Phase equilibria in the SrS-Ga2S3 system

Article abstract of DOI:10.1134/S0036023610080231

In the SrS-Ga₂S₃ system, there exist two individual compounds: SrGa₂S₄ (a = 2.084 nm, b = 2.050 nm, c = 1.220 nm; congruent melting at 1530 K) and Sr₂Ga₂S₅ (a = 1.253 nm, b = 1.203 nm, c = 1.117 nm; peri- tectic melting at 1330 K); both are orthorhombic. We discovered a compound of composition Sr₄Ga ₂S₇; this compound crystallizes in cubic system with the unit cell parameter a = 0.6008 nm, space group Pa3, and decomposes by a solid-phase reaction at 870 K. Eutectic compositions are 42 and 73 mol % Ga ₂S₃; eutectic melting temperatures are 1210 and 1170 K, respectively. The SrS solubility in ?-Ga₂S₃ at 1070 K reaches 4 mol %. Pleiades Publishing, Ltd., 2010.

Full text of DOI:10.1134/S0036023610080231

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2010, Vol. 55, No. 8, pp. 1283–1286. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © A.V. Kertman, N.V. Kraeva, 2010, published in Zhurnal Neorganicheskoi Khimii, 2010, Vol. 55, No. 8, pp. 1359–1363.
PHYSICOCHEMICAL ANALYSIS
OF INORGANIC SYSTEMS
Phase Equilibria in the SrS–Ga2S3 System
A. V. Kertmana and N. V. Kraevab
a Tyumen State University, Tyumen, Russia
b Tyumen State University of Architecture and Civil Engineering, Tyumen, Russia
Received April 1, 2008
Abstract—In the SrS–Ga2S3 system, there exist two individual compounds: SrGa2S4 (
= 1.220 nm; congruent melting at 1530 K) and Sr2Ga2S5 ( = 1.253 nm, = 1.203 nm,
tectic melting at 1330 K); both are orthorhombic. We discovered a compound of composition Sr4Ga2S7; this
compound crystallizes in cubic system with the unit cell parameter = 0.6008 nm, space group Pa , and
decomposes by a solidꢀphase reaction at 870 K. Eutectic compositions are 42 and 73 mol % Ga2S3; eutectic
a
= 2.084 nm,
b = 2.050 nm,
c
a
b
c
= 1.117 nm; periꢀ
a
3
melting temperatures are 1210 and 1170 K, respectively. The SrS solubility in
4 mol %.
γ
ꢀGa2S3 at 1070 K reaches
DOI: 10.1134/S0036023610080231
The SrS–Ga2S3 system with an equimolar compoꢀ
nent ratio forms a phase of composition SrGa2S4, this
phase crystallizing in orthorhombic system with the
EXPERIMENTAL
SrS was prepared from SrSO4 (chemically pure
grade) by reducing it in a hydrogen stream at 1170 K.
Gallium sesquisulfide was synthesized from Ga2O3
(chemically pure grade) in a CS2 stream at 1120–1170 K.
Samples of the SrS–Ga2S3 system were synthesized
from the constituent binary sulfides in quartz
ampoules evacuated to 0.1 Pa and sealed off. After
synthesis, samples were subjected to homogenizing
annealing along three isothermal sections at 570, 820,
and 1070 K for 3000, 1000, and 750 h, respectively.
Equilibration was monitored by powder Xꢀray diffracꢀ
tion. Microstructure was observed on polished and
etched sections in the reflected light on a Metam RVꢀ
22 metallographic microscope. ASM diamond pastes
of various grain sizes were used for polishing. Dilute
hydrochloric acid and acetic acid solutions were used
as etchants. Microhardness was measured on a PMTꢀ
3 tester using a routine procedure; the load on the
unit cell parameters а = 2.093 nm, b = 2.054 nm, c =
1.222 nm, space group Fddd [1]. When the component
ratio is 2SrS : 1Ga2S3 (mol/mol), an orthorhombic
phase of composition Sr2Ga2S5 exists with the unit cell
parameters a = 1.252 nm, b = 1.203 nm, c = 1.114 nm,
space group Pbca [2]. Neither the melting characters
and melting temperatures of phases nor the geometriꢀ
cal image of the phase diagram are known for the SrS–
Ga2S3 system.
SrS crystallizes in NaClꢀtype cubic structure with
the unit cell parameter
а = 0.6015 nm, space group
Fm , and melts congruently at 2590 K [3]. Gallium
3m
sesquisulfide Ga2S3 melts at 1400 K [4] or at 1380 [5]
and exists as three polymorphs, only the highꢀtemperꢀ
ature phase being stable. The lowꢀtemperature cubic
phase
the unit cell parameter
which transforms at 825–875 K to the mediumꢀ
αꢀGa2S3 has a sphalerite defect structure with
indenter
Р = 0.020 kg. Microhardness measurement
precision was 3–5% of the measured value. Xꢀray difꢀ
fraction patterns were recorded on a DRONꢀ3M difꢀ
а
= 0.517 nm, space group
F43m,
temperature wurtziteꢀtype hexagonal phase
having the unit cell parameters = 0.3682 nm,
63mc. The transition to the
ꢀGa2S3 is observed at 1290 K;
fractometer (Niꢀfiltered Cu
K
radiation) and a
α
βꢀGa2S3
DRONꢀ6 diffractometer (Feꢀfiltered Cо
K radiation)
α
а
с =
at room temperature. When necessary, powdered sinꢀ
gleꢀcrystal silicon was used as a reference. The unit cell
parameters of cubic compounds were determined with
an error of 0.0001–0.0002 nm; for orthogonal crystal
systems, the error was 0.001–0.003 nm. The unit cell
parameters were calculated using quadratic forms with
reference to the PDFꢀ2 database with the use of
Powder2 software and PDWin4.0 Xꢀray diffraction
analytical program package. Differential thermal
analysis (DTA) up to 1500 K was carried out in quartz
ampoules evacuated to 0.1 Pa and sealed off. Visual
thermal analysis (VTA) was used to determine the liqꢀ
uefaction and complete melting temperatures in the
0.6031 nm, space group
highꢀtemperature phase
P
γ
this phase crystallizes in hexagonal system with the unit
cell parameters = 0.6385 nm, = 1.804 nm, space
group 61, or in monoclinic system with the unit cell
parameters = 1.114 nm, = 0.641 nm, = 0.7038 nm,
= 121.22°, space group Cc [4].
а
с
P
а
b
с
β
This work was undertaken to study the phase diaꢀ
gram of the SrS–Ga2S3 system by means of physicoꢀ
chemical methods and to construct its geometrical
image.
1283

