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Magnetic structure of the new layered ferromagnetic chromium hexatellurosilicate Cr2Si2Te6
Journal of Magnetism and Magnetic Materials 94 (1991) 127-133 North-Holland
127
Magnetic structure of the new layered ferromagnetic hexatellurosilicate Cr,Si ,Te,
chromium
V. Carteaux, G. Ouvrard Laboraioire de Chimie des Solides, Facultc! des Sciences de Nantes, 2 rue de la Houssi&re,
44072 Nantes Cedex, France
J.C. Grenier Laboratoire de Chimie des Solides du CNRS,
351 cows de la Libkration, 33405 Talence Cedex, France
and Y. Laligant Luboratoire des Fluorares, URA CNRS 449, Universit6 da Maine, 72017 Le Mans Cedex, France Received 5 June 1990
The magnetic structure and properties of Cr,Si,Te, has been investigated by neutron powder diffraction and magnetization measurements. Below 33 K, Cr,Si,T% is ferromagnetic with spins aligned along the c axis (p(Cr3+ ) = 2.73(5)pa at T = 1.2 K). The thermal evolution of the magnetic moment below T, is given.
ibility, magnetization measurements.
1. Introduction The chromium hexatellurosilicate Cr,Si,T% is the first example of a layered material with Si, pairs. Tellurium atoms build up a hexagonal close packing in a AB sequence, The crystal structure of Cr,Si,TG, determined by single crystal X-ray diffraction [1,2] (space group R-3, a = b = 6.7578(6) A, c = 20.665(3) A) is shown in fig. 1. Between the anionic layers, chromium atoms and Si, pairs fill up the octahedral sites in a 2 : 1 ratio (fig. la), leaving alternate octahedral site planes empty (fig. lb). Then, in a slab with all octahedra fully occupied by Cr and Si, pairs, the chromium lattice builds up a honeycomb subnetwork (each chromium octahedron sharing edges with three other chromium octahedra). This work is devoted to the determination of the magnetic structure of Cr,Si,T% using suscept0304-8853/91/$03.50
and
neutron
diffraction
2. Experimental Powdered Cr,Si,Te, (sample 1) was prepared by heating the elements Cr’, Si” and Tea in the desired formula ratio in a sealed evacuated Pyrex tube at 500°C for 10 days, followed by a 10 h slow cooling. Cr,Si,T% platelets (sample 2) can be grown in a tellurium melt in the same conditions. Between 4.2 and 300 K, the magnetic susceptibility was measured by the Faraday method. Magnetization measurements were performed on a “homemade” vibrating sample magnetometer in the temperature range 4.2-300 K. Neutron diffraction patterns were collected on the DlB powder diffractometer of the high-flux
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
of Cr,Si,Te,
V. Carteaux et al. / Magnetic stnrctwe
128
C) C1
a
1 1
( (
0 0
-0-o
0
Fig. 1. (a) Projection along c of Cr,Si,T.q showing one slab, in which $ of the octahedral sites are occupied by the Si, pairs (open circles). The other octahedral sites (at the center of the represented octahedra) are occupied by chromium atoms. (b) Perspective view of the structure of Cr,Si,Tq, pointing out its 2D-dimensionality.
reactor of the Institut Laue-Langevin at Grenoble, using a wavelength of 2.525 A. The diffractometer is equipped with a position-sensitive detector (PSD) recording simultaneously 80” in
X”
28 of the powder diffraction pattern. The sample was contained in a cylindrical vanadium can (diameter 10 mm) held in a vanadium tailed liquid helium cryostat. The absence of diffraction peaks
X
(emu /mole)
(emu/mole) H=0.3T
H=0.3T
I
a /,
50
100 T
(K)
150
2
0
50
100
150 T
200
250
3
(K)
Fig. 2. (a) evolution of the inverse susceptibiliiy of a powder sample of Cr,Si,Te, (sample 1) versus temperature. (b) Evolution of the direct susceptibility. One can notice the presence of a magnetic impurity which presents an ordering temperature of about 240 K.
