Solid-state equilibria and thermodynamic properties of compounds in the Bi-Te-I system

Article abstract of DOI:10.1134/S0020168508100099

The Bi-Te-I system has been studied by differential thermal analysis, x-ray diffraction, and emf measurements in the temperature range 300-400 K using concentration cells of the type (-)Bi(s) | glycerol + KI + BiI₃ | Bi-Te-I(s)(+). The results

Full text of DOI:10.1134/S0020168508100099

ISSN 0020-1685, Inorganic Materials, 2008, Vol. 44, No. 10, pp. 1076–1080. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © Z.S. Aliev, M.B. Babanly, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 10, pp. 1203–1207.
Solid-State Equilibria and Thermodynamic Properties
of Compounds in the Bi–Te–I System1
Z. S. Aliev 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 Bi–Te–I system has been studied by differential thermal analysis, x-ray diffraction, and emf
measurements in the temperature range 300–400 K using concentration cells of the type (–)Bi(s) | glycerol +
KI + BiI3 | Bi–Te–I(s)(+). The results have been used to construct the 300-K section of the Bi–Te–I phase dia-
gram. The existence of the ternary compounds BiTeI, Bi2TeI, and Bi4TeI1.25 has been confirmed, and the posi-
tion of the phase fields involving these compounds has been accurately determined. Using emf data, we have
evaluated the partial thermodynamic functions (∆G, ∆H, and ∆S ) of the bismuth in the alloys studied, the
standard thermodynamic functions of formation of the ternary compounds, and their standard entropies.
DOI: 10.1134/S0020168508100099
1
INTRODUCTION
et al. [12]: if one of two intersecting joins has a eutectic,
at least one of them is not pseudobinary.
Group V-A chalcohalides are attractive semicon-
ducting materials. Many of such compounds and
related phases offer good thermoelectric, photoelectric,
and ferroelectric properties [1–5].
Using chemical vapor deposition, Savilov et al. [13]
obtained for the first time two lower bismuth telluride
iodides: Bi2TeI and Bi4TeI1.25. Melt-grown Bi2TeI sin-
gle crystals had a monoclinic structure with lattice
parameters a = 7.586 Å, b = 4.380 Å, c = 17.74 Å, and
β = 98.20°.
Since the phase-diagram data for some of the joins
in the system in question are contradictory, it is of inter-
est to determine the stability regions of the known bis-
muth telluride iodides in the í–ı–Û phase diagram, to
check whether or not the system contains other ternary
compounds, and to study phase equilibria over the
entire Bi–Te–I system. In earlier reports, we presented
data for the systems Bi2Te3–BiTeI–Te [14], BiI3–
BiTeI–Te [15], and BiI3–TeI4–Te [16]. The first two
systems were shown to have a ternary eutectic. The
Bi3–TeI4–Te system has four primary crystallization
fields, those of Te, BI3, TeI4 and TeI, which are involved
in a number of uni- and invariant peritectic and eutectic
equilibria. Below the solidus, the system consists of two
three-phase fields: BiI3–TeI–Te and BipI3–TeI–TeI4.
The ternary system Bi–Te–I has been the subject of
extensive studies [6–13]. Most effort has been concen-
trated on the phase equilibria in the pseudobinary sys-
tem Bi2Te3–BiI3 [6–10]. The results obtained in [6–8]
are in excellent agreement and indicate that the í–ı
phase diagram of this system has a distectic point cor-
responding to the compound BiTeI, which melts at
828 5 K and has a narrow homogeneity range. The
results reported by Belotskii et al. [9, 10] differ from
those in [6–8] as to the melting point of BiTeI (743 K)
and some of the phase relations in the Bi2Te3–BiI3 sys-
tem.
BiTeI crystallizes in hexagonal symmetry (CdI2
structure, sp. gr. P3m1) with lattice parameters a = 4.31 Å
and c = 6.83 Å (Z = 1) [2, 6].
The BiI3–Te and BiTeI–Te joins are also pseudobi-
nary, with simple eutectics [11]. As shown by Belotskii
et al. [12], the Bi2Te3–I2 join of the Bi–Te–I system is
pseudobinary and contains two congruently melting
ternary compounds: Bi2Te3I2 (693 K) and Bi2Te3 · 3I2
(653 K). In that study, single crystals of these com-
pounds were grown by the Bridgman method [12].
In this paper, we report the solid-state equilibria in
the Bi–Te–I system and the thermodynamic properties
of the ternary compounds BiTeI, Bi2TeI, and Bi4TeI1.25
.
EXPERIMENTAL
The results reported by Evdokimenko and Tsypin
[11] are inconsistent with those obtained by Belotskii
Given that the solid-state equilibria in the composi-
tion region Bi2Te3–BiI3–Te of the Bi–Te–I system can
be inferred from earlier results [14–16], we prepared
Bi–Bi2Te3–BiI3 alloys.
1
Presented in part at the XII Conference High-Purity Substances
and Materials: Preparation, Analysis, and Application, Nizhni
Novgorod, Russia, May 28–31, 2007.
1076

