Phase equilibria in the systems SrS-Cu2S-Ln2S 3 (Ln = la or Nd)

Article abstract of DOI:10.1134/S0036023607040237

Phase equilibria in the systems SrS-Cu₂S-Ln₂S ₃ (Ln = La or Nd) have been studied along the isothermal section at 1050 K and vertical sections CuLnS₂-SrS and Cu₂S- SrLnCuS₃, which are partially quasibinary joins. Compounds SrLnCuS₃ with Ln = La or Nd have been synthesized for the first time. They crystallize in orthorhombic space group Pnma, the BaLaCuS₃ structure type, with the following unit cell parameters: for SrLaCuS ₃, a = 1.1157(2) nm, b = 0.41003(6) nm, c = 1.1545(2) nm; for SrNdCuS₃, a = 1.1083(1) nm, b = 0.40887(7) nm, c = 1.1477(2) nm. Noticeable homogeneity regions for SrLnCuS₃ are not found. The compounds melt congruently by the reaction SrLnCuS₃ SrS + L at 1365 K for SrLaCuS₃ and 1400 K for SrNdCuS₃. The tie-lines at 1050 K in the systems SrS-Cu₂S-Ln₂S₃ radiate from SrLnCuS₃ toward phases SrS, Cu₂S, CuLnS₂, and SrLn₂S₄, lying between the phases CuLnS₂ and compositions from the γ-Ln₂S₃-SrLn ₂S₄ solid-solution field. Eutectics are formed between the compounds CuLaS₂ and SrLaCuS₃ at 21.0 mol % SrS, T = 1345 K; between the compounds CuNdS₂ and SrNdCuS₃ at 31.0 mol % SrS, T = 1310 K; and between the phases Cu₂S and SrLnCuS ₃ at 14.0 mol % SrLaCuS₃, T = 1075 K and 8.0 mol % SrNdCuS₃, T = 1055 K. Nauka/Interperiodica 2007.

Full text of DOI:10.1134/S0036023607040237

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 4, pp. 605–609. © Pleiades Publishing, Inc., 2007.
Original Russian Text © N.V. Sikerina, O.V. Andreev, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 4, pp. 665–669.
PHYSICOCHEMICAL ANALYSIS
OF INORGANIC SYSTEMS
Phase Equilibria in the Systems SrS–Cu2S–Ln2S3 (Ln = La or Nd)
N. V. Sikerina and O. V. Andreev
Tyumen State University, Tyumen, Russia
Received August 1, 2006
Abstract—Phase equilibria in the systems SrS–Cu2S–Ln2S3 (Ln = La or Nd) have been studied along the iso-
thermal section at 1050 K and vertical sections CuLnS2–SrS and Cu2S–SrLnCuS3, which are partially quasi-
binary joins. Compounds SrLnCuS3 with Ln = La or Nd have been synthesized for the first time. They crystal-
lize in orthorhombic space group Pnma, the BaLaCuS3 structure type, with the following unit cell parameters:
for SrLaCuS3, a = 1.1157(2) nm, b = 0.41003(6) nm, c = 1.1545(2) nm; for SrNdCuS3, a = 1.1083(1) nm,
b = 0.40887(7) nm, c = 1.1477(2) nm. Noticeable homogeneity regions for SrLnCuS3 are not found. The com-
pounds melt congruently by the reaction SrLnCuS3
SrS + L at 1365 K for SrLaCuS3 and 1400 K for
SrNdCuS3. The tie-lines at 1050 K in the systems SrS–Cu2S–Ln2S3 radiate from SrLnCuS3 toward phases SrS,
Cu2S, CuLnS2, and SrLn2S4, lying between the phases CuLnS2 and compositions from the γ-Ln2S3–SrLn2S4
solid-solution field. Eutectics are formed between the compounds CuL‡S2 and SrL‡CuS3 at 21.0 mol % SrS,
í = 1345 K; between the compounds CuNdS2 and SrNdCuS3 at 31.0 mol % SrS, í = 1310 K; and between the
phases Cu2S and SrLnCuS3 at 14.0 mol % SrLaCuS3, í = 1075 K and 8.0 mol % SrNdCuS3, í = 1055 K.
DOI: 10.1134/S0036023607040237
No data exist on phase equilibria in the quasi-ter-
nary systems SrS–Cu2S–Ln2S3 with Ln = La or Nd.
