The phase equilibria in the Bi-S-I ternary system and thermodynamic properties of the BiSI and Bi19S27I3 ternary compounds

Article abstract of DOI:10.1016/j.jallcom.2014.05.015

Phase equilibria in the entire Bi-S-I ternary system were determined experimentally by means of differential thermal analysis (DTA), X-ray diffraction (XRD) techniques and EMF measurements. Several vertical sections, an isothermal section at 300 K, and a liquidus surface projection of the system were constructed and two ternary compounds BiSI and Bi₁₉S ₂₇I₃ reported earlier were confirmed. The primary crystallization fields of all phases, the types and coordinates of invariant and monovariant equilibria were determined. Thermodynamic properties of the both BiSI and Bi₁₉S₂₇I₃ ternary compounds were studied by electromotive force measurements (EMF) with the bismuth electrode. From the EMF measurements, the partial molar functions of bismuth in alloys and the standard integral thermodynamic functions of BiSI and Bi₁₉S ₂₇I₃ were calculated.

Full text of DOI:10.1016/j.jallcom.2014.05.015

Accepted Manuscript
The phase equilibria in the Bi-S-I ternary system and thermodynamic properties
of the BiSI and Bi19S27I3 ternary compounds
Ziya S. Aliev, Sabina S. Musayeva, Farhad Y. Jafarli, Imamaddin R.
Amiraslanov, Andrei V. Shevelkov, Mahammad B. Babanly
PII:
S0925-8388(14)01081-0
DOI:
http://dx.doi.org/10.1016/j.jallcom.2014.05.015
JALCOM 31204
Reference:
To appear in:
Received Date:
Revised Date:
Accepted Date:
4 February 2014
22 April 2014
3 May 2014
Please cite this article as: Z.S. Aliev, S.S. Musayeva, F.Y. Jafarli, I.R. Amiraslanov, A.V. Shevelkov, M.B. Babanly,
The phase equilibria in the Bi-S-I ternary system and thermodynamic properties of the BiSI and Bi19S27I3 ternary
compounds, (2014), doi: http://dx.doi.org/10.1016/j.jallcom.2014.05.015
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The phase equilibria in the Bi-S-I ternary system and thermodynamic properties of the
BiSI and Bi19S27I3 ternary compounds
Ziya S. Alieva,b, Sabina S. Musayevaa, Farhad Y. Jafarlia, Imamaddin R. Amiraslanova,b,
Andrei V. Shevelkov c,*, Mahammad B. Babanlya,*
aBaku State University, General and Inorganic Chemistry Department, Baku, Azerbaijan
bInstitute of Physics, Azerbaijan National Academy of Science, AZ1143 Baku, Azerbaijan
cLomonosov Moscow State University, Chemistry Department, Moscow, Russia
* Corresponding authors,
Leninskie Gory 1-3, Moscow 119991, Russia; e-mail: shev@inorg.chem.msu.ru (A.V.S)
Z. Khalilov Str. 23, Baku, AZ 1148, Azerbaijan; e-mail: babanly_mb@rambler.ru (M.B.B)
Phase equilibria in the entire Bi–S–I ternary system were determined experimentally by
means of differential thermal analysis (DTA), X-ray diffraction (XRD) techniques and EMF
measurements. Several vertical sections, an isothermal section at 300 K, and a liquidus
surface projection of the system were constructed and two ternary compounds BiSI and
Bi19S27I3 reported earlier were confirmed. The primary crystallization fields of all phases, the
types and coordinates of invariant and monovariant equilibria were determined.
Thermodynamic properties of the both BiSI and Bi19S27I3 ternary compounds were studied by
electromotive force measurements (EMF) with the bismuth electrode. From the EMF
measurements, the partial molar functions of bismuth in alloys and the standard integral
thermodynamic functions of BiSI and Bi19S27I3 were calculated.
Keywords: phase diagram; bismuth sulfide halides; bismuth triiodide; bismuth sulfide;
electromotive forces; thermodynamic functions
1. Introduction
The study of the phase diagram and thermodynamic properties of the Bi-S-I system is
part of the systematic investigation of the AV–X–I (AV = As, Sb, Bi; X = S, Se, Te) systems
[1–4]. These systems were investigated repeatedly in the last four decades because many
ternary phases in these systems are promising high-performance materials for ferroelectrics,
piezoelectrics, thermoelectrics, photoconductors, piezoelastics [5–7] and recently discovered
topological insulators or materials showing a giant Rashba-type spin splitting [8]. With just a
few exceptions, many of these compounds either melt incongruently or decompose in the
1