SOLID-STATE EQUILIBRIA AND THERMODYNAMIC PROPERTIES OF COMPOUNDS
1077
First, we synthesized Bi2Te3, BiTe, Bi2Te, BiTeI,
Bi2TeI, Bi4TeI1.25, BiI3, and BiI by melting stoichio-
metric high-purity elemental mixtures in silica ampules
sealed off under a vacuum of ~10–2 Pa. Bismuth tellu-
rides were synthesized at ~900 K and then slowly
cooled. Given the high iodine vapor pressure, bismuth
iodides and bismuth telluride iodides were synthesized
in an inclined two-zone furnace. The temperature of the
upper, “cold” zone was ~370 K, and that of the lower,
“hot” zone was 20–30 K above the melting point of the
compound.
The first equilibrium emf values were measured
after ~30 h; subsequently, measurements were taken
every 4–5 h after the temperature had stabilized.
RESULTS AND DISCUSSION
From DTA results for thoroughly homogenized
Bi2TeI and Bi4TeI1.25 (first obtained by Savilov et al.
[13] using chemical vapor deposition), we determined
their melting points and melting behavior. These com-
pounds melt incongruently at 723 and 688 K, respec-
tively. According to powder XRD data, the lattice
parameters of BiTeI are a = 4.311 Å and c = 6.831 Å
(Z = 1), and those of Bi2TeI are a = 7.585 Å, b = 4.381 Å,
c = 17.740 Å, and β = 98.20°, in agreement with earlier
results [2, 13].
After most of the iodine entered into the reaction,
the ampule was lowered to the hot zone, and the reac-
tion was thoroughly stirred. Next, the ampule was
slowly (~2–3 K/min) cooled. In this way, the congru-
ently melting compounds Bi2Te3, BiI3, and BiTeI were
obtained in a homogeneous state. To fully homogenize
the incongruently melting compounds BiTe, Bi2Te, BiI,
Bi2TeI, and Bi4TeI1.25, they were annealed 30–50 K
below the corresponding peritectic temperature for
500–800 h (the peritectic decomposition temperatures
of Bi2TeI and Bi4TeI1.25 were determined to be 723 and
688 K, respectively). Every 200 h of annealing, the
alloys were reground and pressed into pellets. The com-
pletion of reactions was checked by differential thermal
analysis (DTA) and x-ray diffraction (XRD) using ear-
lier results [13, 17, 18].
The present experimental data in combination with
earlier data for the constituent binaries Bi–Te [17], Bi–
I [18], and Te–I [18] and the system Bi2Te3–BiI3–I–Te
[14–16] were used to construct the 300-K section of the
Bi–Te–I phase diagram (figure). The system contains
three ternary compounds: BiTeI (I), Bi2TeI (II), and
Bi4TeI1.25 (III).
BiI3, the most thermodynamically stable compound
in the system, plays a key role in determining the phase
compatibility diagram. BiI3 is in equilibrium with Te,
TeI4, TeI, and the three ternary compounds.
In addition, Bi–Bi2Te3–BiI3 alloys were synthesized
by vacuum-melting the constituent phases, followed by
annealing to the above schedule and then at 500 K for
an additional 200 h.
Since bismuth, bismuth tellurides, and the ternary
compounds have similar XRD patterns, the phase fields
in the Bi–BiI3–Bi2Te3 region are difficult to identify by
XRD, but the problem can easily be solved using emf
measurements. In the figure, we indicate the 300-K emf
(mV) of cell (1) for some of the phase fields in the
Bi−Te–I system. The emf is composition-independent
within each three-phase field and changes jumpwise in
going from one three-phase field to another. For exam-
ple, the emf in the BiI3–III–II region differs from that in
BiI3–II–I by 73 mV (the accuracy in our emf measure-
ments was 0.2 mV).
The alloys were characterized by DTA (NTR-72
pyrometer, Chromel–Alumel thermocouples), XRD
(DRON-2 diffractometer, CuKα radiation), and emf
measurements using concentration cells of the type
(–)Bi(s) | glycerol + KI + BiI3 | Bi–Te–I(s)(+). (1)
Such concentration cells are widely used in thermo-
dynamic studies of ternary chalcogenides [19].
From emf data for cell (1), we were able to evaluate
the thermodynamic properties of the ternary com-
pounds. To this end, we used emf data for the three-
phase regions BiI3–I–Te, BiI3–I–II, and BiI3–II–III,
which correspond to potential-forming reactions
involving the ternary compounds [19, 20].
The left electrode in cell (1) was metallic bismuth,
and the right electrode was an equilibrium alloy of the
system under investigation. The composition of the
right electrode was chosen using phase-diagram data as
described elsewhere [19]. The electrolyte used was a
saturated KI solution in glycerol with the addition of
0.1 wt % BiI3.
The temperature dependences of the emf in these
phase regions were found to be almost linear. This con-
firms that the phases coexisting in these regions have
constant compositions in the temperature range of our
emf measurements and allows the partial entropy and
enthalpy to be evaluated from the temperature coeffi-
cient of emf [19, 20]. The emf data were represented by
least squares linear equations of the form [21, 22]
The procedures used to prepare the electrolyte and
electrodes and to set up the electrochemical cell were
similar to those described earlier [19, 20]. The cell was
mounted in a vertical tubular furnace. The emf of cell
(1) was measured in the temperature range 305–400 K
by a compensation technique using a V7-34A digital
voltmeter. The temperature was measured by a
Chromel–Alumel thermocouple and a mercury ther-
mometer with 0.2°C divisions.
1/2
S2E
n
E = a + bT t ----- + S2b(T – T)2
,
(2)
INORGANIC MATERIALS Vol. 44 No. 10 2008

