Thermal expansion and phase transformations of copper sulfides

Article abstract of DOI:10.1134/S0020168507060027

The microstructure and phase transformations of copper sulfides have been studied by thermal analysis, microstructural analysis, and x-ray diffraction during heating in an inert atmosphere. The unit-cell parameters of Cu _1.8S, CuS, Cu_1.96S, and a Cu_1.8+x S (0 < x ≤ 0.2) solid solution have been determined as functions of temperature.

Full text of DOI:10.1134/S0020168507060027

ISSN 0020-1685, Inorganic Materials, 2007, Vol. 43, No. 6, pp. 573–578. © Pleiades Publishing, Inc., 2007.
Original Russian Text © E.N. Selivanov, R.I. Gulyaeva, A.D. Vershinin, 2007, published in Neorganicheskie Materialy, 2007, Vol. 43, No. 6, pp. 653–658.
Thermal Expansion and Phase Transformations
of Copper Sulfides
E. N. Selivanov, R. I. Gulyaeva, and A. D. Vershinin
Institute of Metallurgy, Ural Division, Russian Academy of Sciences, ul. Amundsena 101, Yekaterinburg, 620016 Russia
e-mail: pcmlab@sky.ru
Received July 31, 2006; in final form, December 11, 2006
Abstract—The microstructure and phase transformations of copper sulfides have been studied by thermal anal-
ysis, microstructural analysis, and x-ray diffraction during heating in an inert atmosphere. The unit-cell param-
eters of Cu S, CuS, Cu1.96S, and a Cu1.8 + xS (0 < x ≤ 0.2) solid solution have been determined as functions of
1
.8
temperature.
DOI: 10.1134/S0020168507060027
INTRODUCTION
The temperature dependence of the lattice parame-
ter for the cubic phase γ-Cu S (derived from the 738-
2
Physicochemical and crystallographic information
about the Cu–S system is crucial for understanding the
chemical and metallurgical processes underlying cop-
per ore processing. There are considerable literature
and 773-K values [7]) is given by
–
5
a = 0.5514 + 2.857 × 10 T.
(4)
The thermal expansion data for the cubic sulfide
data on the phase diagram of the Cu–S system [1–3], Cu1.82S in the temperature range 373–773 K (four data
the compounds and minerals of this system [1–5], and
the thermodynamic [5], physical, and chemical proper-
ties of solid [2] and molten [6] sulfides.
points) can be represented by the equation
–
5
–9
2
a = 0.5518 + 1.8515 × 10 T – 3.778 × 10 T . (5)
In the measurement procedure used by Djurle [7], lat-
tice parameters were determined with an accuracy of
The results of extensive structural studies of copper
sulfides [7–13] were systematized by Vaughan and
Craig [1] and Samsonov and Drozdova [2]. At the same
time, because of the complex phase relations in the Cu–S
system, the large number of polymorphs, including
metastable phases, with a broad homogeneity range,
solid solutions, phase transitions, and peritectoidal
reactions, there are limited thermal expansion data
for copper sulfides, which were not included in
reviews [14].
±
0.0003–0.0010 nm and the temperature was main-
tained with a stability of ±1%. It is probably for this rea-
son that he did not evaluate thermal expansion coeffi-
cients from his data.
The purpose of this work was to study the unit-cell
thermal expansion and phase transformations of meta-
stable copper sulfides during heating in an inert atmo-
sphere.
The lattice parameter of cubic Cu2 – xS at 773 K is
related to the deviation from stoichiometry by [7]
EXPERIMENTAL
X-ray diffraction (XRD) measurements were per-
a = 0.5732 – 0.0556x.
(1) formed on a DRON-2.0 powder diffractometer (CoKα
radiation, diffracted-beam graphite monochromator)
The temperature-dependent lattice parameters of the
equipped with a UVD-2000 high-temperature attach-
hexagonal phase β-Cu S (data from Djurle [7] at 425, ment. XRD scans of powder samples were made during
2
5
73, and 733 K) are given by
heating in helium from room temperature to 1073 K
and subsequent cooling. The XRD data were used to
evaluate the lattice parameters of the sulfides. The lat-
tice parameters of the phases of interest (Cu1.96S and
CuS) in the as-prepared samples were determined by
least squares fitting among 13 to 20 reflections, using
single-crystal silicon (a = 0.543106 nm) and germa-
–
5
a = 0.3900 + 1.430 × 10 T,
(2)
(3)
–
5
c = 0.6586 + 3.010 × 10 T.
Note that the c/a ratio in β-Cu S varies little in the tem- nium (a = 0.56571 nm) internal standards. The error of
2
perature range in question.
determination was ±0.0001–0.0003 nm. The lattice
5
73