Only their boundary binary systems are described in the
literature: Cu2S–Ln2S3, Ln2S3−SrS(Ln = La, Nd), and
Cu2S–SrS. The systems Cu2S–La2S3 and Cu2S–Nd2S3
form complex sulfides CuLaS2 and CuNdS2, which
melt incongruently at 1471 and 1465 K, respectively. A
eutectic is formed between the phases Cu2S and
CuLnS2 [1, 2]. The systems La2S3–SrS and Nd2S3–SrS
form the complex compounds SrLa2S4 and SrNd2S4,
EXPERIMENTAL
The compound Cu2S was prepared from the high
purity grade constituent elements (copper os.ch. 11-4
and sulfur os.ch. 16.5; Russia) in evacuated and sealed
off fused silica ampoules [5]. The sulfides SrS and
Ln2S3 were synthesized from SrSé4 (chemically pure
grade, Russia) and Ln2é3 (LaO-D and NO-D grade
samples, Russia) in flowing H2S and CS2 at 1300 K [6].
SrS–Cu2S–Ln2S3 (Ln = La, Nd) alloys were alloyed
from SrS, Cu2S, and Ln2S3 in a graphite crucible inside
which melt incongruently at 2290 and 2255 K, respec- an open fused silica reactor. The reactor was preevacu-
ated and purged with argon. The crucible was induc-
tively heated in an RF generator. A sulfur-containing
gas atmosphere was created in the reactor, which was
necessary for avoiding the thermal dissociation of the
starting sulfides [6]. Samples with compositions lying
along the joins Cu2S–SrLnCuS3 were synthesized in a
sealed-off fused silica ampoule. The samples, con-
tained in evacuated and sealed off fused silica
ampoules, were annealed at 1050 K for 500 h. At
1350−2000 K, the alloys were exposed for 20–30 min
to a sulfur-containing gas atmosphere in a graphite cru-
cible inductively heated inside an open reactor. The
annealing time ensured the equilibration of the sam-
ples.
Samples of the complex sulfides SrLaCuS3 and
SrNdCuS3 were prepared by alloying the constituent
sulfides in a graphite crucible placed inside a sealed-off
fused silica ampoule.
Powder X-ray diffraction was carried out on a DRON 6
diffractometer using CÓKα radiation. Microstructure
tively. A continuous solid solution with a Th3P4-type
structure is formed between the phases γ-Ln2S3 and
SrLn2S4 [3]. The system Cu2S–SrS has a eutectic phase
diagram with limited solubility on the basis of ëu2S.
The melting temperature of the eutectic is 1095 K; the
composition is 21.5 mol % SrS [4].
The investigations of phase equilibria in the systems
SrS–Cu2S–Ln2S3 (Ln = La, Nd) are topical for the fol-
lowing reasons: this combination of s-, d-, and 4f-metal
sulfides creates prerequisites for the formation of new
compounds; at the same time, this provides the basis for
choosing synthesis parameters for these new com-
pounds.
One goal of this work was to study phase equilibria
in the systems SrS–Cu2S–Ln2S3 with Ln = La or Nd
along the 1050 K isothermal section and vertical sec-
tions. The other goal was to determine the composition
and structurally characterize new complex sulfides.
605

606
SIKERINA, ANDREEV
La S
2
Nd S
2
3
3
4
3
4
3
2
2
5
5
1
SrNdCuS
1
SrLaCuS
3
3
1050
1095
SrS
1050
1090
SrS
Cu S
Cu S
2
2
γ-Cu S + L
γ-Cu S + SrS
2
2
21.5%
21.5%
1450
1850
2250
1450
1850
2250
γ- Cu S + L
SrS + L
γ-Cu S + L
2
2
SrS + L
L
L
2650
T, K
2650
T, K
Fig. 1. Tie-lines in the systems SrS–Cu S–La S and SrS–Cu S–Nd S at 1050 K. Dots indicate experimentally studied composi-
2
2
3
2
2 3
tions.
was examined on a Metam PB microscope. Visual poly- SrLnCuS3–SrS, SrLnCuS3–CuLnS2, and Cu2S–SrLnCuS3,
thermal analysis (VPA) and differential thermal analy-
sis (DTA) were carried out as described previously [6].
The precision was up to 1% of the measured value in
VPA and 0.1–0.2% in DTA. Edstate 2D and Edstate 3D
software was used for graphic design. The unit cell
parameters were calculated using Powder2 software.
