Physical and optical properties of the quaternary sulfides SrCu2MS4 and EuCu2MS4 (M=Ge and Sn)

Article abstract of DOI:10.1016/S0022-4596(03)00093-8

Four quaternary sulfides SrCu₂MS₄ and EuCu₂MS₄ (M=Ge and Sn) were prepared from a thoroughly ground mixture of EuS or SrS, Cu, or Sn, and S in stoichiometric proportions. Electrical conductivity measurements on pressed pellets showed that all the phases are semiconductors. The optical band gaps were assessed at 2.8eV for SrCu₂GeS₄, 2.1eV for SrCu₂SnS₄, 2.2eV for EuCu₂SnS₄, and 1.6eV for EuCu₂GeS₄. Both Sr-based compounds present a temperature-independent paramagnetism, of about +135×10^-6 and +92×10^-6emu/mol, for SrCu₂SnS₄ and SrCu₂GeS₄, respectively. In the case of the europium compounds, they follow a Curie-Weiss dependence above 1.8K (EuCu₂GeS₄) and above 4K (for EuCu₂SnS₄), with values of the magnetic effective moment μ_eff and the Curie-Weiss temperature Θ, equal to 6.27μ_B and -2.8K for EuCu₂GeS₄, and 6.81μ_B and +0.7K, for EuCu₂SnS₄. The experimental magnetic moments confirm that the europium ion is in divalent state, similar to Sr in the related compounds.

Full text of DOI:10.1016/S0022-4596(03)00093-8

Journal of Solid State Chemistry 173 (2003) 78–82
Physical and optical properties of the quaternary sulfides SrCu MS4
2
and EuCu MS (M=Ge and Sn)
2
4
a,ꢀ
a
Jaime Llanos, Carlos Mujica, Vı
a
nchez, and Octavio Pen
b
´
ctor Sa
´
˜
Departamento de Quimica, Facultad de Ciencias, Universidad Cat o´ lica del Norte, Casilla 1280, Antofagasta, Chile
a
a
b
Laboratoire de Chimie du Solide et Inorganique Mol e´ culaire, UMR 6511-CNRS, Universit e´ Rennes 1-Institut de Chimie de Rennes,
3
5042 Rennes Cedex, France
Received 8 October 2002; received in revised form 20 January 2003; accepted 31 January 2003
Abstract
Four quaternary sulfides SrCu
or SrS, Cu, or Sn, and S in stoichiometric proportions. Electrical conductivity measurements on pressed pellets showed that all
the phases are semiconductors. The optical band gaps were assessed at 2.8 eV for SrCu GeS , 2.1 eV for SrCu SnS , 2.2 eV for
EuCu SnS , and 1.6 eV for EuCu GeS . Both Sr-based compounds present a temperature-independent paramagnetism, of about
135 ꢁ 10 and +92 ꢁ 10 emu/mol, for SrCu SnS and SrCu GeS , respectively. In the case of the europium compounds, they
2 4 2 4
MS and EuCu MS (M=Ge and Sn) were prepared from a thoroughly ground mixture of EuS
2
4
2
4
2
4
ꢂ6
2
4
ꢂ6
+
2
4
2
4
follow a Curie–Weiss dependence above 1.8 K (EuCu
moment meff and the Curie–Weiss temperature Y; equal to 6.27 mB and ꢂ2.8 K for EuCu
EuCu SnS . The experimental magnetic moments confirm that the europium ion is in divalent state, similar to Sr in the related
2
GeS
4
) and above 4 K (for EuCu
2
SnS
4
), with values of the magnetic effective
2
GeS , and 6.81 mB and +0.7 K, for
4
2
4
compounds.
r 2003 Elsevier Science (USA). All rights reserved.
Keywords: Chalcogenides; Inorganic compounds; Semiconductors; Chemical synthesis; Impedance spectroscopy; X-ray diffraction; Electrical
properties; Magnetic properties; Optical properties
1
. Introduction
synthesis of two new phases EuCu SnS and
2 4
EuCu GeS , which are isostructural with the analog
2
4
The chemistry of ternary and quaternary germanium
Ba and Sr-containing sulfides previously described by
Teske [6].
and tin chalcogenides has been extensively studied in the
last decade. Many alkaline-earth and rare-earth ternary
germanium and tin sulfides have been reported [1–5].
Quaternary tin and germanium sulfides were reported
the first time by Ch. Teske in 1976 [6], and recently
Johrendt and Kanatzidis [7,8] have systematically
studied complex sulfides of transition metals and XIV
group elements. These studies include, for example, the
synthesis and characterization of compounds such as
Rb Cu SnS , Na Cu Sn S , and BaCu Ge S .
The crystal structure of these phases can be schema-
tically described as a three-dimensional framework of
ꢂ7
ꢂ4
corner sharing [CuS4] , and [MS4]
(M=Sn, Ge)
tetrahedra. The divalent ions are in eightfold coordina-
tion forming a distorted cubic antiprism polyhedron.
According to the usual electron counting rules, that
implies the following Sr (+II) and Eu (+II), Cu (+I),
Sn (+IV) and Ge (+IV) oxidation states, and from
the color of the series of compounds (green for the
germanium sulfides, and the reddish brown for the tin
sulfides), it is possible to predict a semi-conducting
behavior for all these compounds, with an absorption
band in the visible part of the optical spectrum. These
predictions, however, have not been demonstrated
experimentally, because neither the electrical nor the
magnetic properties have been investigated.
2
2
4
2
2
2
6
6
2 8
The interest in these compounds is based on their
potential applications in nonlinear optics phenomena,
optical storages, solar energy conversion, and catalysis
9–12].
The investigation in our laboratory of the quaternary
rare-earth transition metal chalcogenides led us to the
[
ꢀ
In the present paper, we have undertaken an
investigation of the electrical and magnetic properties
Corresponding author. Fax: +56-55-355632.
E-mail address: jllanos@ucn.cl (J. Llanos).
0022-4596/03/$ - see front matter r 2003 Elsevier Science (USA). All rights reserved.
doi:10.1016/S0022-4596(03)00093-8