5
74
SELIVANOV et al.
cooled at a rate of ꢀ1.0 K/s. The sulfur content of sam-
ple 1 was 21.3 wt %.
Cu1.96
S
CuS (sample 2) was prepared by slowly (200 h)
heating an equimolar elemental powder mixture to
7
0
53 K in a silica ampule sealed off under a vacuum of
.1 Pa. The sample contained 31.8 wt % S.
Cu1.81
S
RESULTS AND DISCUSSION
According to XRD and microstructural data
Fig. 1), sample 1 consisted predominantly of a gray
(
1
00 µm
(
a)
phase of composition Cu1.96S, isostructural with meta-
stable, tetragonal djurleite [3, 12, 16]. The gray phase
was pierced with a small amount of blue platelike pre-
cipitates isostructural with the metastable low-temper-
ature sulfide Cu1.81S [16].
CuS
During heating to 823 K, the DSC curve of sample 1
Fig. 2) showed three endothermic events, in accor-
(
dance with the Cu–S phase diagram reported by Blach-
nik and Muller [12]. The lowest temperature event,
with onset at 347 K and a peak temperature of 353 K, is
the transformation of Cu1.81S from its low-temperature
(tetragonal) to high-temperature (cubic) form [17]. The
thermal event with an onset temperature of 365 K and a
peak temperature of 370 K is the tetragonal-to-cubic
phase transition of Cu1.96S. The broad endotherm with
onset at 429 K and a peak temperature of 446 K is
attributable to the formation of a Cu1.8 + x digenite-based
solid solution [3, 18].
Cu7S4
(
b)
Fig. 1. Microstructures of synthesized copper sulfide sam-
ples: (a) sample 1, (b) sample 2.
parameters were evaluated by a standard procedure
14], which was also used in earlier studies of sulfide
systems [15]. At high temperatures, the lattice parame-
ters were determined using five to ten reflections, with
an accuracy of ±0.0002–0.0004 nm. The temperature
was measured and maintained with an accuracy
of 0.7%.
The same thermal events were observed during
cooling of the sample, but they were shifted to higher
temperatures, especially those with peak temperatures
of 467 and 428 K. The large thermal hysteresis of the
phase transitions suggests that the polymorphic trans-
formations are first-order and may be accompanied by
the formation of metastable phases [14].
[
Microstructures were examined on a Neophot-2
optical microscope using SIMAGIS image process-
ing software. The temperatures of phase transitions
in sulfide samples were determined with a Netzsch
STA 449C Jupiter thermal analyzer during heating
and cooling at a rate of 5.0 K/min in flowing argon
From the XRD data for sample 1, the room-temper-
ature lattice parameters of the Cu1.96S in this sample
(
tetragonal djurleite structure, sp. gr. P4 2 2) were
3 1
determined to be a = 0.4003 nm and c = 1.1262 nm.
Prolonged (300 h) annealing at 920 K in evacuated sil-
ica ampules and subsequent cooling had an insignifi-
cant effect of the lattice parameters of Cu1.96S (a =
.40025 nm, c = 1.1261 nm). Our results agree well
with earlier data [1, 2, 6].
(
30 ml/min). The samples were contained in alumina
crucibles for differential scanning calorimetry
DSC). The equipment and software used enabled us
0
(
to detect thermal events leading to temperature dif-
ferences between the sample and reference substance
as small as 0.1 K and to determine their temperatures
with an accuracy of ±1.5 K.
According to our high-temperature XRD data
(
Fig. 3), heating sample 1 to above 393 K leads to the
formation of high-temperature, cubic digenite, which
The starting chemicals used were electrolytic cop- can be thought of as a cation-deficient solid solution
per powder (PM 1 copper, 99.7% purity) and elemental extending from Cu S to ꢀCu1.73S [1]. The reflections
2
sulfur (OSCh 16-6, 99.98% purity). Cu1.96S (sample 1) from cubic Cu1.96S persist to 453 K. From our measure-
was synthesized by melting (1473 K) copper powder ment results (Fig. 4), we derived linear best fit equa-
with an excess (10%) of sulfur in an alundum crucible tions for the temperature-dependent (393–1073 K)
under an N –25% CO atmosphere. The sample was unit-cell parameters of the high-temperature digenite-
2
INORGANIC MATERIALS Vol. 43 No. 6 2007