solid state, and therefore, their synthesis and, especially, crystal growth become a tricky
problem [5]. The knowledge of the phase relationships and constructed phase diagrams can
helps to solve these problems by choosing proper initial compositions for growing large single
crystals of binary and ternary phases from melt.
Up to present, the phase diagram of the Bi–S–I ternary system has not yet been
reported. Only the pseudo-binary BiI3–Bi2S3 system was studied by Ryazantsev et al. [9].
According to that paper, this system is quasi-binary and includes two ternary compounds BiSI
and Bi19S27I3 melting peritectically at 808 K and 990K, respectively. The compositions of the
peritectic points lie at 35 and 80 mol % Bi2S3, respectively. The eutectic reaction was found to
occur at 668 K and 4 mol % Bi2S3. The thermodynamic properties of the both ternary
compounds in this system were obtained by Oppermann [10] using vapor pressure
measurements.
The crystal structures of both compounds are well studied. BiSI crystallizes in the
orthorhombic space group Pnma with the lattice parameters a = 8.529, b = 4.172, c = 10.177
Å and z = 4 [11]. Bi19S27I3 has a hexagonal lattice, space group P63/m, with a = 15.640, c =
4.029(2) Å, and z = 2/3. BiSI crystallizes in the BiSCl structure type (also referred to as the
SbSBr type) and features one-dimensional BiSI strands with the considerably elongated Bi–I
distance of 3.0 Å that run along the c axis. The slabs are joined into a 3D structure through
non-covalent Bi–I interactions at 3.7 Å (Figure 1a). In the crystal structure of Bi19S27I3, the
slabs running along the c axis are formed only by bismuth and sulfur atoms; they are further
connected through disordered Bi(1) atoms, whereas the iodine atoms sit in the voids, with the
Bi–I distances exceeding 3.5 Å (Figure 1b). Taking into account the details of the crystal
structure of Bi19S27I3, it is frequently formulated as Bi(Bi2S3)9I3 [12].
The Bi–I, Bi–S and S–I binary systems, which form the boundaries of the Bi–S–I
ternary system were studied in detail [13-21].
The phase diagram of the Bi–S system in the entire composition range is given in Ref.
[13]. It is characterized by the presence of monotectic and eutectic eqiulibria, with only one
intermediate compound Bi2S3 that congruently melts at 1048 K. Bi2S3 crystallizes in the
orthorhombic Sb2S3-type system, space group Pbnm, z = 4, with the unit cell parameters a =
11.150; b = 11.300; c = 3.981 Å [14].
In the Bi–I system, four equilibrium compounds exist, BiI3, BiI, Bi14I4 и Bi18I4 [13], as
well as a metastable Bi16I4 [15]. Only BiI3 melts congruently at 681 K, whereas the other
thermodynamically stable compounds decompose at 603, 573 and 568 K by peritectic
reactions, respectively. The phase transition of BiI was observed to occur at 568 K. BiI3
2

