Magnetic properties and 151Eu Moessbauer effects of mixed valence europium copper sulfide, Eu2CuS3

Article abstract of DOI:10.1016/j.jssc.2004.04.034

Ternary europium copper sulfide Eu₂CuS₃ have been investigated by X-ray diffraction, ¹⁵¹Eu Moessbauer spectroscopy, magnetic susceptibility, magnetization, and specific heat measurements. In this compound, Eu²⁺ and Eu³⁺ ions occupy two crystallographically independent sites. The ¹⁵¹Eu Moessbauer spectra indicate that the Eu²⁺ and Eu³⁺ ions exist in the molar ratio of 1:1, and the Debye temperatures of Eu²⁺ and Eu ³⁺ are 180 and 220K, respectively. In its magnetic susceptibility, the divergence between the zero-field cooled and field cooled susceptibilities appears below 3.4K. The specific heat has a λ-type anomaly at the same temperature. From the field dependence of magnetization at 1.8K, the Eu ²⁺ ion was found to be in the ferromagnetic state with the saturation magnetization M_S=6.7μ_B.

Full text of DOI:10.1016/j.jssc.2004.04.034

ARTICLE IN PRESS
Journal of Solid State Chemistry 177 (2004) 3853–3858
1
51
Magneticproperties and
¨
Eu Mossbauer effects of mixed valence
europium copper sulfide, Eu CuS3
2
Fumito Furuuchi, Makoto Wakeshima, and Yukio Hinatsu*
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
Received 10 November 2003; accepted 22 April 2004
Available online 21 September 2004
Abstract
1
51
Ternary europium copper sulfide Eu
susceptibility, magnetization, and specific heat measurements. In this compound, Eu
2 3
¨
CuS have been investigated by X-ray diffraction, Eu Mossbauer spectroscopy, magnetic
2
+
3+
and Eu ions occupy two
3+
ions exist in the molar
1
51
2+
crystallographically independent sites. The
ratio of 1:1, and the Debye temperatures of Eu
Eu Mo
+
¨
ssbauer spectra indicate that the Eu
3+
and Eu
are 180 and 220 K, respectively. In its magnetic susceptibility, the
2
and Eu
divergence between the zero-field cooled and field cooled susceptibilities appears below 3.4 K. The specific heat has a l-type anomaly
at the same temperature. From the field dependence of magnetization at 1.8 K, the Eu ion was found to be in the ferromagnetic
2
+
state with the saturation magnetization MS ¼ 6:7 m
B
.
r 2004 Elsevier Inc. All rights reserved.
Keywords: Magneticproperties; Mo
¨
ssbauer spectroscopy; Magnetic susceptibility; Specific heat; Europium; Copper; Sulfide
1
. Introduction
Ternary europium copper sulfide Eu CuS has been
anion, or a second weakly electronegative cation. For
2
+
3+
compounds containing mixtures of Eu
51
and Eu
Eu Mossbauer spectro-
,
1
Eu S [7] and EuPd S [4],
3
¨
2
3
4
3 4
reported to crystallize in the orthorhombic structure
with space group Pnma [1]. The schematic structure of
Eu CuS is illustrated in Fig. 1. This compound has the
scopic measurements were performed to investigate the
mixed-valence state of Eu. These compounds show the
occurrence of the electron transfer and/or the electron
2
3
2
+
3+
+
2À
2+
3+
formal oxidation state of Eu Eu Cu S and the
3
hopping between Eu
3+
and Eu , which may stabilize
ions. The situation for Eu CuS is different
2
+
3+
Eu and Eu ions occupy crystallographically differ-
ent sites. In the case of sulfides, the mixed-valence
compounds show an interesting electronic behavior as
the valence state and the magnetic interaction. For
example, Eu S has the Verwey transition at 186 K [2]
the Eu
2
3
from the case for the above-mentioned sulfides, i.e.,
Eu CuS has two crystallographically distinguishable
2
3
2
+
3+
sites for Eu
and Eu
ions. However, none of the
studies on the electronic and magnetic properties of this
mixed valence compound have been carried out.