THERMAL EXPANSION AND PHASE TRANSFORMATIONS OF COPPER SULFIDES
575
DSC, µV
(a)
3
54
428
437
Cooling
4
3
2
1
0
1
2
3
4
4
67
EXO
3
59
489
–
–
–
–
3
47 365
429
Heating
3
53
446
3
70
3
00
400
500
600
700
800
DSC, µV
EXO
0
0.5
1.0
1.5
2.0
(
b)
–
–
–
–
3
46
Heating
360
3
00
340
380
420
400
T, K
Fig. 2. DSC curves of samples (a) 1 and (b) 2 recorded during heating and cooling at a rate of 5.0 K/min in argon.
based solid solution (determination coefficients of
.975 and 0.976, respectively):
The copper sulfide synthesized by the sealed-
ampule technique (sample 2) was nonuniform in phase
composition (Fig. 1) and consisted of covellite (CuS)
0
–
5
[
16] and anilite (Cu S ) (Fig. 3). Thermal analysis of
a = 0.5520 + 2.17 × 10 T,
(6)
(7)
7 4
sample 2 at temperatures below 463 K (Fig. 2) revealed
a small endotherm with an onset temperature of 346 K
and a peak temperature of 360 K, attributable to anilite
decomposition [18], in accordance with phase-diagram
data [12].
–
5
V = 0.1678 + 2.10 × 10 T.
The thermal expansion coefficients of this phase are
–
5
–1
–4 –1
α = 1.22 × 10 K and α = 1.18 × 10 K . The high
a
V
value of α indicates that, in addition to thermal expan-
V
The majority phase in sample 2, CuS, had a hexag-
sion proper, there is chemical strain associated with the onal structure (sp. gr. P6 /mmc) with lattice parameters
3
formation of a solid solution of Cu S in digenite. Cool-
2
a = 0.3792 nm and c = 1.6344 nm, in reasonable agree-
ing the sample to room temperature causes the high- ment with earlier data [7]. The only effect of heating the
temperature digenite-based solid solution to decom- sample to 373 K was the thermal expansion of CuS, as
pose into tetragonal Cu1.96S and, depending on the evidenced by the constant axial ratio c/a = 4.31. The ‡-
annealing temperature, Cu S (T
Cu1.81S (Tann = 870 K).
= 580 K) or and Ò-axis thermal expansion coefficients in the range
7
.2
4
ann
–
6
–1
293–373 K were determined to be α = 1.76 × 10 K
a
INORGANIC MATERIALS Vol. 43 No. 6 2007