crystallizes in the hexagonal system, space group R-3, with the unit cell parameters a = 7.535
and c = 20.703 Å [16]. BiI crystallizes in the monoclinic system, space group C2/m; its low
temperature α-modification has the following lattice parameters with the same space group: a
= 14.245, b = 4.428, c = 19.968 Å, β = 92.960, whereas its high temperature β-modification
0
has the lattice parameters a = 14.444, b = 4.456, c = 10.493 Å, =107.87 [17, 18]. Bi14I4,
β
Bi16I4, and Bi18I4 possess crystal structures featuring corrugated slabs of bismuth atoms
limited by iodine atoms on the sides [15, 19, 20]. In general, with exception of BiI3, all
bismuth iodides feature Bi–Bi bonds with the interatomic distance of 3.0–3.1 Å.
The phase diagram of the S–I system was reported to be a simple eutectic system in Ref.
[21]. No binary compound exists in this system. The eutectic composition has the melting
point of 338 K at about 80 at. % S.
The aim of the present work is to experimentally determine phase equilibria in the Bi–
S–I ternary system in order to reveal phase relationships in this system and to provide more
accurate experimental data for preparing pure and high quality materials. Also, we present
thermodynamic properties of the BiSI and Bi19S27I3 ternary compounds experimentally
obtained by EMF measurements.
2. Experimental Part
2.1. Synthesis. Binary BiI, BiI3 and ternary BiSI, Bi19S27I3 were synthesized from the starting
elements of a high purity grade (not less than 99.999%) in sealed (~10-2 Pa) silica ampoules
with the minimized free volume at 650 K (BiI), 700 K (BiI3), 910 K (BiSI) and 1050 K
(Bi19S27I3), respectively, according to the literature procedures [1, 9] in a two-zone furnace.
Because BiI, BiSI, and Bi19S27I3 melt by peritectic reactions, further heat treatments at 585,
750, and 950 K for 500 h, respectively, were performed in a vertical tube furnace. Bi2S3 was
also synthesized by melting the stoichiometric mixture of bismuth and sulfur at 1100 K in a
two-zone tube furnace because of a high vapor pressure of elemental sulfur at high
temperatures. As a rule, all investigated samples (total mass, 0.5 g) were prepared from initial
elemental components or preliminary synthesized compounds. After melting most of the
alloys were annealed at 400 K and 360 K for 500 h.
2.2. Analysis. Differential thermal analysis (DTA) and X-ray powder diffraction (XRD) were
used to analyze the samples. DTA of annealed alloys was carried out using a NETZSCH 404
F1 Pegasus system. The measurements were performed between room temperature and ~1100
K depending on composition with a heating and cooling rate of 10 K min–1. Temperatures of
3

thermal effects were taken mainly from the heating curves. XRD analysis was done with a
Bruker D8 ADVANCE diffractometer (Cu-Kα1 radiation).
2.3. EMF measurements. For the electro-motive force (EMF) measurements, the following
concentration chains were assembled:
(–) Bi (solid) / glycerin + KI + BiI3 / (Bi–S–I) (solid) (+)
(1)
In the chains of type (1), metallic bismuth was the left (negative) electrode, while equilibrium
alloys of the Bi–S–I system were exploited as right (positive) electrodes. A saturated glycerin
solution of KI with the addition of 0.1 mass % of BiI3 was used as the electrolyte. The
assembly of an electrochemical cell and measurements are described in detail elsewhere [22].
EMF was measured by the compensation method in the temperature range of 300–390 K
with the accuracy of 0.1 mV, using the high-resistance universal B7-34A digital voltmeter.
3. Results and discussion
3.1. Quasi-binary sections. The Bi–S–I system is represented by two quasi-binary sections.
For the purpose of easy comparison, the quasi- and non-quasibinary polythermal sections are
expressed using the normalized compositions, meaning that the number of atoms in initial
components is equalized. The thermodynamic functions are expressed per mole of formula
units.
3.1.1. The quasi-binary system BiI3–Bi2S3. This system (Figure 2) was first reported by
Ryazantsev and co-authors [9] as we shown above. In the present investigation, DTA and
XRD results for the selected compositions agree well with the result of that work. EMF
measurements also confirm the existence of both ternary compounds BiSI and Bi19S27I3. As
can be seen from Figure 2b, they are well determined on the Е–х diagram. The isothermal Е–х
curve at 300 K consists of three horizontal straights with the EMF values of 370, 353, and 343
mV. Such a stepwise changing of the EMF values at the composition of both compounds
indicates that these compounds are stable at room temperature. We did not observe
appreciable solid solubility based on both initial compounds.
3.1.2. The quasi-binary system BiI3–4S. This system (Figure 3) relates to the simple eutectic
type. The eutectic degenerates nearest of pure sulfur. The section liquidus is represented only
by one curve of the primary crystallization of BiI3. No solid solubility fields based on both
initial compounds were detected.
3.2 The isothermal section at 300 K. The isothermal section of the Bi–S–I ternary system at
300 K (Figure 4) confirms the formation of both ternary compounds, Bi19S27I3 (C1) and BiSI
(C2). Ten three-phase equilibria were found in this system at 300 K. As shown in Figure 4,
4