In this study, we synthesized the polycrystalline
samples of Eu CuS and have measured the magnetic
3
4
and the ferromagnetictransition at 3.1 K [3]. EuPd S
3
4
2
+
3+
shows an electron hopping between Eu and Eu and
has the antiferromagnetictransition at 3 K [4].
Eu Sn S is metamagneticand has two field-dependent
2
3
susceptibility, magnetization, specific heat measure-
1
5
3 12
51
antiferromagneticphases at low temperatures [5].
Generally, the divalent state of Eu is stable in sulfides,
but some compounds contain trivalent europium.
Flahaut noted that these were classified into two types
ments and
will be discussed in detail.
¨
Eu Mossbauer spectroscopy. The results
3
+
[6]. In the case that compounds containing only Eu
ions, there is necessarily a strongly electronegative
2. Experimental
A ternary europium copper sulfide, Eu CuS , was
2
3
synthesized by a solid-state reaction. Europium mono-
sulfide (EuS), copper powders, and sulfur were used as
*
Corresponding author. Fax: +81-11-706-2702.
E-mail address: hinatsu@sci.hokudai.ac.jp (Y. Hinatsu).
0022-4596/$ - see front matter r 2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2004.04.034

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F. Furuuchi et al. / Journal of Solid State Chemistry 177 (2004) 3853–3858
3854
(
Laboratory Equipment Co.) using a radiation source
51
1
SmF (1.85 GBq). The sample lapped in an aluminum
3
foil was cooled down to each temperature and the
isomer shift was determined relative to the shift of
europium fluoride (EuF3).
Magneticsus ce ptibility measurements of the powder
sample weighing B20 mg were performed with a
SQUID magnetometer (Quantum Design MPMS mod-
el) from 1.8 to 300 K. The applied field was 0.1 T. In the
neighborhood of the magnetictransition, the magnetic
susceptibility was measured under zero-field cooled
Eu(1)S octahedron
6
(
ZFC) and field cooled (FC) conditions with 5 mT.
The field dependence of magnetization was measured at
.8 K by changing the applied magnetic field between À5
and 5 T.
The specific heat measurement was carried out using a
(a)
CuS tetrahedron
4
c
1
a
b
relaxation technique supplied by commercial specific
heat measurement system (Quantum Design, PPMS) in
the temperature range from 1.8 to 300 K. The sample in
the form of pellet (B7 mg) was mounted on an
aluminum plate with apiezon for better thermal contact.
Eu(1)
Cu
Eu(2)S monocapped prism
7
3. Results and discussion
The Eu CuS compound was obtained as a single
2
3
phase. The X-ray diffraction profile was indexed on an
orthorhombic ce ll with the spa ce group Pnma, and its
(
b)
crystallographic parameters were refined by the Rietveld
method. Fig. 2 shows the X-ray diffraction profile of
Eu CuS . The calculated profile was in good agreement
with the observed one (Rwp ¼ 8:10%; RI ¼ 2:95%). The
refined lattice and positional parameters are listed in
Table 1, and the selected interatomic distances are listed
in Table 2. These values for the lattice parameters and
the interatomicdistan ce s agree with those reported by
Lemoine et al. [1].