129
V. Carteaux et al. / Magnetic structure of Cr,Si,Te,
H=O.lT
3.0I-
2.5
20
I \
1.5
1.0
0.5 -7 0
T (W
Fig. 3. Thermal evolution of direct susceptibility of some Cr,Si,Tee platelets (sample 2). Notice the presence of a magnetic impurity which orders at about 170 K.
at very low angles was first checked during a preliminary run at 1.4 K in the range 5O < 28 < 85 O. The PSD was then positioned to record the diffraction pattern in the range 18” < 28 < 98”. The patterns were collected at various tempera-
---z-l cycle of Cr,Si,Ter,.
30
40
1
T (K) Fig. curve tion. could
5. Thermal evolution of the magnetization. The dashed would correspond to a “classical” behavior of magnetizaThe presence of a significant magnetization above T, be due either to ZD-short range magnetic ordering or to the magnetic impurity shown in fig. 2b.
tures between 1.2 K and the Curie temperature, in order to detect possible anomalies in the thermal evolution of the magnetic moment. Finally, longer
I
800
600
ho
zoo B
Fig. 4. Hysteresis
20
10
The remanent
magnetization
is q = 0.3079(5)pa
I 0
2&
(IO-~
T)
400
and the coercive
600
field H,=183(2)~10-~
T.
V. Carteaux et al. / Magnetic structure of Cr,Si,Te,
130
Table 1 Irreductible representations of R-3 (k = 0). w = exp 2ni/3 E
C3
c3”
I-,
1
I-, r, r, r,
1 1 1 1
1 1 w w* w
1 1 w* w w*
r,
1
w*
w
I
ss5
s6
1
1
1 -1
-1
-1
w* w -w*
w w* -w
-1
-w
-w*
-1 1 1
Table 2 Bertaut’s modes for Cr,Si,Tq+ The 6(c) cation position is taken from the International Table of Crystallography (vol. 1): 0 0 .r (I), f f f + z (2), f + f + .r (3) 00-z(4),;++-2(5),$ff-z(6)
r,
xOvplane
L
-
s,,
+ -
r2
_
s,,
r3 r4
_ s,,
+ -
r5 r6
s2, %,
+ -
s3x -
s4x
_
s6x
_
_ s,,
+ +
s2, %x
+ +
s3,
+
&x _
s6x
1.6
s2, %,
+ +
2ooo-( Intensity
+ -
s2, %
-
s4z
~
: 3 Fig. 7. Thermal evolution of the integrated intensities of some magnetic peaks. The intensity variation indicates that the Curie temperature is 33(l) K.
s6z +
+
s3,
_I
s3z + s6z
s.4,
data acquisitions were performed above T, (50, 100, 200 and 268 K) to record data in the paramagnetic state. The diffraction patterns were analyzed by the Rietveld method [3] as modified by Hewat [4]. The nuclear scattering lengths and magnetic form fac-
.
Fig. 6. Belov and Kouvel curves for a powder sample of Cr,Si,Te, deduced from magnetization versus magnetic field measurements. A linear interpolation gives an ordering temperature of 32.9(5) K.
V. Carteaux et al. / Magnetic structure of Cr,Si,Te,
tors were taken from Koester and Rauch [5] and Watson and Freeman [6], respectively. Bertaut’s representation theory [7] was used to identify the possible models of magnetic structure.
131
3. Magnetic study Magnetic measurements on sample 1 were first performed by the Faraday method. Fig. 2 presents
l-
a
b
26.0
36.0
48.0
50.0
66.0
76.0
66.0
Fig. 8. Comparison between observed and calculated intensities of diffraction in the paramagnetic (50 K (a)) and ordered magnetic state (1.2 K (b)). The bottom line is the difference pattern at the same scale. Magnetic peaks are identified by stars. For sake of clarity, hkl have w omitted for 26 z- 55 O.