The phases SrLaCuS3 and SrNdCuS3 were identified
with reference to the Powder Diffraction File PDF2
(JCPDS-ICDD, Release 2002). X-ray structure refine-
ment was carried out using GSAS software [7]. The
starting model used included data for the isostructural
compound BaLaCuS3 [8].
as well as between the phase CuLnS2 and compositions
from the γ-Ln2S3–SrLn2S4 solid solution (ss) field. Five
subordinate triangles were recognized in each of the
systems SrS–Cu2S–Ln2S3; in Fig. 1, they are indicated
by figures. The compounds SrLnCuS3 are in equilib-
rium with the phases CuLnS2 and SrS; they are simul-
taneously formed along the join CuLnS2–SrS, which
makes the investigation of this join pertinent. Of the
other phases in equilibrium with the compound
SrLnCuS3, the only congruently melting phase is Cu2S.
The character of phase equilibria along the join
Cu2S−SrLnCuS3 determines many features of equilib-
ria in the relevant portion of the triangle, which makes
it necessary to construct a phase diagram for this join.
RESULTS AND DISCUSSION
Previously unknown compounds SrLnCuS3 (Ln =
La or Nd) are formed in the systems SrS–Cu2S–Ln2S
(Ln = Ln or Nd) when the component ratio is 2SrS :
1Cu2S : : 1Ln2S3. The compounds SrLaCuS3 and
SrNdCuS3 have a BaLaëuS3-type structure, space
group Pnma. The parameters of the orthorhombic unit
cell for SrLaCuS3 are ‡ = 1.1157(2) nm, b = 0.41003(6)
nm, Ò = 1.1545(2) nm; for SrNdCuS3, ‡ = 1.1083(1) nm,
Joins CuLaS2–SrS and CuNdS2–SrS
In the systems CuLnS2–SrS (Fig. 2), complex sul-
fides SrLnCuS3 are formed when the component ratio is
1CuLnS2 : 1SrS. These complex sulfides melt incon-
gruently by the reaction SrLnCuS3
ssSrS + L at
1365 K for SrLaCuS3 and 1400 K for SrNdCuS3.
Homogeneity ranges for compounds SrLnCuS3 and
b = 0.40887(7) nm, Ò = 1.1477(2) nm. Microstructure CuLnS2 were not found. Samples with chemical com-
observation and powder X-ray diffraction confirmed
that complex sulfide SrLnCuS3 is in equilibrium with
simple and complex sulfides of the boundary systems
Cu2S–Ln2S3, Ln2S3–SrS, and Cu2S–SrS. This fact enabled
positions approaching phases SrLnCuS3 and CuLnS2
consist of two phases, and the amount of the second
phase correlates the position of the sample in the dia-
gram. The positions of reflections of compounds
us to determine the positions of tie-lines at 1050 K SrLnCuS3 and CuLnS2 in the two-phase samples were
(Fig. 1). The tie-lines were drawn between the conjugate
phases CuLnS2–SrLn2S4, SrLnCuS3–SrLn2S4,
the same as in homogeneous and stoichiometric sam-
ples. SrS is the base of a limited solid solution. Within
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 4 2007

PHASE EQUILIBRIA IN THE SYSTEMS SrS–Cu2S–Ln2S3
607
1
2
3
4
T, K
T, K
2300
2300
L
Nd S + L
2
3
L
La S + L
2
3
CuNdS + Nd S + L
2
2 3
CuLaS + La S + L
2
2 3
1900
1900
SrLaCuS
L + SrS
L + SrS
3
SrNdCuS
3
CuNdS + L
2
CuLaS + L
2
L + SrNdCuS
1500
1100
700
1500
1100
700
3
L + SrLaCuS
3
1365
1400
21%
1345
31% 1310
CuLaS + SrLaCuS
SrLaCuS + SrS
CuNdS + SrNdCuS
SrNdCuS + SrS
3
2
3
3
2
3
CuLaS
20
40
60
80
SrS CuNdS
20
40
60
80
SrS
2
2
mol % SrS
mol % SrS
Fig. 2. CuLaS –SrS and CuNdS –SrS phase diagrams. (1) DTA data; (2) full melting of the sample according to VPA; (3) single-
2
2
phase samples; (4) two-phase samples as determined by powder X-ray diffraction and microstructure examination.
the solid solution, the experimental SrS unit cell 1310 K. A characteristic feature of the system
parameter versus composition data show a scatter of CuLaS2−SrS is the closeness of the temperature of the
0.006−0.007 nm; this scatter did not allow us to estab-
lish a correlation. The extent of the solid solution was
determined from microstructure and powder X-ray dif-
fraction data. The solvus line at 1050 K and the solidus
line at 1650 K pass through the point 97.0 mol % SrS.