1078
ALIEV, BABANLY
I
90
10
TeI
20
4
80
BiI
3
30
70
60
40
BiI
50
TeI
50
(514)
(167)
40
60
I
(89.5)
30
70
II
(16.5)
III
Bi I
7 2
80
20
γ + II
Bi I
9 2
γ + I
10
90
β + III
β' + III
γ
Te
Bi
10
20
30
40
50
mol %Te
60
Bi Te
70
80
90
β
2
3
300-K section of the Bi–Te–I phase diagram: (I) BiTeI, (II) Bi TeI, (III) Bi TeI ; numbers in parentheses indicate the emf (mV).
2
4
1.25
where n is the number of data points (Ö and í), SE is the It follows from schemes (3)–(5) that the standard ther-
modynamic functions of the ternary compounds can be
evaluated using the relations
variance of an individual emf measurement, Sb is the
variance of b, T is the average absolute temperature,
and t is Student’s t. For a 95% confidence interval and
n ≥ 20, we have t ≤ 2.
The equations thus obtained are given in Table 1.
From these equations, we evaluated partial thermody-
namic functions of bismuth in the three-phase regions
in question using well-known thermodynamic relations
(Table 2).
∆G0(BiTeI) = 1.667∆G(Bi) + 0.333∆G0(BiI3), (6)
∆G0(Bi2TeI) = ∆G (Bi) + ∆G0(BiTeI),
(7)
∆G0(Bi4TeI1.25) = 1.917∆G(Bi)
+ ∆G0(Bi2TeI) + 0.083∆G0(BiI3),
(8)
It can be seen from the figure that these partial func-
tions correspond to the following reactions [19, 20]:
and their standard entropies can be found as
S0(BiTeI) = 1.667(∆S(Bi) + S0(Bi))
+ 0.333S0(BiI3) + S0(Te),
Bi + 0.5BiI3 + 1.5Te = 1.5BiTeI,
Bi + BiTeI = Bi2TeI,
(3)
(4)
(9)
Bi + 0.522Bi2TeI + 0.043BiI3 = 0.522Bi4TeI1.25. (5)
S0(BiTe2I) = ∆S (Bi) + S0(Bi) + S0(BiTeI),
(10)
INORGANIC MATERIALS Vol. 44 No. 10 2008

SOLID-STATE EQUILIBRIA AND THERMODYNAMIC PROPERTIES OF COMPOUNDS
1079
Table 1. Temperature-dependent emf of cell (1) for some of the phase fields in the Bi–Te–I systems (T = 300–400 K)
1/2
S2E
E = a + bT t ----- + S2b(T – T)2
Phase field in the figure
n
1/2
BiI3 + BiTeI + Te
1.84
26
E = 168.1 – 0.004T 2 --------- + 1 . 4 × 1 0 –4(T – 342.4)2
1/2
BiI3 + Bi2TeI + BiTeI
7.26
26
E = 97.06 – 0.025T 2 --------- + 6 . 7 × 1 0 –4(T – 346.6)2
1/2
BiI3 + Bi4TeI .25 + Bi2TeI
1
2.68
26
E = 12.4 – 0.020T 2 --------- + 3 × 1 0 –4(T – 354.5)2
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(11)
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