Fig. 1. Polyhedral representation of the Eu
2
CuS
3
structure. (a)
tetra-
CuEu(1)S layers composed of Eu(1)S octahedra and CuS
hedra. (b) The double chains of the Eu(2)S monocapped trigonal
3
6
4
2
3
7
prisms with Eu(1) and Cu represented by the larger and the smaller
spheres.
starting materials. To obtain the europium monosulfide,
europium sesquioxide (Eu O ) was heated on a graphite
2
3
The crystal structure of Eu CuS is shown in Fig. 1. In
2
3
boat at 1273 K in a flow of the mixed gas of CS and N ,
2
2
this structure, the Eu ions occupy two crystallographi-
cally independent sites. The Eu(1) ions are coordinated
which was obtained by bubbling the N gas through
2
liquid CS at room temperature. Stoichiometric mix-
2
tures of the starting materials were ground and pressed
into a pellet, and then heated in a quartz ampoule. The
inner wall of the ampoule was, in advance, coated with a
carbon film by decomposing benzene. The reaction was
carried out at 1173 K for 12 h, with one interval
regrinding.
Powder X-ray diffraction measurements were per-
formed with CuKa radiation on a Rigaku MultiFlex
diffractometer equipped with a curved graphite mono-
chromator. Intensity data were collected by step
4
000
000
3
2000
1000
0
20
40
60
80
100
120
ꢀ
ꢀ
scanning in the range 2y ¼ 10 2120 at intervals of
2
θ / º
ꢀ
0.02 . The structure and lattice parameters were refined
with a Rietveld program RIETAN 2000 [8].
Fig. 2. Powder X-ray diffraction patterns and Rietveld refinements for
Eu CuS . The bottom trace is a plot of the difference between
2
3
151
The Eu Mo
¨
5 and 300 K with a Mo
ssbauer spectra were measured between
ssbauer spectrometer VT-6000
observed+(cross makers) and calculated (solid line) intensities. All
allowed Bragg reflections are shown by vertical lines.
1
¨

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F. Furuuchi et al. / Journal of Solid State Chemistry 177 (2004) 3853–3858
3855
Table 1
100
2 3
Lattice parameters and atomic positions of Eu CuS
Space group: Pnma
Z ¼ 4
9
8
˚
˚
˚
15 K
a ¼ 10:3596ð2Þ A, b ¼ 3:9586ð1Þ A, c ¼ 12:8194ð3Þ A
2
B (A )
˚
96
00
Atom
Site
x
y
z
Eu(1)
Eu(2)
Cu
4c
4c
4c
4c
4c
4c
0.0164(2)
0.7801(2)
0.2321(4)
0.0505(6)
0.4068(7)
0.2635(7)
1/4
1/4
1/4
1/4
1/4
1/4
0.7374(1)
0.0014(2)
0.2225(3)
0.1157(5)
0.1021(5)
0.8280(5)
0.46(5)
0.55(5)
0.80(9)
0.55(17)
0.30(16)
0.57(16)
1
S(1)
S(2)
S(3)
9
8
6
100 K
9
hX
X
i
1=2
2
2
Rwp ¼
wðjFoj À jFcjÞ =
wjFoj
¼ 8:10%;
100
X
X
RI ¼
RF ¼
jIo À Icj=
Io ¼ 2:95%;
X
X
200 K
98
jFo À Fcj=
Fo ¼ 2:13%:
Table 2
Selected interatomic distances (A) of Eu
100
˚
2
CuS
3
Eu(1)ÀS(2)
Eu(1)ÀS(3)
Eu(1)ÀS(3)
Eu(1)ÀS(1)
2.795(5) Â 2
2.795(8)
2.812(7)
3
00 K
98
2.818(5) Â 2
-20
-10
0
10
20
Eu(2)ÀS(3)
Eu(2)ÀS(1)
Eu(2)ÀS(2)
Eu(2)ÀS(1)
2.985(5) Â 2
3.041(5) Â 2
3.070(5) Â 2
3.121(6)
Velocity / mm s-1
151
Fig. 3.