about all phases are in equilibrium with BiI3 and BiSI; this indicates that they are most stable
phases and play a key role in the distribution of the phase areas in the subsolidus area. No
solid solution fields were detected in this system based on initial and intermediate phases.
The EMF method allows to clearly differentiating these areas. Figure 4 shows the EMF values
(in parenthesis, mV) of the chains of type (1) in appropriate phase areas of the system Bi–S–I
at 300 K where equilibrium alloys were taken as right electrodes. The measurements show
that under specified temperature, in the range of each three-phase area, the EMF has a strict
constant value, which is independent of the general composition of alloys, but changes jump-
like upon the transition from one three-phase area to another. No other ternary compounds
were found in this system during the present investigation. In particular, no Bi-rich
compounds similar to those found in the Bi–Te–I system [23] were detected in the Bi–S–I
system. This can be clearly seen from the powder XRD patterns for different alloys of the Bi–
BiSI subsystem (Figure 5) which all contain a set of diffraction lines corresponding to the
Bi+BiSI mixture with varying intensities depending on the composition.
3.3. The liquidus surface projection. The liquidus surface projection of the Bi–S–I system
was constructed using the experimental and literature data [13, 21] (Figure 6). It consists of
ten primary crystallization fields corresponding to the five binary and two ternary compounds
and three elemental components. The primary crystallization fields of the Bi2S3, Bi19S27I3,
BiSI, and BiI3 are very large and occupy major part of the total area of the Bi–S–I triangle.
The primary crystallization of elemental iodine from the melt takes place in a very narrow
composition range along the binary border system S–I. Two wide immiscibility fields are
/
observed in this system. The first one (L1 + L2) starts from the Bi–I system (m2m2 ) and
spreads into the Bi–BiI3–Bi2S3 subsystem whereas the second one (L2+L3) starts from the
/
Bi2S3–S subsystem (m1m1 ) and spreads into the Bi2S3–BiI3–S subsystem. Subsequently, the
monovariant eutectic (e5) and peritectic (p5 and p4) curves from the BiI3–Bi2S3 quasi-binary
system pass through these fields and transform the quadriphase monotectic equilibria M1, M2,
M3 and M4.
The primary crystallization fields of Bi, S, BiI, Bi7I2 and Bi9I2 as well as some invariant
and monovariant equilibria are degenerate very close to the elemental bismuth and sulfur
corners of the composition triangle. Schematic descriptions of these areas are given in Figure
6 as blow-ups.
Table 1 summarizes the types and coordinates of innvariant equilibria, including the
binary border subsystems, whereas Table 2 compiles the types and temperature intervals for
the monovariant equilibria.
5

3.4. The vertical sections. In addition to the quasi-binary sections shown in Figure 6 by
dashed lines, the vertical sections BiI–2S (Figure 7) and Bi–S0.5I0.5 (Figure 8) were studied
in order to determine the nature and exact position of the monovariant processes and
coordinates of the invariant points on the T–x–y diagram.
3.4.1. The vertical section BiI-2S. This section passes through the stoichiometric
composition of BiSI. In the subsolidus, this compound is in equilibrium with both initial BiI
and S phases (Figure 7). The crystallization of melts on this section is much more complex
with respect to the incongruent melting character of BiI and BiSI. This section crosses the
independent systems Bi–BiI3–Bi2S3 and Bi2S3–BiI3–S and is characterized by two immis-
cibility areas L1+L2 and L2+L3 in the region of 80–100 mol % BiI and 6–58.5 mol % BiI,
respectively. In the compositional range of <80 mol % BiI, Bi19S27I3 primarily crystallizes
from two different liquid phases L2 and L3 or two immiscibility fields of them. Crystallization
over the both immiscibility fields were observed to occur on the monotectic reactions M1, M2
and M4 (Table 1). The primary crystallization of BiSI was found to occurs by monotectic
reactions L2 + Bi19S27I3 ↔ L1 + BiSI (M1) in the compositional range of >80 mol % BiI. The
peritectic curves p5М1 and p5М4 confirm the crystallization of BiSI by the peritectic reaction
L2(L1)(L3) + Bi19S27I3 ↔ BiSI (Table 2, № 3,4). Horizontal lines at 790, 725 and 610 K,
respectively, reflect the quadriphase monotectic (M4, M1 and M2) equilibria (Table 1). The
crystallization process concludes by the invariant transition reactions L + Bi19S27I3 ↔ BiSI + S
(U7) and L + BiI3 ↔ BiI + BiSI (U1) at 385 and 600 K in the range of <66.7 and >66.7 mol %
BiI, respectively. Consequently, the two-phase areas BiSI+S and BiI3+S were found in the
subsolidus. Schematic descriptions of some degenerate non- and monovariant equilibria and
the phase areas near BiI and S are given in Figure 7 as blow-ups.
3.4.2. The vertical section Bi–S0.5I0.5. This section crosses both the quasi-binary sections
BiI3–Bi2S3 and BiI3–S of the Bi–S–I system at 33 and 15 at.% Bi, respectively (Figure 8). It
passes through two immiscibility areas L1+L2 and L2+L3 in the regions of 47–83 and 17.5–
30.5 at.% Bi, respectively. Bi19S27I3 primarily precipitates from the liquid phases in a wide
range of composition (> 20 at %), whereas BiSI crystallizes from the Bi-poor liquid.
Crystallization continues monovariantly by the peritectic reaction L1(L2) + Bi19S27I3 ↔ BiSI
(Table 2) and invariantly by the monotectic reactions M4 and M1 at 790 and 725 K,
respectively (Table 1). A horizontal line at 650 K corresponds to the onset of monovariant
crystallization of the eutectic BiSI + BiI3 mixture. Crystallization finishes by the invariant
eutectic E3 and E2 and transition U5 reactions at 337, 383 and 541 K, respectively (Table 1).
Consequently, three different phase areas were found in the subsolidus; they are BiI3 + S + I2
6