1284
KERTMAN, KRAEVA
T, K
T, K
2590
2500
2500
2100
1700
1300
1
2
3
4
L
2100
1700
1300
SrS +
L
Sr2Ga2S5 +
L
1330
SrGa2S4 +
L
1170
1210
L
+ γꢀGa2S3
γ
42 %
SrS + Sr2Ga2S5
73 %
870
900
500
900
500
Sr2Ga2S5
SrS
+
Sr2Ga2S5
+
+
SrGa2S4 + γꢀGa2S3
SrGa2S4
Sr4Ga2S7
Sr4Ga2S7
SrS
25
50
mol %
75
Ga2S3
Fig. 1.
T
–
x
diagram of the SrS–Ga S system. Notations: (
1
) DTA data, (
2
) VTA data, (3) singleꢀphase samples, and (4) twoꢀ
2 3
phase samples as determined by Xꢀray diffraction and microstructure examination.
sample. The temperature determination precision was structure symmetry of SrGa2S4 relative to
0.4% in DTA and 0.7% in VTA. due to the fillingꢀin of cation vacancies arranged parꢀ
allel to axis in the ꢀGa2S3 structure, this axis being
γꢀGa2S3 is
c
γ
one of the rhombohedral axes. The existence of such
vacancies causes contraction of sulfur tetrahedra
RESULTS AND DISCUSSION
The Т–х diagram shown in Fig. 1 is the first one
constructed for the SrS–Ga2S3 system. Individual
compounds exist in the system, namely, SrGa2S4,
around these vacancies in the direction of axis
result, a monoclinic cell is formed in . Fillingꢀin of
tetrahedral interstices by Sr2+ ions increases the unit
cell size along axis = +0.5162 nm for SrGa2S4
against ꢀGa2S3) and generates an orthorhombic strucꢀ
ture with the unit cell parameters = 2.084 nm,
2.050 nm, = 1.220 nm.
c; as a
с
с
(Δс
Sr2Ga2S5, and Sr4Ga2S7. SrGa2S4 melts congruently,
γ
Sr2Ga2S5 is formed by a peritectic reaction, and
a
b =
Sr4Ga2S7 is formed by a solidꢀphase reaction. ꢀGa2S3
γ
c
is the base of a limited solid solution.
Homogeneous and heterogeneous fields in the
Apart from the SrGa2S4 phase, we confirmed the
SrS–Ga2S3 system were determined in studying fused existence of the Sr2Ga2S5 phase in the SrS–Ga2S3 sysꢀ
and annealed samples by a set of physicochemical tem; this phase crystallizes in orthorhombic system with
methods. SrGa2S4 is formed for the equimolar compoꢀ the unit cell parameters
а = 1.253 nm, b = 1.203 nm, and
nent sulfide ratio. All samples containing 50 mol % = 1.117 nm. Samples containing 66.66 mol % SrS
с
Ga2S3 and annealed along various isothermal sections and 33.33 mol % Ga2S3 annealed at 1070 and 820 K
comprise a single phase as identified by microstructure comprised a single phase as identified by microstrucꢀ
observation and powder Xꢀray diffraction. SrGa2S4 is ture observation; all reflections in the Xꢀray diffraction
stable up to its congruent melting temperature, which patterns of these samples were assigned to Sr2Ga2S5
.
is 1530 K as determined by VTA. The higher crystal The peritectic melting character of Sr2Ga2S5 is supꢀ
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 8 2010