00 K.
¨
2 3
Eu Mossbauer spectra of Eu CuS at 15, 100, 200, and
3
CuÀS(1)
CuÀS(2)
CuÀS(3)
2.344(7)
2.363(7)
˚
(0.37 A), respectively, and d means the interatomic
distance between cation i and anion j. The valences of
the Eu(1), Eu(2), and Cu ions were determined to be
2.397(4) Â 2
ij
+3.04, +1.73, and +0.971, respectively. This result
indicates that the trivalent and divalent europium ions
occupy the Eu(1) and Eu(2) sites, respectively.
by six sulfur ions and the Eu(1)S6 octahedra share
corners along the a-axis and edges along the b-axis. The
1
51
Eu(1)S6 octahedra and CuS4 tetrahedra form
CuEu(1)S layer perpendicular to the c-axis. The Eu(2)
a
¨
2 3
Fig. 3 shows the Eu Mossbauer spectra of Eu CuS
between 15 and 300 K. Two absorption peaks appear at
3
À1
ions are coordinated by seven sulfur ions between the
CuEu(1)S layers and make the Eu(2)S monocapped
trigonal prisms. The bases of the Eu(2)S monocapped
ca. À12 and 0 mm s in these spectra, indicating the
presence of the divalent and trivalent Eu ions. To
simplify this analysis, each peak was fitted with a single
3
7
7
2
+
trigonal prisms, S(1), S(2), and S(3), are shared by each
other to make a chain along the b-axis and one corner of
this base, S(1), is also coordinated by the Eu(2)
belonging to the neighboring chain.
Lorentzian.
(dB À 12:4 mm s ) and Eu
The
À1
isomer
shifts
of
Eu
3+
À1
(dB0.68 mm s ) are
nearly constant between 15 and 300 K. It is known that
and
decreases with increasing the temperature in
mixed valence europium sulfides, Eu S [7], EuPd S
3 4
2
+
the difference in the isomer shifts between Eu
3
+
The valences of the cations are calculated by the bond
valence sum (V ) [9]:
Eu
i
3
4
ꢀ
ꢁ
[
hopping between Eu
dependence of the isomer shifts for Eu CuS means that
2
4], and (EuS)1.173NbS [10] because of the electron
2
R0 À dij
2
+
3+
and Eu . The temperature
vij ¼ exp
;
ð1Þ
ð2Þ
b
3
X
the electron hopping does not occur in this compound.
Fig. 4 shows the temperature dependence of the
Vi ¼
vij;
j
2
+
absorption area of the intensity curves of the Eu and
3
Eu ions. Both intensities decrease monotonously with
+
where R0 and b are known as the bond valence
parameters of various cations and a constant value
2
+
increasing temperature. The intensities for Eu
and

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F. Furuuchi et al. / Journal of Solid State Chemistry 177 (2004) 3853–3858
3856
1
4
2
0
8
6
4
2
0
50
1
1
5
0
5
0
Eu3+
Eu2+
ZFC
FC
1
1
4
3
2
1
0
0
0
0
0
ꢀD = 220 K
0
5
10
T / K
ꢀD = 180 K
0
100
200
300
T / K
0
100
200
T / K
Fig. 4. Temperature dependence of the absorption area of intensities
of Eu CuS
. The solid line is the theoretical curve (YD ¼ 180 K)
normalized to A(Eu ) at 15 K and the broken line is the theoretical
300
Fig. 5. Temperature dependence of the reciprocal magnetic suscept-
À1
ibility w of Eu
2
CuS
3
. A solid line is the fitting result by using Eq. (4)
(see text). The inset shows w of Eu CuS below 10 K measured in the
magneticfield of 5 mT.
2
3
2
3
2+
3+
curve (YD ¼ 220 K) normalized to A(Eu ) at 15 K.
distances for Eu(1)S as compared with those for the
6
2
+
2À
Eu ÀS bonds of Eu(2)S7.
3
+
Eu are equal at 15 K but the ratio of the intensities for
Fig. 5 shows the reciprocal magnetic susceptibilities
À1
2
+
3+
Eu
difference may be due to a difference in the Debye–
and Eu
is found to be 2:3 at 300 K. This
(w ) of Eu CuS as a function of temperature in the
2
3
magneticfield of 0.1 T. The molar sus ec ptibilities of
Eu CuS is given by
2
+
3+
Waller factors between Eu and Eu . The area of the
intensity curve is proportional to the recoil-free fraction.