(<15 at.% Bi), BiSI + BiI3 + S (15–33.3 at.% Bi) and Bi + BiSI (>33.3 at.% Bi). Schematic
descriptions of some degenerate monovariant equilibria and the phase areas very close to the
elemental bismuth is given in Figure 8 as blow-ups.
3.2. Thermodynamic properties of BiSI and Bi19S27I3. The results of the EMF
measurements of the chains of type (1) also allowed calculating thermodynamic properties of
BiSI and Bi19S27I3. The analysis showed the linearity of the EMF dependences upon
temperature for various alloys belonging to the heterogeneous fields BiI3–C2–S and C2–C1–S.
Accordingly, the linear least-square treatment of the data was performed [24] and the results
were expressed according to the literature recommendations [25] as



2 1/2
2
SE
n
(2)
E, mV = a + bT t
+ S2b
(T − T
)


where n is the number of pairs of Е and Т values; SE and Sb are the error variances of the EMF
readings and b coefficient, respectively;T is the mean absolute temperature; t is the Student’s
test. At the confidence level of 95% and n ≥ 20, the Student’s test is t ≤ 2 [22, 24]. By using
the obtained equations of type (2) (Table 3) and the following thermodynamic expressions
[26]:
∆GBi = −zFE
∆H Bi = −zFa
∆S Bi = zFb
the partial molar functions of bismuth in the alloys at 298 K were calculated. The results are
listed in Table 4. According to Ref. [26] and Figure 5, the partial molar thermodynamic
values for bismuth in the above-mentioned phase areas are the response to the following
potential-forming reaction:
Bi (solid) + 0.5BiI3 (solid) + 1.5S (solid) = 1.5BiSI (solid) (3)
Bi (solid) + 0.1875BiSI (solid) + 1.5S (solid) = 0.0625Bi19S27I3 (solid)
(4)
According to these equations, the standard thermodynamic functions of BiSI and Bi19S27I3
were calculated as:
∆f Z0 ( BiSI ) = 0.667∆Z( Bi )+ 0.333∆f Z0 ( BiI3 )
∆ f Z 0 (Bi19 S27 I3 ) =16∆Z(Bi) + 3∆ f Z 0 (BiSI)
(5)
(6)
( ∆f Z0 is the ∆G2098 and ∆H2098 values for the corresponding compound, whereas ∆Z Bi is
,
∆GBi ∆H Bi
). The standard entropies were calculated as:
7

