurides adopt the struc-
ture of CdI2-type. TiSe1.60−xTex forms a continuous series of
hexagonal solid solutions. Fig. 1 shows typical XRD patterns
of the samples x = 0.16, 0.80, 1.44.
˚
˚
Lattice parameters a = 3.845 A, c = 6.388 A for TiTe1.60
agree with the data of the previous work [2]. Table 1 shows
powder XRD data for hexagonal TiSe1.60 with the lattice pa-
˚
˚
rameters a = 3.574 A and c = 5.984 A. It is worth noticing that
previously [4,5] only monoclinic structure was determined
for the selenide.
The hexagonal lattice parameters and the unit cell volume
are shown as function of composition in Fig. 2. There is only
a small negative and positive deviation from the Vegard’s law
for the a- and c-axis, respectively.
Fig. 2. Latticeparametersandunitcellvolumeofhexagonaltitaniumchalco-
genides TiSe1.60−xTex.

O.Yu. Pankratova et al. / Thermochimica Acta 428 (2005) 91–94
93
Fig. 4. Powder XRD patterns for TiSe0.16Te1.44. Dashed line: sample before
heating, solid line: sample after heating in TGA to 1273 K under Ar; dotted
line: sample after heating in TGA to 1273 K under N2. The bars mark: CaF2
internal standard.
pure gaseous tellurium and other tellurides, which are richer
in metal than the initial compound:
TiTe1.60(s) → TiTe1.60−z(s) + z/2 Te2(g)
(1)
According to the mass loss in TGA, the composition of
the telluride after heating should be about TiTe1.14. However,
Fig. 3. TG curves of TiSe1.60−xTex in Ar and N2 atmospheres:
(1) TiSe1.44Te0.16/Ar; (2) TiSe1.6/N2; (3) TiSe1.44Te0.16/N2; (4)
TiSe1.6/Ar; (5) TiSe0.80Te0.80/Ar; (6) TiSe0.16Te1.44/Ar; (7) TiTe1.6/Ar; (8)
TiSe0.80Te0.80/N2; (9) TiSe0.16Te1.44/N2; (10) TiTe1.60/N2.
this composition disagrees with XRD data that give TiTe1.5
.
Second possibility for the thermal dissociation is that, dis-
proportion reaction takes place and two tellurides are formed:
one is richer in chalcogen and the other is richer in metal than
the initial telluride. We propose that in argon atmosphere at
temperature about 1173–1273 K, the following reaction may
take place:
2.2. TGA measurements
Fig. 3 shows the TG curves for 10 experiments.
Earlier data about thermal decomposition of the titanium
selenides and tellurides have only been reported for stoichio-
metric compounds TiChy, where y ≥ 2. Suzuki and Wahlbeck
[9] investigated Ti–Te system by evaporation in the temper-
ature range from 573 to 1523 K and determined that evap-
oration of tellurides is incongruent and elemental tellurium
is released. For the Ti–Se system, the same investigation has
not been carried out. Only a few authors [10] used thermal
dissociation of sesqui- and diselenide to prepare selenides
with lower Se content.
TiTe1.60(s) → 0.8 TiTe1.50(s) + 0.2 TiTe2(g)
(2)
This hypothesis is in good accordance with our quantita-
tive data. According to TGA (Fig. 3), the mass loss in the ex-
periment was 23.14%; our calculation with reaction (2) gives
about 24%. The literature data also supports this proposal
because TiTe2 is evaporating easier than the other tellurides
[9].
2.4. Nitrogen atmosphere
2.3. Argon atmosphere
Nitrogen atmosphere is not inert for the TiSe1.60−xTex
compounds, the pure selenide (x = 0) being an exception.
TGA experiment showed that the composition of TiSe1.60
did not change and the results were similar in both nitrogen
and argon atmospheres.
For seleno-tellurides in N2 atmosphere, the disproportion
reaction (2) and a reaction of ditelluride with nitrogen (3)
take place simultaneously:
In argon atmosphere only thermal dissociation took place.
TGA of TiSe1.60 (x = 0) are performed in Ar revealed that
the sample is the most stable one in the studied series of
chalcogenides. It practically does not show any mass loss.
This observation is in good agreement with XRD patterns that
are very similar for TiSe1.60 before and after heating in Ar. In
contrast, themasslossofTiTe1.60 (x = 1)afterheatinginargon
atmosphere is the biggest among the five chalcogenides, and
the telluride compositions before and after heating differ.