PHASE EQUILIBRIA
1285
ported by thermal analysis and microstructure obserꢀ
vation in samples from the existence regions of SrS
and Sr2Ga2S5 phases. The microstructure of an
Sr2Ga2S5 phase cooled from the melt reveals SrS microꢀ
inclusions, which disappear upon homogenizing annealꢀ
ing. Polished sections of samples from the 1–30 mol %
Ga2S3 region annealed at 1070 K reveal ovalꢀshaped
80 mol % SrS – 20 mol % Ga2S3
Annealing 1070 K
1330
80 mol % SrS – 20 mol % Ga2S3
Annealing 570 K
dark grey grains of the SrS phase (
H
= 2070 MPa) in
= 1420 MPa).
the field of the grey Sr2Ga2S5 phase (
Н
Temperature–time thermal curves for these samples
feature heating arrests, caused by the vigorous melting
of the most part of the sample as shown by VTA. The phase
870
1330
66.7 mol % SrS – 33.3 mol % Ga2S3
Annealing 1070 K
melting reaction is as follows: Sr2Ga2S5
→
2SrS + L.
The onset temperature of the peritectic melting reacꢀ
tion remains virtually unchanged, while the percentꢀ
age content of the Sr2Ga2S5 phase in the sample
changes, and equals 1330 K as derived by matching the
DTA (Fig. 2) and VTA results. All properties (the unit
cell parameters and microhardness) of the SrS and
Sr2Ga2S5 conjugate phases in the twoꢀphase region
remain unchanged.
There is a eutectic between the SrGa2S4 and
Sr2Ga2S5 phases at 42 mol % Ga2S3; this composition
was determined by the visual inspection of samples
annealed at 1070 K and the construction of the Tamꢀ
mann triangle. In the microstructure of a 40 mol %
Ga2S3 sample, we observed a grey field of the Sr2Ga2S5
phase and the eutectic; in the 45 mol % Ga2S3 sample,
we observed a light grey field of the SrGa2S4 phase and
the eutectic. A value of 1210 K was determined for the
eutectic temperature by averaging VTA and DTA data
(Fig. 2).
1330
60 mol % SrS – 40 mol % Ga2S3
Annealing 1070 K
1210
20 mol % SrS – 80 mol% Ga2S3
Annealing 1070 K
1310
Exo
1170
Endo
1230
800 900 1000 1100 1200 1300
1400
, K
T
Fig. 2. DTA curves for samples of the SrS–Ga S system
recorded on a DTAꢀ1M setup at a heating rate of 20 K/min.
2 3
The composition of the SrGa2S4 and ꢀGa2S3
γ
sition 70 mol % CaS–30 mol % Ga2S3, Xꢀray diffracꢀ
tion patterns showed reflections from Sr4Ga2S7 and
Sr2Ga2S5 (Fig. 3). Sr4Ga2S7 was indexed in terms of a
eutectic was 73 mol % Ga2S3 as derived from microꢀ
structure observation in samples from this region. In
polished sections of 55, 60, and 70 mol % Ga2S3 samꢀ
ples, eutectic grains were observed in the light grey
field of the SrGa2S4 phase; in polished sections of 75,
80, 85, 90, and 95 mol % Ga2S3 samples, eutectic
grains were observed in the light yellow field of the
faceꢀcentered cubic unit cell with the parameter
а =
0.6008 nm as calculated using Powder2.0 software.
The Xꢀray diffraction data for this compound are tabꢀ
ulated. A 80 mol % SrS–20 mol % Ga2S3 sample
annealed at 1070 K comprised two phases as deterꢀ
mined by powder Xꢀray diffraction and microstructure
examination, signifying the solidꢀphase decomposiꢀ
tion of the Sr4Ga2S7 phase. Indeed, the DTA heating
curve for a sample containing Sr4Ga2S7 featured an
exotherm associated with the decomposition of the
compound; this peak did not appear on the cooling
curve (Fig. 2). The Sr4Ga2S7 decomposition reaction is
γ
ꢀGa2S3 phase. The eutectic temperature is 1170 K as
determined by averaging DTA (Fig. 2) and VTA data.
We discovered an unknown phase of composition
Sr4Ga2S7 in the SrS–Ga2S3 system. Xꢀray diffraction
patterns for 80 mol % SrS–20 mol % Ga2S3 samples
annealed at 570 and 820 K, are qualitatively similar.
Reflections in these Xꢀray diffraction patterns do not
belong to any phase known to exist in this system, but
are very close to the structure of the SrS phase (Fig. 3).
Microstructure examination in these samples showed
Sr4Ga2S5(s)
and occurs at 870 K as determined by DTA.
A narrow field of a ꢀGa2S3ꢀbased solid solution is
2SrS(s) + Sr2Ga2S5(s)
one light grey colored phase (
Н
= 1100 MPa). From
γ
this, we suggested that an individual compound of formed in the SrS–Ga2S3 system; this solid solution
composition Sr4Ga2S7 is formed in the SrS–Ga2S3 sysꢀ crystallizes in monoclinic structure. By the strength of
tem for the component ratio 4SrS : 1Ga2S3, as in the microstructure and Xꢀray diffraction data,
CaS–Ga2S3 system [6] where a compound of compoꢀ 1070 K dissolves 4 mol % SrS. The unit cell volume
sition Sr4Ga2S7 exists. Xꢀray diffraction patterns for increases systematically from
= 0.503 nm3 (
= 0.7038 nm, = 121.22
= 0.513 nm3 (
= 1.122 nm, = 0.645 nm,
= 0.709 nm, = 121.22 ) for the endꢀmember of the
γꢀGa2S3 at
V
a =
) for
90 mol % CaS–10 mol % SrGa2S4 samples annealed 1.114 nm,
b
V
= 0.641 nm,
c
β
°
at 570 and 820 K, showed reflections from SrS and
γ
ꢀGa2S3 to
c
a
b
Sr4Ga2S7; for the same isothermal sections of compoꢀ
β
°
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 8 2010