J. Llanos et al. / Journal of Solid State Chemistry 173 (2003) 78–82
79
as well as some of the optical properties of these family
of compounds, in particular the diffuse reflectance
measurements for the sulfides of formula SrCu MS or
The magnetic measurements were performed on a
SHE-VTS-906 SQUID magnetometer, from 2 K up
to room temperature, under a static field of 1 kOe
(Eu-based samples) or 5 kOe (Sr-based compounds).
The powdered samples were contained in gelatin
capsules whose contributions were subtracted at each
temperature point.
2
4
EuCu MS (M=Sn, Ge).
2
4
2
. Experimental
Polycrystallines SrCu MS and EuCu MS (M=Ge,
3. Results and discussion
2
4
2
4
Sn) phases were prepared from a thoroughly ground
mixture of EuS or SrS, Cu (Aldrich, 99.5% pure), Ge
The least-squares refinements (trigonal cell) of the
PXD patterns yielded the unit-cell parameters, which
are presented in Table 1 along with the previously
reported unit-cell parameters for SrCu SnS and
(
Aldrich, 99.99% pure) or Sn (Aldrich, 99.8% pure),
and S (Aldrich, 99.5% pure) in stoichiometric propor-
tions. The mixtures were placed in an alumina boat and
heated in a CS /Ar stream at 723 K for about 36 h, with
2
4
SrCu GeS . Whereas SrCu SnS and EuCu SnS crys-
2 4 2 4 2 4
2
two intermittent grindings. All phases are colored:
SrCu GeS and EuCu GeS , green, and SrCu SnS
4
tallize in the space group P3 (No. 144), the space group
1
of the Ge-derivatives is P32 (No. 145) [6–14]. Latter
studies suggest that the appropriate space group for this
type of compounds is P3221 (No. 154) or its enantiomer
P3121 (No. 153) [15].
2
4
2
4
2
and EuCu SnS , reddish brown, and stable in air.
2
4
The SrS and EuS were prepared by heating SrCO3
Aldrich, 99.995% pure) or Eu O (Aldrich, 99.99%
pure), respectively, in an alumina crucible at 900 K,
(
2
3
The curves log s ¼ f ðTÞ illustrated in Fig. 1 for
EuCu GeS (curve 1), SrCu GeS (curve 2), SrCu SnS
4
under continuous flow of CS /Ar.
2
2
4
2
4
2
To check phase purity, powder X-ray diffraction
PXD) data were collected with a Siemens D-5000
(curve 3), and EuCu SnS (curve 4) in the temperature
2 4
(
range 293–450 K are typical for semi-conducting mate-
rials. The room temperature resistivities and activation
energy values are shown in Table 1. All these values are
typical for this type of conduction.
difractometer fitted with a graphite monochromator,
using CuKa radiation (l ¼ 154:057 pm), with quartz as
internal standard. The experimental powder patterns
were almost in perfect agreement with those calculated
from the single-crystal data using the program Lazy-
Pulverix [13]. First inspection of the powder pattern
revealed a close relationship between all phases.
The electrical conductivity measurements were carried
by using the electrochemical impedance spectroscopy
EIS) technique from room temperature to 450 K. A
The UV-Vis spectra of all compounds presented here
show a very strong absorption band in the wave-number
range 450–800 nm from which the optical band gap can
be derived (Fig. 2). The optical band gap corresponds to
the intersection point between the base line along the
energy axis and the extrapolated line from the linear
portion of the spectrum. With these data, the band gaps
Eg can be assessed at 2.8 eV for SrCu GeS , 2.1 eV
(
VoltaLab40 PGZ301 universal potentiostat was used for
all measurements. The applied signal amplitude was
2
4
for SrCu SnS , 2.2 eV for EuCu SnS , and 1.6 eV for
4
2
4
2
1
0 mV in the 10 Hz to 1 kHz frequency range. Cylind-
EuCu GeS . The Eg values are consistent with the
2 4
2
rical pellets (area=0.29 cm , and thickness between 0.05
and 0.15 cm) were made from pressed powders of
SrCu MS and EuCu MS . Impedance measurements
were made in a conductivity cell provided with two
identical electrodes. The two opposite flat surfaces of
the samples were covered with gold by sputtering
and sandwiched between the gold electrodes of the
conductivity cell.
Diffuse reflectance spectra were recorded in order to
determine the gap between the valence and conduction
bands. A Perkin-Elmer Lambda 20 UV/Vis spectro-
photometer equipped with a Labsphere RSA-PE-20
diffuse reflectance accessory was used to measure the
diffuse reflectance spectra of SrCu MS and EuCu MS
4
over the range 200 nm (6.2 eV) to 600 nm (2.1 eV). The
samples were ground into powder and pressed to form
a very thin layer onto MgO on a sample holder. MgO
pressed powder was used as reference.
citreous color of SrCu GeS , the green color of the
2 4
EuCu GeS and the red-brown color of the Sn-
2
4
derivatives.
Noticeable is that the band gap of EuCu GeS
2
4
2
4
2
4
(1.6 eV) is relatively small and comparable to those of
CuInS2 (1.55 eV) or the related phase Rb Cu Sn S
2 6
2
2
(1.44 eV). These values suggest that this compound is
suitable for efficient absorption of most of the solar
radiation making it potential candidate for solar cell
applications [8].
The Sr-based compounds present both a diamagnetic
ꢂ6
ꢂ6
signal of about ꢂ80 ꢁ 10 and ꢂ105 ꢁ 10 emu/mol
at 300 K, for SrCu SnS and SrCu GeS , respectively.
2
4
2
4
After subtraction of the diamagnetic contributions
of the constituent atoms [16], the respective room
temperature magnetic susceptibilities amount to
2
4
2
ꢂ6
ꢂ6
+135 ꢁ 10
and +92 ꢁ 10 emu/mol. These values
were used for the analysis of the magnetic susceptibility