5
76
SELIVANOV et al.
Cu1.96
Cu1.81
S
S
(a)
5
4
3
2
1
2
4
32
40
48
56 2θ, deg
(b)
CuS
Cu7S4
5
4
3
2
1
3
2
40
48
56
2θ, deg
Fig. 3. Portions of XRD patterns from copper sulfides: (a) sample 1 at (1) 298, (2) 393, (3) 503, (4) 673, and (5) 973 K; (b) sample 2
at (1) 298, (2) 513, (3) 613, (4) 683, and (5) 873 K.
and α = 3.47 × 10–6 K . Reflections from Cu1.8 + xS ficients were determined to be α = 7.4 × 10 K , α =
–1
–6 –1
c
a
c
–
6
–1
–5 –1
were detected in our measurements starting at 393 K. In
9
.4 × 10 K , and α = 2.42 × 10 K .
V
the temperature range 393–613 K, CuS and Cu1.8 + x
coexisted, and the CuS content was found to decrease
with increasing temperature. The CuS Cu1.8 + x
phase transition was accompanied by the release of ele-
mental sulfur into the gas phase and was irreversible in dependent unit-cell parameters (a, c, and V) and ther-
the flow-type measuring cell we used. Above 393 K, the mal expansion coefficients (α and α ) of these phases
S
For the temperature stability ranges of CuS (mea-
surement range 393–613 K) and Cu1.8 + xS (683–928 K),
we obtained best fit polynomials for the temperature-
S
a
V
unit-cell parameters of CuS rise steeply (Fig. 5), and the (table):
c/a ratio varies constantly, attesting to the development
of chemical strain in CuS. The thermal expansion coef-
–
5
–8
2
P = K
+ K
× 10 T + K
× 10 T .
(8)
0
1
2
INORGANIC MATERIALS Vol. 43 No. 6 2007

THERMAL EXPANSION AND PHASE TRANSFORMATIONS OF COPPER SULFIDES
577
‡, nm
‡, nm
0
0
0
.3804
.3796
.3788
0
0
0
.574
.566
.558
Ò, nm
1
.6392
.6372
.6352
V, nm3
1
1
1
0
.188
0
0
.180
.6332
.172
3
V, nm3
50
550
750
950
í, ä
0
0
0
.205
.204
.203
Fig. 4. Temperature dependences of unit-cell parameters for
a high-temperature digenite-based solid solution (heating of
sample 1 in helium).
2
70
370
470
570
í, ä
In the temperature range 683–928 K, copper mono-
sulfide fully converts to a digenite-based cubic solid
solution, Cu1.8 + xS. The unit-cell parameters of
Cu1.8 + xS rise steadily with temperature (Fig. 6). Cool-
ing sample 2 after annealing in this temperature range
leads to consecutive formation of phases from Cu S to
Fig. 5. Temperature dependences of unit-cell parameters for
CuS (heating of sample 2 in helium).
Our results demonstrate that heating sample 2 gives
1
.8
Cu1.96S and Cu S. In particular, the samples cooled rise to a variety of phase transitions, which can be rep-
2
after annealing at temperatures from 683 to 813 K were resented by the overall scheme CuS
found to contain Cu S and Cu1.96S. After annealing at that cooling leads to the Cu1.8 + x
Cu1.8 + xS, and
S
Cu1.96S (Cu S)
1
.8
2
8
73 K, the samples contained Cu1.96S and Cu S.
phase transition.
2
Temperature-dependent unit-cell parameters and thermal expansion coefficients of CuS and Cu1.8 + xS (sample 2)
Sulfide
Parameter
K0
K1
K2
R2
CuS
a
0.37896
1.63065
0.2023
0.0667
1.5377
–
0.965
0.901
0.943
0.998
0.998
0.999
0.999
c
–
–
V
0.4969
Cu1.8 + x
S
a
0.50124
0.1202
12.722
–5.13
–4.69
–
V
11.899
αa
αV
0.000127
0.000119
–0.01026
–0.00938
–
INORGANIC MATERIALS Vol. 43 No. 6 2007