S 0 (BiSI) = 0.667∆S(Bi) + 0.667S 0 (Bi) + 0.333S 0 (BiI3 ) + S 0 (S)
(7)
(8)
0.0625S 0 (Bi19 S27 I3 ) =16∆S(Bi) +16S 0 (Bi) + 3S 0 (BiSI) + 24S 0 (S)
31.923 0.209 J·K-1·mol-1) and
0
The following thermodynamic data for BiI (Table 5), S (
S =
3
Bi (S0 = 56.902 0.418 J·K-1·mol-1) [27, 28] were used for calculations according to (7) and
(8). The results of the calculations are presented in Table 5. In all cases the estimated standard
deviations were calculated by accumulating the errors. For comparison, Table 5 also contains
thermodynamic data reported by Oppermann [10] on the basis of vapor pressure
measurements. Apparently, our results within an error are in good agreement with those
reported in Ref. [10] and have higher accuracy.
Acknowledgment: This work was supported by the Science Development
Foundation under the President of the Republic of Azerbaijan, Grant № EIF-2011-1(3)-
82/69/4-M-50.
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9

Table 1. Invariant equilibria in the Bi–S–I system
Point on
Figure 6
Equilibria
Composition, аt.%
T,K
S
I
D1
60
-
1048
L ⇔ Bi 2S3
L ⇔ BiI3
D2
e1
е2
е3
е4
е5
e6
e7
p1
p2
-
75
1
681
544
542
388
386
668
386
338
603
573
99
L ⇔ Bi + Bi2S3
>99
>99
>98
3.5
-
-
L ⇔ Bi + Bi9I2
-
L ⇔ Bi2S3 +S
-
L ⇔ BiI3 +S
70
>99
20
4
L ⇔ BiSI + BiI3
L ⇔ BiI3 + I2
80
-
L ⇔ S + I2
L+ BiI3 ⇔ BiI
-
2.5
2
L + BiI ⇔ Bi7 I2
p3
p4
-
568
990
808
540
383
337
600
570
565
542
541
386
L + Bi 7 I2 ⇔ Bi 9 I2
L + Bi 2S3 ⇔ Bi19 S27 I3
L+ Bi19S27I3 ⇔ BiSI
L ⇔ Bi+ Bi9I2 + BiSI
L ⇔ BiI3 + BiSI + S
L ⇔ BiI3 + S + I2
L + BiI3 ⇔ BiI+ BiSI
L + BiI ⇔ Bi7I2 + BiSI
L + Bi7I2 ⇔ Bi9I2 + BiSI
L + Bi2S3 ⇔ Bi + Bi19S27I3
L+ Bi19S27I3 ⇔ Bi+ BiSI
L + Bi2S3 ⇔ Bi19S27I3 +S
45
15
<1
>99
80
<1
<1
<1
<1
<1
>99
19
56.5
<1
<1
19
4
p5
E1
E2
E3
U1
U 2
U 3
U 4
U 5
U 6
2.5
2
<1
<1
<1
10

U 7
>99
<1
385
990
625
725
610
950
790
L+ Bi19S27I3 ⇔ BiSI+S
L1 ⇔ L3 + Bi2S3
/
m1(m1 )
67 (99)
-
-
/
m2(m2 )
60 (5)
50 (6)
58 (4)
17 (<1)
50 (<2)
L2 ⇔ L1 + BiI3
/
M1(M1 )
11 (3)
2 (<1)
51 (>99)
25 (>98)
L2 + Bi19S27I3 ⇔ L1 + BiSI
L2 + BiSI ⇔ L1 + BiI3
L2 + Bi2S3 ⇔ L3 + Bi19S27I3
L2 + Bi19S27I3 ⇔ L3 + BiSI
/
M2(M2 )
/
M3(M3 )
/
M4(M4 )
Notes: Conjugate invariant points and their compositions are in parentheses.
11

