Thermodynamic properties and homogeneity regions of Tl6SCl 4 and Tl5Se2Cl

Article abstract of DOI:10.1134/S0020168509020046

In this activity system Tl-Tl₂X-X (X = S, Se)are studied using emf measurements of concentration chains relative thallic electrode. The solid phase diagrams of these systems are clarified, homogeneity areas of the compounds Tl₆SCl

Full text of DOI:10.1134/S0020168509020046

ISSN 0020-1685, Inorganic Materials, 2009, Vol. 45, No. 2, pp. 129–134. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © G.M. Guseinov, D.M. Babanly, F.M. Sadygov, M.B. Babanly, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45, No. 2, pp. 165–170.
Thermodynamic Properties and Homogeneity Regions
of Tl SCl and Tl Se Cl
6
4
5
2
G. M. Guseinov, D. M. Babanly, F. M. Sadygov, and M. B. Babanly
Baku State University, Baku, Azerbaijan
e-mail: Babanly_mb@rambler.ru
Received September 14, 2007
Abstract—In this activity system Tl–Tl X–X (X = S, Se)are studied using emf measurements of concentration
2
chains relative thallic electrode. The solid phase diagrams of these systems are clarified, homogeneity areas of
the compounds Tl SCl and Tl Se Cl are determined. On the basis of emf measurement results, relative partial
6
4
5
2
0
0
molar functions of thallium in alloys and standard integral thermodynamic functions (∆G (298 K), ∆H (298 K),
S (298 K)) of the ternary compounds Tl SCl and Tl Se Cl and phases of variable composition based on the
0
∆
6
4
5
2
latter are calculated.
DOI: 10.1134/S0020168509020046
INTRODUCTION
According to [5], the quasibinary system TlCl–Tl Se is
2
characterized by formation of one ternary compound,
Development and optimization of the processes for
obtaining and purifying multicomponent inorganic
materials is based on data on the phase equilibria and
thermodynamic properties of the corresponding sys-
tems.
Chalcogenides of metals, in particular, thallium, and
phases on their basis are of interest for obtaining new
semiconducting and thermoelectric materials.
Tl Se Cl, incongruently melting by a syntectic reaction at
5
2
7
25 K. At the syntectic temperature, the immiscibility
region ranges from 15 to 80 mol % Tl Se. The compound
2
Tl Se Cl forms eutectic point with TlCl (5 mol % Tl Se,
5
2
2
6
89 K) and a peritectic point with Tl Se (~93 mol %
2
2
Tl Se, 684 K). According to the peritectic reaction L +
Tl Se Cl
α, solid solutions based on Tl Se with a
5
2
2
concentration of TlCl up to 18 mol % are formed [5].
The ternary system Tl–S–Cl is studied in [1–4].
According to the data of [1], the quasibinary section
The compound Tl Se Cl crystallizes into a tetrago-
5
2
nal structure of the Cr B type with the following lattice
TlCl–Tl S is characterized by formation of the ternary
5 3
2
parameters: ‡ = 8.565, Ò = 12.741 Å, Z = 4, space group
compound Tl SCl incongruently melting by a peritec-
6
4
P4/nnc [6]. It is interesting that the lower selenide of thal-
tic reaction at 683 K. The compound Tl SCl crystal-
6
4
lium Tl Se (0.06 < ı < 0.5) has the same structure [9].
lizes in a tetragonal lattice of the type Tl HgBr (space
5
3 – x
6
6
group P4/mnc). In [2–4], the system Tl–S–Cl is studied
It is shown in [7] that the system TlCl–TlSe forms a
in the area of compositions Tl–TlCl–S. An elaborated phase diagram of the simple eutectic type with degen-
version of the í–ı–diagram of TlCl–Tl S system is pre- erate eutectics near TlSe.
2
sented; according to it, the coordinates of the peritectic and
It is found by a repeated investigations [8] that this
eutectic points are 685 K, 55 mol % and 680 K, 60 mol % section is characterized by eutectic (85 mol % TlSe,
Tl S [4]. These compositions differ significantly from 593 K) and monotectic (685 K) equilibrium. At the
2
the data of [1] (33 and 35 mol % Tl S). In addition, monotectic temperature, the immiscibility region
2
ranges from 15 to 70 mol % TlSe.
according to [4], in the system TlCl–Tl S, there is a
2
wide region of immiscibility (13–50 mol % Tl S) with
In [8], a complete í–ı–Û diagram of the system
2
a temperature of monotectic equilibrium of 690 K. Tl−TlCl–Se, a number of its polythermal sections, and
Polythermal sections of TlCl–TlS and TlCl–Tl S are an isothermal section at 400 K are constructed; fields of
4
3
primary crystallization of phases and types and coordi-
nates of nonvariant equilibria are determined. Accord-
nonquasibinary, but they are stable below the solidus;
that is, they consist of two-phase mixtures of the initial
compounds. In [4], a complete picture of phase equilib-
ing to [8], the compound Tl Se Cl, in contrast to the
5
2
ria in the system Tl–TlCl–S is obtained; various poly- data of [5], is a nonstoichiometric phase and has a wide
thermal sections, an isothermal section at 300 K, and a region of homogeneity, which almost completely cov-
projection of the liquidus surface are constructed.
The ternary system Tl–Se–Cl is also studied in details
ers the elementary triangle Tl
The aim of the present paper is to study the solid phase
X–X (X = S, Se) and
2 5 3 5 2
Se–Tl Se –Tl Se Cl.
in the region of the compositions Tl–TlCl–Se [5−8]. equilibrium in the systems TlCl–Tl
2
1
29