V. Carteaux et al. / Magnetic structure of Cr,Si,Te,
132
respectively the Cr,Si,T% inverse and direct susceptibility versus temperature. The first curve indicates a ferromagnetic behavior at about 30 K. The latter shows another ferromagnetic behavior with an ordering temperature of about 240 K. Our first problem was to determine whether the Curie temperature of Cr,Si,T% was 30 or 240 K. Susceptibility measurements performed on sample 2 were very helpfull (fig. 3). From figs. 2 and 3, we can conclude that the Cr,Si,T% Curie temperature is 30 K, and that the higher ordering temperatures correspond to magnetic impurities. They are chromium telluride Cr,Te, in sample 2, whereas the study of the nature of the impurity existing in sample 1, and which orders ferromagnetically at about 240 K, is currently in progress. The magnetization versus magnetic field curve (fig. 4) shows the hysteresis cycle of Cr,Si,T%. It presents a rather low remanent magnetization a, = 0.3079(5)~, and an important coercitive field H, = 183(2) x 1O-4 T. The magnetization versus temperature curve (fig. 5) reveals a “non-classical” behavior: we can observe a significant magnetization above the ordering temperature of about 30 K. This phenomenon could be due either to short range 2D magnetic ordering or to the presence of the magnetic impurity cited above. In order to determine accurately the Curie temwe used the Belov and perature of Cr,Si,Te,, Kouvel method [8,9]. This method is based on the
I
I
1.0
2.0
3’0
J
T(K)
evolution of the refined magnetic moments of cr’+.
- Fig. 9. Thermal
calculations of isothermal curves H/a versus u2. The measurements were performed with high magnetic fields (up to 2 T) for different temperatures. With this method, the ordering temperature is determined univocally by the isothermal curve which passes through the origin. Considering the curves reported on fig. 6, and with a sample interpolation, we find an ordering temperature of 32.9(5) K. The neutron diffraction study below shows that the magnetic peaks (corresponding to the magnetic ordering) disappear at 33(l) K, in excellent agreement with the temperature deduced from the Belov and Kouvel method.
C
20.88-
b I_( 100
Fig. 10. Magnetostriction
I 200
T(K)
I
100
I
I 200
I
J T(K)
for the cell parameters of Cr,Si,Tq+ Figs. 10a and b show respectively the thermal evolution of the parameters a and c.
V. Carteaux et al. / Magnetic structure of Cr,Si,Te,
4. Neutron diffraction study The neutron diffraction patterns were collected at various temperatures in the range 1.2-50 K, and the thermal evolution of the intensities of some magnetic peaks appears in fig. 7. This indicates that the Curie temperature corresponds to the minimum of X-‘(T) observed at 33 K. Below 33 K, new purely magnetic peaks appear which can be indexed in the nuclear cell. Furthermore, the absence of any magnetic contribution on the (0 0 1) reflections gives strong evidence of moments lying along the c axis. The magnetic structure has been solved using the representation analysis method of Bertaut [7]. For the space group R-3 (k = 0), the irreducible representations given in table 1 lead to the modes summarized in table 2. r, and r, representations are ruled out because they lead to a star configuration of spins in the (a, 6) plane for Cr3+. Only the r, and r, representations are in agreement with moments lying along the c axis: (1) the I’, representation corresponds to an antiferromagnetic behavior. (2) for the r, representation, the ferromagnetic behavior is due to the collinear configuration of spins lying along the c axis. Atomic coordinates and the spin component were refined simultaneously at each temperature. The best fit (Rmag = 0.095 at T = 1.2 K) corresponds to the I” mode as expected. The component of the magnetic moment on the c axis of the cell is listed table 3 with atomic coordinates. The comparison between observed and calculated profiles is given in fig. 8. The spins of Cr3+ exhibit a value 1_1=2.73(5)pa at T = 1.2 K, which is in excellent agreement with the results of the magnetic study. The thermal evolution of the magnetic moments, refined between 1.2 and 32 K, is presented on fig. 9. From fig. 10, which gives the thermal
133
Table 3 Refined cell parameters and atomic coordinates of Cr,SirT% at 50 K and 1.2 K. (Values at 1.2 K correspond to the second line.) a = 6.763(2) A, c = 20.582(3) A, (a = 6.773(2) A, c = 20.528(2) A) Atom
position
x
Y
z
&
0.3302(8) 0.3313(6) 0.0569(6) 0.0566(6) 0.2488(4) 0.2491(4)
0.30 0.15 0.30 0.15 0.30 0.15
Cr
6(c)
0 0
Si
6(c)
Te
18(f)
0 0 0.666(2) 0.666(2)
0 0 0 0 0.977(l) 0.977(l)
Rt
R Prof
R WProf
R N”d
R Mat?