80
J. Llanos et al. / Journal of Solid State Chemistry 173 (2003) 78–82
Table 1
Unit-cell parameters and the electrical, optical and magnetic properties of SrCu
2
MS
4
and EuCu
2
MS
4
(M=Ge and Sn)
Eg (eV)
˚
a (A)
˚
c (A)
Compound
r (RT) (O cm)
Ea (eV)
y (K)
meff (MB)
EuCu
EuCu
2
GeS
SnS
4
6.102(3)
6.249(8)
6.143(2)
6.268(2)
15.22(8)
15.48(7)
15.282(6)
15.533(4)
24.21
862.1
403.2
27.10
0.79
0.94
1.75
1.04
1.6
2.2
2.8
2.1
ꢂ2.7
0.7
6.27
6.81
—
2
4
SrCu
SrCu
2
GeS
SnS
4
TIP
TIP
2
4
—
TIP: temperature-independent paramagnetism.
The unit-cell parameter for SrCu
from this work.
2
GeS
4
and SrCu
2
SnS
4
were taken from the literature [6,14] and for EuCu
2
GeS
4
and EuCu
2
SnS
4
were obtained
T (K)
T (K)
360
4
40 420
400
380
360
340
4
20 400
380
340
320
-
-
-
-
-
-
-
-
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2
EuCu GeS4
-
-
-
2
3
4
2
SrCu GeS4
Ea =1.75eV
Ea = 0.79eV
-5
2
.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0
2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1
1000/T(K)
1000/T(K)
Curve 1
Curve 2
T (K)
340
T (K)
350
4
00
375
325
300
4
20 400 380 360
320
300
-
-
-
-
5.2
5.6
6.0
6.4
-
-
-
-
-
1.0
1.5
2.0
2.5
3.0
SrCu SnS4
Ea = 1.04eV
EuCu SnS4
2
2
Ea = 0.94 eV
2
.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3
000/T(K)
2.5 2.6 2.7 2.8 2.9 3.0 3.1
1
1000/T(K)
Curve 3
Curve 4
Fig. 1. Electrical conductivity as a function of temperature.
of the Eu-based compounds, by subtracting the bare
susceptibility of the SrCu SnS from the EuCu SnS
The experimental magnetic moments are lower
2
+
than the one expected for a Eu
but much higher than an eventual van Vleck contribu-
free ion (7.94 m ),
B
2
4
2
4
data and, similarly, SrCu GeS
from that of
EuCu GeS . Fig. 3 shows the corrected data of the
2
4
7
tion due to the admixture of the F and F levels of the
7
2
4
0
1
3
+
inverse susceptibility of EuCu SnS4 and EuCu GeS ,
4
Eu ion (experimental moments of 3.2–3.6 m [17,18]).
2
2
B
measured after zero-field cooling. Assuming a Curie–
Weiss dependence in the paramagnetic state, these data
were fitted by a least-squares refinement in the
temperature range 1.8–300 K for EuCu GeS , and 4–
We may confirm then, that europium is at its 2+
oxidation state, similar to Sr in the isostructural
compounds.
At low temperatures, the magnetic behavior of the
Eu-based samples differs from one compound to the
other (inset of Fig. 3). While EuCu GeS follows a
2
4
3
00 K for EuCu SnS . The obtained values of the
2 4
magnetic effective moment meff and the Curie–Weiss
temperature Y were equal to 6.27(70.10) mB and
ꢂ2.8(70.2) K for EuCu GeS , and 6.81(70.05) m and
2
4
Curie–Weiss dependence down to our lower experimen-
tal temperature, extrapolating to Y ¼ ꢂ2:8ð70:2Þ K,
EuCu SnS shows a ferromagnetic-like behavior: its
2
4
B
+
0.7(70.1) K for EuCu SnS .
2 4
2
4