1
30
GUSEINOV et al.
the thermodynamic properties of thio- and selenochlo-
rides of thallium by the emf method.
The electrolyte was a glycerin solution of KCl with
TlCl added. Taking into account that moisture and oxy-
gen must not be present in the electrolyte, the glycerin
(
analytically pure) was thoroughly dehydrated and
EXPERIMENTAL AND RESULTS
evacuated at a temperature of ~450 K; anhydrous,
For planning of the experiments, we have used the chemically pure KCl and TlCl were used.
results of [4, 8] that allow us to determine efficient com-
positions of samples for studies by the emf method and
to select conditions for their synthesis and thermal
treatment.
Techniques for assembling the electrochemical cell
and for measuring the emf are described in detail in [8,
1
2]. The emf was measured by the compensation method
usingV-7-27 digital voltmeter in the range 300–380 K (the
We have composed concentration chains of the type system TlCl–Tl S–S) or 300–430 K (the system TlCl–
2
Tl Se–Se). Taking into account the recommendations
+
2
(
–)Tl(sol.) liquid electrolyte, Tl
TlCl–Tl X–X)(sol.)(+),
of [12], the first equilibrium values of the emf were
measured 60–80 h after the cell holding at 380–400 K;
subsequent values were measured every 3−4 h after
achievement of the required temperature. The emf val-
ues were considered equilibrium if upon repeated mea-
surements at the given temperature they did not differ
(
1)
(
2
in which the left electrode is pure metallic thallium and
the right electrodes are equilibrium alloys from differ-
ent phase regions of the systems TlCl–Tl X–X.
2
In order to prepare the right electrodes of the chains from each other by more than 0.5 mV, irrespective of
of type (1), initial compounds TlCl, Tl Se, and Tl S the temperature variation direction.
2
2
were synthesized at first. The synthesis of Tl Se and
The measurements have shown that the emf values
2
Tl S was carried out by the ampule method from high- for the chains of type (1) obtained by heating and cool-
2
purity primitive components (impurity content of at ing coincide with an accuracy of ±3 mV; their varia-
–
3
–2
tions with temperature are nearly linear. Reversibility
of the chains of type (1), along with reproducibility of
emf values, is also confirmed by an almost complete
absence of loss in the electrode weight after the exper-
iments.
The results of the emf measurements, processed in
approximation of their linear temperature dependence
by the least squares method [13], are presented in the
form of equations of the type [14] (Table 1)
most 10 wt %) in vacuum conditions of ~10 Pa. The
alloying temperature was 750 K.
Thallium monochloride was obtained by the tech-
nique described in [10]: first, by dissolution of metallic
thallium at ~350 K in diluted (~7–10 mol %) sulfuric
acid, a solution of Tl SO was obtained. Diluted HCl
2
4
was added to a boiling 2% solution of Tl SO until com-
2
4
plete sedimentation. After cooling of the mother solu-
tion, TlCl was separated, washed with icy distilled
water, and dried in a drying oven at 390–400 K for a
long period of time.
The synthesized compounds were identified by the
methods of differential thermal and x-ray phase anal-
ysis.
2
E
1/2
S
2
2
b
E = a + bT + 2 ---- + S (T – T)
,
n
2
2
where S and S are the dispersions of the constant b
b
E
and separate measurements of emf, n is the number of
Via the alloying of initial compounds and elementar
sulfur (selenium) in various ratios in evacuated quartz pairs of Ö and í values, and T is the mean value of tem-
vessels, alloys of the systems TlCl–Tl X–X were pre- perature.
2
pared; they were subjected to a long-term stepped
homogenizing annealing under the conditions described
in [4, 8]; then, they were additionally held at ~380 K for
00 h.
In order to measure the emf of the chains of type (1),
electrodes, electrolyte, and electrochemical cells were
prepared. The left electrode was made by attaching
metallic thallium (purity of at least 99.99%) to a molyb-
denum current collector.
On the basis of the data of Table 1, according to
known thermodynamic relations, relative partial molar
thermodynamic functions of thallium in the alloys at
3
2
98 K were calculated (Table 2).
Figure 1 depicts the solid-phase equilibrium dia-
grams for the systems TlCl–Tl S–S and TlCl–Tl Se–Se
2
2
constructed in [4–8], with indicated values of emf (mV)
in different phase regions at 300 K. The measurement
results have shown that, in the system TlCl–Tl S–S, the
2
Taking into account oxidization of thallium even at
room temperature, before assembling electrochemical
cell, the left electrodes were kept in glycerin, since
metallic thallium does not directly interact with it [11].
emf values in each three-phase region at the given tem-
perature are constant irrespective of alloy composition
and that they vary in steps at the transition from one
three-phase region into another (Fig. 1a, Table 1). The
The right electrodes were prepared by pressing of stepwise variation in emf from 410 to 426 mV at the
powdered equilibrium alloys of the systems under stoichiometric composition of the Tl SCl compound
6
4
study into current collectors in the form of cylindrical (Fig. 1a) confirms the data of [1–4] on the absence of an
tablets with a weight of ~0.5 g.
appreciable region of homogeneity of this compound.
INORGANIC MATERIALS Vol. 45 No. 2 2009