The Debye temperatures for Eu
2
3
2þ
3þ
wM ¼ w ðEu Þ þ w ðEu Þ þ w0;
ð4Þ
2
+
3+
M
M
and Eu
are
estimated from the recoil-free fraction. The recoil-free
fraction is represented by [11]
where w is the temperature-independent term contain-
ing the diamagneticterm. The ground state of the Eu
8
ion is S7/2. Therefore the orbital angular momentum
0
2
+
"
(
)#
ꢀ
ꢁ Z
2
YD=T
À6ER
kBYD
1
4
T
x dx
x
vanishes, and the crystal field does not affect the
2+
f ¼ exp
þ
;
ð3Þ
YD
ðe À 1Þ
magneticproperties of Eu
2
M
compounds. The magnetic
2+
is represented by
0
+
susceptibility (w (Eu )) of Eu
2
eff
where k is the Boltzmann’s constant, Y is the Debye
N m =3k ðT2Y Þ; where m is the effective magnetic
B
D
A
B
W
eff
temperature, and ER is the free-atom recoil energy. The
theoretical curves with YD ¼ 180 K and YD ¼ 220 K
using this equation are in good agreement with the
moment and YW is the Weiss constant. The ground state
7 3+ 7
( F ) of Eu is nonmagnetic, and the excited states F
0
J
(J ¼ 1; 2; y; 6) are close enough to give energy differ-
ences comparable to kBT at room temperature. Thus, in
consideration of the excited states, the magnetic
2
+
3+
experimental data for Eu
Debye temperature for the Eu(1) ion indicates that the
and Eu . The higher
Eu ÀS2À bonds in the Eu(1)S6 octahedron are
3
+
susceptibility of Eu can be written as
3+
2
B
N m
3
wMðEu3þÞ ¼
A
kBgT
4 þ ð13:5g À 1:5Þe þ ð67:5g À 2:5Þe þ ð189g À 3:5Þe þ ?;
Àg
À3g
À6g
À6g
2
ð5Þ
Àg
À3g
1
þ 3e þ 5e þ 7e þ ?
2
+
2À
bonds in the Eu(2)S7
stronger than the Eu ÀS
where g ¼ l=kBT is 1/21 of the ratio of the over all
monocapped trigonal prism, which is attributable to the
small coordination-number and the short interatomic
multiplet width to k T [12]. On the assumption that the
B
screening number is 34, the theoretical value of l=k is
B

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F. Furuuchi et al. / Journal of Solid State Chemistry 177 (2004) 3853–3858
3857
À1
À1
3
63 cm . If l=kB was fixed to be 363 cm [12], the
harmoniclatti ce series in odd powers of T:
2
+
values of meff and YW of Eu
7.96(1) mB and +4.74(2) K, respectively. The effective
magneticmoment is in good agreement with the
were determined to be
3
5
7
Clat ¼ B T þ B T þ B T þ ?:
ð6Þ
3
5
7
On the assumption that the Cmag is negligible above
16 K, the constants B , B , B were determined by fitting
theoretical one (7.94 m ). The positive Weiss constant
B
3
5
7
indicates the existence of the ferromagnetic coupling of
Eq. (6) to the observed specific heat data between 16 and
25 K. The Cmag is obtained by subtracting Clat from the
total C : A dashed line in the C À T curve represents
2
+
Eu ions.
The inset of Fig. 5 shows the magneticsus ce ptibility
measured in the applied field of 5 mT. The divergence
between the ZFC and FC magneticsus ce ptibility have
been observed below 3.4 K. To clarify this behavior, we
measured the field dependence of the magnetization at
p
p
the extrapolated specific heat below 1.8 K. This is
3
=2
calculated by fitting to the function C ¼ BT þ C
p
lat
in the temperature range 1.8 KpTp2.2 K, because C
mag
for a ferromagneticordering is proportional to T3 [13].