Table 2. Monovariant equilibria in the Bi–S–I system
№
Curves on Figure 6
Equilibria
Temperature
ranges, K
/
1
2
3
4
5
6
7
8
9
e5M2 (M2 U1)
668-610 (610-600)
L2 (L1) ⇔ BiSI+ BiI3
L2 ⇔ BiSI+ BiI3
e5E2
668-383
/
p5M1 (M1 U5);
808-725 (725-541)
808-790 (790-385)
990-542
L2 (L1) + Bi19S27I3 ⇔ BiSI
L2 (L3)+ Bi19S27I3 ⇔ BiSI
L+ Bi2S3 ⇔ Bi19S27I3
L2 (L3)+ Bi2S3 ⇔ Bi19S27I3
L ⇔ Bi2S3 +S
/
p5M4 (M4 U7)
p4U4
/
p4M3 (M3 U6)
990-950 (950-386)
388-386
e3U6
U6U7
386-385
L ⇔ Bi19S27I3 +S
U7E2
e4E2
385-383
386-383
L ⇔ BiSI+S
L ⇔ BiI3 +S
10
11
12
13
14
e4E3
e6E3
e7E3
p1U1
386-337
386-337
338-337
603-600
L ⇔ BiI3 +S
L ⇔ BiI3 + I2
L ⇔S+ I2
L+BiI3 ⇔ BiI
L ⇔ BiI+BiSI
L + BiI ⇔ Bi7I2
15
16
U1U2
p2U2
600-570
573-570
17
18
19
20
21
22
23
U2U3
p3U3
U3E1
e2E1
570-565
568-565
565-540
542-540
544-542
542-541
541-540
L ⇔ Bi7I2 +BiSI
L+ Bi7I2 ⇔ Bi9I2
L ⇔ Bi9I2 +BiSI
L ⇔ Bi9I2 +Bi
L ⇔ Bi+Bi2S3
L ⇔ Bi+Bi19S27I3
L ⇔ Bi+BiSI
e1U4
U4U5
U5E1
Notes: Conjugate monotectic curves are in parentheses.
12

Table 3. Temperature dependences of the EMF for the chains of type (1) for alloys of the
Bi-S-I system in the temperature range of 300-390 K
1/ 2
Phase area
S2
2


E
2
+ Sb T −T
(
)
E,mV = a + bT
t




n
BiI3-BiSI+S
4.76

1/ 2
E = 364.32+0.017Т
E = 366.98−0.040Т
2
+2.4⋅10−4(Т −352.88)2




26
BiSI+Bi19S27I3+S
1.45
26


1/ 2
2
+1.1⋅10−4 (Т −349.54)2



Тable 4. Relative partial thermodynamic functions of bismuth in the alloys of the Bi-
S-I system at 298 К.
− ∆H
Bi
− ∆GBi
∆S Bi
J⋅ К-1⋅mol-1
4.92 8.97
-11.58 6.07
kJ⋅mol-1
Phase area
BiI3-BiSI+S
106.92 0.55
102.77 0.33
105.46 3.15
106.22 2.13
BiSI+Bi19S27I3+S
Table 5. Standard integral thermodynamic functions of BiSI and Bi19S27I3at 298К
− ∆f G0 ( 298 K )
− ∆f H 0 ( 298K )
kJ⋅mol–1
S0 ( 298K )
Compound
J⋅К–1⋅mol–1
224.2 6.3
BiI3 [25, 26]
BiSI
148.8 8.0
120.9 3.1
150.6 6.3
120.5 3.4
148.0 8.6
116.3 5.9 [10]
151.0 10.5 [10]
Bi19S27I3
2007 14
2061 42
1936 124
1943.05 62.80 [10]
2103.29 62.80 [10]
13

Figure Captions
Figure 1. Crystal structures of BiSI (a) and Bi19S27I3 (b) shown as projections onto the
respective ab planes. (color online)
Figure 2. Phase diagram (а) and EMF dependence upon composition (b) for the quasi-binary
BiI3–Bi2S3 system.
Figure 3. Phase diagram of the quasi-binary BiI3–4S system.
Figure 4. Isothermal section of the Bi–S–I system at 300K.
Figure 5. XRD patterns for different alloys of the 3Bi–BiSI sub-system. (color online)
Figure 6. The liquidus surface projection of the Bi–S–I system. Primary crystallization fields
of the phases: 1, Bi2S3; 2, BiSI; 3, Bi19S27I3; 4, BiI3; 5, BiI; 6, Bi7I2; 7, Bi9I2; 8, Bi; 9, S; 10, I2.
(color online)
Figure 7. Phase diagram of the vertical section BiI–2S. (color online)
Figure 8. Phase diagram of the vertical section Bi–S0.5I0.5. (color online)
14

Figure 1.
15

Figure 2.
16

Figure 3.
17

Figure 4.
18

Figure 5.
19

Figure 6.
20

Figure 7.
21

Figure 8.
22

• The Bi–S–I ternary system is experimentally investigated
• The fields of primary crystallization are determined
• The projection of the liquidus surface is presented
• Partial molar thermodynamic functions of Bi are calculated
• Standard integral thermodynamic functions of BiSI and Bi19S27I3 are
calculated
23