THERMODYNAMIC PROPERTIES AND HOMOGENEITY REGIONS
131
Table 1. Temperature dependences of emf of the chains of type (1) in some phase regions of the systems TlCl–Tl X–X
2
No.
Phase region in Fig. 1
TlCl–TlS–Tl2S3
TlCl–Tl S –TlS
E, mV = a + bT ± 2S (T)
E
550.7 – 0.117T ± 2[(2.3/25) + 6.2 × 10–5 (T – 345.8) ]
2 1/2
1
2
3
4
5
6
7
8
9
467.4 – 0.014T ± 2[(2.2/25) + 6 × 10–5 (T – 345.8) ]
2 1/2
4
3
452.1 – 0.089T ± 2[(1.7/25) + 4.6 × 10–5 (T – 345.8) ]
2 1/2
TlCl–Tl SCl –Tl S
6
4
4
3
386.5 + 0.080T ± 2[(1.3/25) + 3.5 × 10–5 (T – 345.8) ]
2 1/2
Tl S–Tl SCl –Tl S
2
6
4
4
3
636.4 – 0.041T ± 2[(4.2/22) + 8.6 × 10–5 (T – 362.8) ]
2 1/2
TlCl–TlSe–Se
473.3 – 0.066T ± 2[(1.6/22) + 5.1 × 10–5 (T – 362.8) ]
2 1/2
TlCl–Tl Se Cl–TlSe
5
2
446.4 + 0.012T ± 2[(1.3/24) + 3.7 × 10–5 (T – 360.6) ]
2 1/2
Tl Se Cl
5
2.2 0.8
433.6 + 0.036T ± 2[(1.8/24) + 5.4 × 10–5 (T – 360.6) ]
2 1/2
Tl Se Cl
5
2.4 0.6
413.0 + 0.074T ± 2[(1.2/24) + 3.2 × 10–5 (T – 360.6) ]
2 1/2
Tl Se Cl
5
2.6 0.4
2 1/2
1
1
0
1
Tl Se Cl
394.7 + 0.091T ± 2[(1.7/24) + 4.7 × 10–5 (T – 360.6) ]
5
2.8 0.2
2 1/2
Tl Se
368.2 + 0.124T ± 2[(1.5/24) + 5 × 10–5 (T – 360.6) ]
5
3
Table 2. Partial thermodynamic functions of thallium in some phase regions of the systems TlCl–Tl X–X at 298 K
2
–
∆G(Tl)
–∆H(Tl)
∆S(Tl)
Phase region in Fig. 1
kJ/mol
J/(K mol)
TlCl–Tl SCl –Tl S
41.06 ± 0.08
44.54 ± 0.08
43.77 ± 0.10
43.42 ± 0.09
42.87 ± 0.10
41.98 ± 0.08
40.70 ± 0.10
39.09 ± 0.09
43.62 ± 0.45
45.95 ± 0.46
45.67 ± 0.55
43.07 ± 0.22
41.84 ± 0.23
39.85 ± 0.21
38.08 ± 0.23
35.53 ± 0.22
–8.59 ± 1.31
–3.67 ± 1.36
–6.37 ± 1.38
1.16 ± 1.17
3.47 ± 1.42
7.14 ± 1.09
8.78 ± 1.32
11.96 ± 1.36
6
4
4 3
TlBr–Tl SBr –TlS
6
4
TlCl–Tl Se Cl–TlSe
5
2
Tl Se Cl
5
2.2 0.8
Tl Se Cl
5
2.4 0.6
Tl Se Cl
5
2.6 0.4
Tl Se Cl
5
2.8 0.2
Tl Se
5
3
In the case of the system TlCl–Tl Se–Se, another formation of a continuous series of solid solutions
2
through this section.
picture is observed. The measurements have shown
that, through the section Tl Se Cl–Tl Se , the emf val-
ues of the chains of type (1) and the partial thermody-
5
2
5
3
It should also be noted that numerical values of emf
and equations of their temperature dependences in
namic functions of thallium are monotonic functions of phase regions 1, 2, 4, and 5 (Table 1) practically coin-
composition (Tables 1, 2; Fig. 2). This is evidence of cide with corresponding data [16, 17] for the binary
INORGANIC MATERIALS Vol. 45 No. 2 2009