The solubility at the incongruent-melting temperature of
compounds SrLnCuS3 is 5.0 mol % CuLnS2. The tem-
perature dependence of the position of the solvus line as
a function of composition is confirmed by the micro-
structure of aliquots of the samples. Inside all primary
strontium sulfide grains, there are needle-like
SrLnCuS3 crystals that appeared during annealing as
a result of the demixing of the SrS ss.
eutectic between phases CuLaS2 and SrLaCuS3 (1345 K)
and the incongruent-decomposition temperature of the
phase SrLaCuS3(1365 ä). Microstructurally, the
eutectic between CuLaS2 and SrLaCuS3 appears as an
alternation of oval crystals of the conjugate phases
10–20 µm in size. The eutectic between the phases
CuNdS2 and SrNdCuS3 consists of coarse grains both
in annealed samples and in those cooled from melt. The
eutectic crystals of the phases CuNdS2 and SrNdCuS3
have an oval shape; their sizes are 60–80 µm.
The DTA traces for all aliquots of the samples with
compositions lying in the region CuLnS2–eutectic dem-
onstrate a peak due to the melting of the eutectic and a
peak due to the melting of primary gains of the com-
pound CuLnS2. The specific feature of the system
CuNdS2–SrS is the diffuse character of the peaks asso-
ciated with the melting of primary CuNdS2 crystals.
The cooling traces display well-defined peaks associ-
ated with the crystallization of primary CuNdS2 grains.
The liquidus temperatures are set equal to the arithmetic
mean between the melting temperatures upon heating
and the crystallization temperatures upon cooling.
The CuLnS2–eutectic and eutectic–SrS liquidus
lines consist of two branches, each built by fitting the
experimental (DTA or DTA plusVPA) data to a second-
order polynomial.
Visual polythermal analysis, to which the systems
CuLnS2–SrS were subjected in order to design parts of
the eutectic–SrS liquidus line, confirms the incongruent
melting character of SrLnCuS3. The appearance of
a liquid phase in aliquots was observed within a narrow
Below the incongruent decomposition temperature
of SrLnCuS3 in the region of 50–100 mol % SrS, phases
SrLnCuS3 and SrS are in equilibrium, and only reflec-
tions of these phases appear in the X-ray diffraction
patterns of annealed samples. The similarity of the
microstructure of aliquots of the samples with compo-
sitions in the range 50–100 mol % SrS confirms the
incongruent melting of SrLnCuS3. The samples contain
chains of SrS grains in the background of crystals of the
phase SrLnCuS3, which is intrinsic to two-phase fields
with a peritectic.
In the region of 0–50 mol % SrS, a eutectic is
formed between the phases CuLnS2 and SrLnCuS3.
The eutectic composition was determined from micro-
structure and DTA data. The coordinates of the eutectic
formed between CuLaS2 and SrLaCuS3 are 21.0 mol %
SrS, T = 1345 K; for the eutectic between CuNdS2 and
SrNdCuS3, the coordinates are 31.0 mol % SrS, í =
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 4 2007

608
SIKERINA, ANDREEV
T, K
T, K
1500
1500
SrS + L
SrS + L
L
L
Cu S + L
Cu S + L
2
SrNdCuS + SrS + L
2
3
SrLaCuS + SrS + L
1300
1100
900
1300
1100
900
3
SrLaCuS + L
3
SrNdCuS + L
3
14%
1075
8%
1055
γ
γ
γ + SrLaCuS
γ + SrNdCuS
3
3
700
500
700
500
620
370
620
370
β + SrLaCuS
β + SrNdCuS
3
3
β
β
α + SrLaCuS
α + SrNdCuS
3
3
α
α
SrLaCuS
SrNdCuS
3
Cu S
20
40
60
80
Cu S
20
40
60
80
3
2
2
mol % SrLaCuS
mol % SrLaCuS
3
3
Fig. 3. Cu S–SrLaCuS and Cu S–SrNdCuS phase diagrams. For the notation, see Fig. 2.