1286
KERTMAN, KRAEVA
90 mol % SrS–10 mol % Ga S
2
3
Sr Ga S
2 7
4
70 mol % SrS–30 mol % Ga S
2
3
20
60
2θ, deg
Fig. 3. Xꢀray diffraction patterns for samples of the SrS–Ga S system annealed and quenched from 820 K (DRONꢀ6,
2 3
Co
K radiation, Fe filter).
α
solid solution ( V
= +0.010 nm3). As the unit cell volꢀ pound in certain stoichiometries obeying the law of
Δ
simple multiple proportions. All these compounds are
daltonide phases and are each indicated by a solid verꢀ
tical line in the phase diagram.
ume increases, the microhardness of samples from the
solid solution field decreases from 4950 and 3600 MPa
(ΔН = –1350 MPa). As temperature decreases, the
extent of the solid solution is reduced systematically to
become 3 mol % SrS at 820 K.
The phase diagram of the SrS–Ga2S3 system was
constructed using Edstate2D graphics editor. The
experimental liquidus lines are constructed on the
basis of averaged DTA and VTA data and fitted by a
secondꢀorder polynomials.
We have not found solid solutions based on SrS or
on SrGa2S4, Sr2Ga2S5, and Sr4Ga2S7 ternary comꢀ
pounds. The absence of solubility based on intermediꢀ
ate ternary compounds is dictated by the character of
their formation via chemical interactions of a comꢀ
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RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 8 2010