1
32
GUSEINOV et al.
TlCl
(a)
4
0
Tl SCl
6
4
2
0
Tl + Tl SCl + Tl S
6
4
2
(
410) (463)
(516)
Tl
20
Tl2S 40 Tl S TlS Tl S 60
Tl S
2 5
80
S
4 3
2 3
‡
t. % S
Tl0.5Cl0.5
(b)
4
0
A – Tl Se Cl
5
2
3
0
2
0
Tl + TlSe + α
1
Ä
(454)
(624)
Tl + α
α + TlSe
1
TlCl + TlSe + Se
1
0
α
Tl
20
Tl0.67Se0.33 40
Tl0.5Se0.5 60
80
Se
‡
t. % Se
Fig. 1. Isothermal section of the phase diagram of the systems Tl–TlCl–S (a) and Tl–TlCl–Se (b) at 300 K (in brackets, the emf
values for the chains of type (1) are given).
compounds TlS, Tl S , Tl S, and TlSe, respectively. On
Tl + 0.5TlCl + TlSe = 0.5Tl Se Cl,
5 2
4
3
2
the one hand, this testifies of reversibility of the chains
of type (1); on the other hand, this indirectly points to
the absence of appreciable regions of solid solutions
based on the aforementioned compounds in the systems
respectively.
According to the equations of these reactions, we
determined ∆Z —the standard thermodynamic func-
tions of formation (∆G , ∆H ):
0
TlCl–Tl X–X.
0
0
2
According to [12], the partial thermodynamic func-
tions of thallium in solid-phase regions 3 and 6 (Table 2,
Figs. 1a, 1b) are thermodynamic functions of the fol-
lowing potential-forming reactions
0
0
∆
Z (Tl SCl ) = 2/3∆Z (Tl) + 4∆Z (TlCl) +
6
4
0
1/3∆Z (Tl S ))
(2)
4
3
and
Tl + 6TlCl + 0.5Tl S = 1.5Tl SCl
4
4
3
6
0
0
0
and
∆Z (Tl
5
S
2
C) =2∆Z (Tl) + ∆Z (TlCl) + 2∆Z (TlSe).(3)
INORGANIC MATERIALS Vol. 45 No. 2 2009