5
78
SELIVANOV et al.
‡
, nm
2. Samsonov, G.V. and Drozdova, S.V., Sul’fidy (Sulfides),
Moscow: Metallurgiya, 1972.
0
0
0
0
.574
3. Drits, M.E., Bochvar, N.R., Guzei, L.S., et al., Dvoinye i
mnogokomponentnye sistemy na osnove medi (Binary
and Multicomponent Copper-Based Systems), Moscow:
.570
.566
.562
Nauka, 1979.
^
4
5
. Kostov, I. and Minceva-Stefanova, J., Sulphide Miner-
als, Sofia: Acad. Sci., 1981.
. Morachevskii, A.G., Ryabko, A.G., and Tsemekh-
man, L.Sh., Termodinamika sistemy med’–sera (Ther-
modynamics of the Copper–Sulfur System), St. Peters-
burg: Politekhn. Univ., 2004.
V, nm3
0
6. Vaisburd, S.E., Fiziko-khimicheskie svoistva i osoben-
nosti stroeniya sul’fidnykh rasplavov (Physicochemical
Properties and Structural Aspects of Sulfide Melts),
Moscow: Metallurgiya, 1996.
.190
.186
.182
7
. Djurle, S., An X-ray Study on the System Cu–S, Acta
Chem. Scand., 1958, no. 12, pp. 1415–1426.
0
8
. Shah, I.D. and Khalafalla, S.E., Thermal Decomposition
of CuS to Cu S, Metall. Trans., 1971, vol. 2, pp. 605–
1
.8
0
0
6
06.
9
. Roseboom, E.H., An Investigation of the System Cu–S
and Some Natural Copper Sulfides between 25° and
.178
6
7
00°C, Econ. Geol., 1966, vol. 61, no. 4, pp. 641–672.
50
750
850
950
í, ä
10. Vanyukov, A.V., Bystrov, V.P., and Snurnikov, V.A,
Phase Equilibria in the Copper–Sulfur System, Tsvetn.
Met., 1971, no. 11, pp. 11–15.
Fig. 6. Temperature dependences of unit-cell parameters for
Cu1.8 + xS (heating of sample 2 to above 613 K).
1
1. Snurnikov, V.A., Bystrov, V.P., and Vanyukov, A.V.,
Thermochemical Properties of the Copper–Sulfur Sys-
tem near Stoichiometric Copper Sulfide, Izv. Vyssh.
Uchebn. Zaved., Tsvetn. Metall., 1971, no. 6, pp. 34–36.
The digenite-based cubic solid solutions obtained
1
1
2. Blachnik, R. and Muller, A., The Formation of Cu2S
from the Elements: 1. Copper Used in Form of Powders,
Thermochim. Acta, 2000, no. 361, pp. 31–52.
3. Blachnik, R. and Muller, A., The Formation of Cu2S
from the Elements: 2. Copper Used in the Foils, Thermo-
chim. Acta, 2001, no. 366, pp. 47–59.
from CuS differ in unit-cell parameters from those
obtained from Cu1.96S, because of the difference
between their compositions.
CONCLUSIONS
Using thermal analysis and XRD data for the syn- 14. Filatov, S.K., Vysokotemperaturnaya kristallokhimiya
(
High-Temperature Crystal Chemistry), Leningrad:
thesized samples, we accurately determined the phase-
transition temperatures of copper sulfides forming dur-
Nedra, 1990.
ing heating in an inert atmosphere and obtained best fit 15. Vershinin, A.D., Selivanov, E.N., and Danilushkin, A.L.,
Thermal Expansion and Decomposition of FeS2 – x and
CuFeS2 – x, Neorg. Mater., 1998, vol. 34, no. 12,
pp. 1423–1427 [Inorg. Mater. (Engl. Transl.), vol. 34,
no. 12, pp. 1198–1202].
polynomials for the temperature dependences of their
unit-cell parameters. The unit-cell parameters of the
high-temperature digenite-based solid solutions are
shown to depend on their composition.
1
6. Powder Diffraction File, Newtown Square: Int. Centre for
Diffraction Data, 2001, cards 17-0449, 41-0959, 6-0464,
ACKNOWLEDGMENTS
3
3-0489.
This work was supported by the Russian Foundation 17. Vaissiere, J. and Roche, F., Contribution a l'étude du dia-
for Basic Research, grant no. 05-03-32156.
gramme de phase du système CuxS (1.75 ≤ x ≤ 2) entre
1
0° et 200°C, Mater. Res. Bull., 1976, vol. 11, pp. 851–
8
56.
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8. Morimoto, N. and Koto, K., Phase Relation of the Cu–S
System at Low Temperatures: Stability of Anilite, Am.
Mineral., 1970, vol. 55, pp. 106–117.
1
. Vaughan, D and Craig, J., Mineral Chemistry of Metal
Sulfides, Cambridge: Cambridge Univ. Press, 1978.
INORGANIC MATERIALS Vol. 43 No. 6 2007