The thermal dissociation of the telluride could be de-
scribed in two different ways. First, the telluride dissociates to
TiTe2(g) + (1/2)N2(g) → TiN(s) + Te2(g)
(3)
Formation of titanium nitride is observed by XRD measure-
ments for seleno-tellurides where x ≥ 0.8 (Fig. 4).

94
O.Yu. Pankratova et al. / Thermochimica Acta 428 (2005) 91–94
The TiTe1.60 shows a mass loss about 55% after TGA
mal dissociation described by Suzuki and Wahlbeck [9]. In
Ar atmosphere only the dissociation of chalcogenides with
an evaporation of ditelluride took place. In N2 atmosphere,
the dissociation of titanium tellurides and reaction with nitro-
gen took place simultaneously. The reaction of tellurides with
residue of nitrogen has been observed [11] in preparation of
titanium ditelluride single crystals by chemical vapor trans-
port method. All the intermediate titanium seleno-tellurides
showed the same tendency in both atmospheres: selenide is
more stable; telluride dissociates and reacts with nitrogen.
heating in nitrogen atmosphere but both XRD patterns before
and after TGA measurement are very similar. The result again
supports that reaction (2) is taking place instead of reaction
(1). The evaluation of TG curve shows that the mass ratio
TiN:TiTe1.50 in the final mixture is about 2:3. In other words,
3/4 of TiTe1.60 mass had time to decay and the TiTe2 reacted
during the TGA experiment. The diffuse reflections at small
angle in XRD patterns (Fig. 4) show that the sample contains
amorphous material. It is partly because the reaction is not
complete after TG measurement. It may well be, that part
of TiN is amorphous. The incompleteness of the reaction is
additionally supported by supplementary TGA experiments
made in nitrogen atmosphere with low heating rate or interval
annealing at 1223 K, the mass loss increased.
For intermediate titanium seleno-tellurides for x = 0.16,
0.80, 1.44, TGAdatashowsthesamegeneraltendencyinboth
atmospheres. When the tellurium content increases, the mass
loss at 1023–1273 K becomes more significant. The bigger
is the tellurium content the larger is the difference between
the mass loss after heating in N2 and Ar. It is directly related
to processes (2) and (3). For x = 0.16 only reaction (2) takes
place in N2 atmosphere. For x = 1.44, reaction (3) is probably
dominant one and nitrogen substitutes tellurium. This is sup-
ported by XRD patterns shown in Fig. 4 for TiSe0.16Te1.44
(x = 1.44) before and after heating in Ar and N2.
Acknowledgment
This work is supported by the grant RF Ministry of Edu-
cation No. E02-5.0-226.
References
[1] H.P.B. Rimmington, A.A. Balchin, J. Cryst. Growth 21 (1974)
171–181.
[2] S. Brunie, M. Chevreton, Mater. Res. Bull. 3 (1968) 309–311.
[3] O.Yu. Pankratova, L.I. Grigorieva, R.A. Zvinchuk, A.V. Suvorov,
Rus. J. Inorg. Chem. 38 (1993) 383–386.
[4] F. Gronvold, F.J. Langmyhr, Acta Chem. Scand. 15 (1961)
1949–1962.
[5] P. Bernusset, C. R. Acad. Sci. 257 (1963) 2840–2842.
[6] O.Yu. Pankratova, V.A. Vladimirova, R.A. Zvinchuk, Rus. J. Inorg.
Chem. 36 (1991) 597–600.
[7] G.V. Pavlova, O.Yu. Pankratova, R.A. Zvinchuk, A.V. Suvorov, Vest-
nik SPbGU, Seriya 4: Fizika, Khimiya (2000) 117–119 (in Russian).
[8] O.Yu. Pankratova, L.I. Grigor’eva, R.A. Zvinchuk, A.V. Suvorov,
Vestnik SPbGU, Seriya 4: Fizika, Khimiya (2000) 119–121 (in Rus-
sian).
[9] A. Suzuki, P.G. Wahlbeck, J. Phys. Chem. 70 (1966) 1914–1923.
[10] F.K. McTaggart, A.D. Wadsley, Aust. J. Chem. 11 (1958) 445–457.
[11] O.Yu. Pankratova, L.I. Grigor’eva, R.A. Zvinchuk, A.V. Suvorov,
Rus. J. Gen. Chem. 68 (1998) 1527–1532.
3. Conclusions
The TiSe1.60−xTex compounds studied formed a contin-
uous solid solution of CdI2-type structure. However, the se-
lenide TiSe1.60 and the telluride TiTe1.60 behaved very differ-
ently under heating in argon, and nitrogen atmospheres. On
the whole, the selenide is much more stable than the telluride.
Thermalbehaviouroftitaniumtelluridesdiffersfromthether-