J. Llanos et al. / Journal of Solid State Chemistry 173 (2003) 78–82
81
65
60
55
EuCu GeS4
60
2
SrCu GeS4
2
Eg = 2.77 eV
Eg = 1.57 eV
50
40
30
20
1.45
1.50
1.55
1.60
1.65
1.70
2.0
2.5
3.0
3.5
(a)
Energy (eV)
(b)
Energy (eV)
1
2
0
2
SrCu SnS4
30
25
2
EuCu SnS4
Eg = 2.05eV
Eg = 2.22 eV
1
8
6
4
20
15
10
1
.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4
1
.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6
(c)
Energy (eV)
(d)
, (b) SrCu
Energy (eV)
Fig. 2. Diffuse reflectance spectra of (a) EuCu
2
GeS
4
2
GeS
4
, (c) SrCu SnS
2
4
, and (d) EuCu
2
SnS
4
(T ¼ 300 K).
8
of the well-isolated S ground state and no intrinsic
effects should be expected. The striking difference in
these magnetic behaviors should be investigated deeper,
either by magnetic data at very low temperature
7
/2
80
60
40
20
1 / X
2
1
(
millikelvin region) or by neutron diffraction in samples
1
53
fabricated from a specific europium isotope ( Eu).
Small differences in the interatomic distances and/or
angles may be responsible of the existence of such
ferromagnetic exchange interactions.
0
5
10
T (K)
EuCu GeS4
2
EuCu SnS4
2
Acknowledgments
0
50
100
150
200
250
300
Temperature (K)
This work was supported by FONDECYT (Chile)
˜
contracts 1990364. O. Pena thanks the financial support
from the France–Chile program PICS 922.
Fig. 3. Inverse molar susceptibility of EuCu
2
4 2 4
GeS and EuCu SnS ,
measured under an applied field of 1 kOe. The inset zooms the low-
temperature data.
susceptibility levels off at about 3 K, while the data
extrapolates to Y ¼ þ0:7ð70:1Þ K.
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oxidation states for the europium ion. The presence of
[
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has never shown such effects, to our knowledge, because
[
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