THERMODYNAMIC PROPERTIES AND HOMOGENEITY REGIONS
S (Tl), J/(mol K)
133
∆
1
0
5
0
5
–
Tl Se Cl
Tl Se
5 3
5
2
0
.2
0.4
0.6
0.8
∆
∆
G (Tl);
H (Tl),
mol % Tl Se
5
3
3
4
4
5
0
5
kJ/mol
1
2
Fig. 2. Dependences of the partial thermodynamic functions in the system Tl Se Cl–Tl Se at 298 K: (1) ∆H (Tl, (2) ∆G (Tl).
5
2
5
3
0
0
0
0
Here, ∆Z (Tl) is the respective partial molar values of S (Tl S C) = 2[(∆S(Tl) + S (Tl)] +S (TlCl) + 2S (TlSe).(5)
5
2
thallium.
In the calculations by relations (2)–(5), in addition
The standard entropies of the ternary compounds
were also calculated on the basis of the potential-form-
ing reactions by the relations
to our own data on ∆Z (Tl), we used more reliable ther-
modynamic data for TlCl [15], Tl S [16], TlSe [17],
4
3
and Tl [15].
0
0
The integral thermodynamic functions of the solid
S (Tl Cl ) = 2/3[∆S(Tl) + S (Tl)]
6
4
(
4) solutions Tl Se Cl were calculated by integrating the
5
2 – x
x
0
0
+
4S (TlCl) + 1/3∆S (Tl S )
Gibbs–Duhem equation by the technique of [12, 18].
4
3
Table 3. Standard integral thermodynamic functions of the ternary phases of the systems Tl–TlCl(Br)–S and Tl–TlCl–Se
0
0
S0(298 K)
–
∆G (298 K)
–∆H (298 K)
Phase
kJ/mol
J/(K mol)
Tl SCl
833.5 ± 3.7
392.8 ± 1.1
378.5 ± 1.2
351.4 ± 1.4
327.5 ± 1.4
293.6 ± 1.5
928.1 ± 14.0
421.6 ± 5.1
395.1 ± 5.3
367.1 ± 5.6
330.6 ± 5.6
290.7 ± 5.7
599 ± 9
434 ± 7
437 ± 8
439 ± 9
444 ± 10
448 ± 11
6
4
Tl Se Cl
5
2
Tl Se Cl
5
2.2 0.8
Tl Se Cl
5
2.4 0.6
Tl Se Cl
5
2.6 0.4
Tl Se Cl
5
2.8 0.2
INORGANIC MATERIALS Vol. 45 No. 2 2009

1
34
GUSEINOV et al.
The calculation results are presented in Table 3. The
errors were found by the error accumulation method.
Tl Se Tetragonal Phase, Kristallografiya, 1980, vol. 25,
5 3
no. 5, pp. 1070–1072.
1
1
0. Brauer, G., Rukovodstvo po neorganicheskomu sintezu
(Guide on Inorganic Synthesis), Moscow: Mir, 1985,
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INORGANIC MATERIALS Vol. 45 No. 2 2009