2
3
2
3
range of temperatures and coincided with the DTA eutectic coordinates are as follows: 14.0 mol %
determinations of the melting temperature for the phase SrLaCuS3, í = 1075 K. The microstructure of a eutectic
SrLnCuS3.
sample displays a eutectic mixture of needle-shaped
crystals of Cu2S and SrLaCuS3. The eutectic grains are
20–40 µm long. The microstructure observations are
confirmed by DTA results. Only one peak at 1075 K
due to the melting of the eutectic is observed in the
DTA trace for a sample containing 14.0 mol %
SrLaCuS3 in addition to the peak associated with the
The systems CuLnS2–SrS are partially quasi-binary
joins of the triangles Cu2S–Ln2S3–SrS (Ln = La, Nd).
The quasi-binary character of the joins is spoiled above
the incongruent-melting temperature of CuLaS2 and
CuNdS2. Below these temperatures, equilibrium in the
systems CuLnS2–SrS involves the conjugate phases
CuLnS2 and SrLnCuS3, and SrLnCuS3 and SrS, respec-
tively.
polymorphic transition α-Cu2S
β-Cu2S. The shape
of this peak implies that in the phase diagram, melting
is represented by invariant phase equilibrium.
Thus, complex sulfides SrLnCuS3 are formed in the
systems CuLnS2–SrS. A eutectic is formed between the
compounds CuLnS2 and SrLnCuS3. There is a limited
solid-solution field on the base of SrS.
All DTA traces for the annealed samples of the sys-
tem Cu2S–SrLaCuS3 demonstrate a peak associated
with the polymorphic transition α-Cu2S
β-Cu2S.
The polymorphic-transition temperature decreases
slightly from 376 to 370 K, proving the existence of a
small field of Cu2S-base solid solution. The insignifi-
cant extent of the solid solution makes microstructure
observation difficult. However, we synthesized a sam-
ple containing 1.0 mol % SrLaCuS3 in which needle-
shaped SrLaCuS3 crystals have larger sizes than eutec-
tic crystals and are uniformly distributed over the sam-
ple. The sizes and distribution of SrLaCuS3 grains
imply that the grains were formed upon the demixing of
the primary solid Cu2S-base solution. The DTA peak
associated with the eutectoid transformation β-Cu2Sss +
Joins Cu2S–SrLaCuS3 and Cu2S–SrNdCuS3
The system Cu2S–SrLaCuS3 (Fig. 3) is a partially
quasi-binary join of the system SrS−Cu2S–La2S3. Its
quasi-binary character is spoiled near the compound
SrLaCuS3 above its incongruent melting temperature.
A field SrS + L appears in the phase diagram, and a
third phase (SrS) is observed in the microstructure of
samples cooled from melt and containing more than
66.7 mol % SrLaCuS3. After annealing at 750°C, SrS
crystals disappear. At temperatures below the
SrLaCuS3 decomposition temperature, the starting
phases Cu2S and SrLaCuS3 are in equilibrium. The
X-ray diffraction patterns of samples annealed at 750 K
display only reflections of α-Cu2S and SrLaCuS3.
SrLaCuS3
γ-Cu2S ss is detected at 620 K only in an
aliquot of the sample that contains 1.0 mol %
SrLaCuS3. The value of this peak only insignificantly
differs from the noise level; this eutectoid transforma-
tion is indicated by a dashed line in the diagram.
The Cu2S–SrLaCuS3 system has a eutectic phase
diagram with a limited Cu2S-base solid solution. The point of Cu2S is drawn through two intermediate points.
The liquidus branch from the eutectic to the melting
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 4 2007

PHASE EQUILIBRIA IN THE SYSTEMS SrS–Cu2S–Ln2S3
609
The trend of the line is nearly linear. The liquidus line
ACKNOWLEDGMENTS
between the eutectic and SrLaCuS3 consists of two
branches, which meet at the incongruent-melting tem-
perature of SrLaCuS3 (at 1365 K). The branches are the
second-order-polynomial best-fit curves obtained for
the DTA data (until the curves meet the peritectic hori-
zontal of SrLaCuS3) and DTA and VPTA data (from the
meeting with the peritectic horizontal to the SrLaCuS3
line).
This work was supported by the Russian Foundation
for Basic Research (project no. 01-03-333-22A) and a
grant-in-aid of the International Center for Diffraction
Data (April 1, 2006).
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formed between phases Cu2S and SrNdCuS3; its com-
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