=2
1
.8 K as shown in Fig. 6. The magnetization linearly
The temperature dependence of the C =T and the
mag
R
magneticentropy h ac nge is estimated to be
increases and then reaches an almost constant value.
Actually it still increases with a further increase of the
magnetic field strength. The coersive force is very small
mag
magnetic entropy calculated by Smag ¼ ðC =TÞ dT
are shown in Fig. 7(b). From the Smag À T curve, the
À1 À1
(B1 mT). The extrapolation of the high-field magnetic
B15 J mol K , and it is close to Rln (2S+1)=R ln 8.
moment to zero field yields a saturation moment of
3
+
6
magnetization of Eu CuS is negligible at 1.8 K because
.7 m . The magneti coc ntribution of Eu
to the
B
20
2
3
1
50
00
50
0
7
of its nonmagneticground state ( F ). This saturation
0
moment corresponds to the ferromagnetic-ordering
2
1
+
magnetization of Eu which is expected to be 7 mB.
15
The temperature dependence of the specific heat C of
p
Eu CuS is shown in Fig. 7(a). The sharp l-type
2
3
anomaly at 3.4 K indicates that the existence of a long-
range magneticordering co rresponds to the ferromag-
netictransition at 3.4 K observed from the magnetic
susceptibility. The specific heat of insulating Eu CuS
1
0
5
0
0
100
T / K
200
300
2
3
consists of a magnetic contribution (Cmag) and a lattice
contribution (C ). The C is represented by the usual
lat
lat
0
5
10
15
20
8
6
4
2
0
(
a)
T / K
6
4
2
0
20
1
5
10
5
-
2
4
6
8
-
-
-
0
20
0
5
10
15
(b)
T / K
Fig. 7. (a) Temperature dependence of the specific heat below 20 K for
Eu CuS . The inset shows the specific heat behavior up to 300 K. (b)
-4
-2
0
2
4
2
3
H / T
Temperature dependence of the magnetic specific heat divided by
temperature (left ordinate) and magneticentropy (right ordinate) for
Fig. 6. Field dependence of magnetization measured at 1.8 K for
Eu CuS
2
3
.
2 3
Eu CuS .

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F. Furuuchi et al. / Journal of Solid State Chemistry 177 (2004) 3853–3858
3858
This result strongly indicates that the ferromagnetic
[5] T.I. Volkonskaya, A.G. Gorobets, S.A. Kizhaev, I.A. Smirnov,
V.V. Tikhonov, M. Guittard, C. Lavenant, J. Flahaut, Phys. Stat.
Sol. (a) 57 (1980) 731.
2
ordering of Eu CuS is caused by only the Eu ion and
+
2
3
the eight-fold degeneracy remains in the ground state of
[
6] K.A. Gschneidner Jr., L. Eyring (Eds.), Handbook on the Physics
and Chemistry of Rare Earth, North-Holland Publishing
Company, Amsterdam, 1979 (Chapter 31).
2
+
Eu , which is consistent with the results obtained
through the magneticsus ce ptibility and the magnetiza-
tion measurements.
[7] B.C. Bunker, R.S. Drago, M.K. Kroeger, J. Am. Chem. Soc. 104
1982) 4593.
(
[
[
8] F. Izumi, T. Ikeda, Mater. Sci. Forum 321–324 (2000) 198.
9] N.E. Bresse, M. O’Keefe, Acta Crystallogr. Sect. B 47 (1991) 192.
[
[
10] L. Cario, P. Palvadeau, A. Lafond, C. Deudon, Y. Moe
Corraze, A. Meerschaut, Chem. Mater. 15 (2003) 943.
¨
lo, B.
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