4.20% 4.96%
8.63% 8.98%
7.83% 7.97%
4.20% 4.21%
9.52%
Refined magnetic moment (in pa) at T =1.2
K: S, = 2.73(5).
evolution of the cell parameters, it is clear that the occurrence of magnetic ordering induces a strong magnetostriction for the cell.
Acknowledgments The authors are very indebted to Professors R. Brec and G. Ferey for helpful discussions and a critical reading of the manuscript, and to Dr J. Pannetier (ILL Grenoble) for the collection of neutron diffraction data.
References (11 G. Ouvrard, E. Sandre and R. Brec, J. Solid State Chem. 73 (1988) 27. [2] R.E. Marsh, J. Solid State Chem. 77 (1988) 190. [3] H.M. Rietveld, J. Appl. Cryst. 2 (1969) 65. [4] A.W. Hewat, Harwell Report AERE, R 7350 (1973). [5] L. Koester and H. Rauch, IAEA Contract 2517/RB (1981). [6] R.E. Watson and J. Freeman, Acta Cryst. 14 (1961) 27. [7] E.F. Bertaut, Acta Cryst. 24 (1968) 217. [S] K.P. Belov and A.N. Goryaga, Fiz. Met. Metall. 2 (1956) 3. [9] J.S. Kouvel, Gen. El. Res. Lab., Report 59 R.L. 1799 (1957).
127
Magnetic structure of the new layered ferromagnetic hexatellurosilicate Cr,Si ,Te,
chromium
V. Carteaux, G. Ouvrard Laboraioire de Chimie des Solides, Facultc! des Sciences de Nantes, 2 rue de la Houssi&re,
44072 Nantes Cedex, France
J.C. Grenier Laboratoire de Chimie des Solides du CNRS,
351 cows de la Libkration, 33405 Talence Cedex, France
and Y. Laligant Luboratoire des Fluorares, URA CNRS 449, Universit6 da Maine, 72017 Le Mans Cedex, France Received 5 June 1990
The magnetic structure and properties of Cr,Si,Te, has been investigated by neutron powder diffraction and magnetization measurements. Below 33 K, Cr,Si,T% is ferromagnetic with spins aligned along the c axis (p(Cr3+ ) = 2.73(5)pa at T = 1.2 K). The thermal evolution of the magnetic moment below T, is given.
ibility, magnetization measurements.
1. Introduction The chromium hexatellurosilicate Cr,Si,T% is the first example of a layered material with Si, pairs. Tellurium atoms build up a hexagonal close packing in a AB sequence, The crystal structure of Cr,Si,TG, determined by single crystal X-ray diffraction [1,2] (space group R-3, a = b = 6.7578(6) A, c = 20.665(3) A) is shown in fig. 1. Between the anionic layers, chromium atoms and Si, pairs fill up the octahedral sites in a 2 : 1 ratio (fig. la), leaving alternate octahedral site planes empty (fig. lb). Then, in a slab with all octahedra fully occupied by Cr and Si, pairs, the chromium lattice builds up a honeycomb subnetwork (each chromium octahedron sharing edges with three other chromium octahedra). This work is devoted to the determination of the magnetic structure of Cr,Si,T% using suscept0304-8853/91/$03.50
and
neutron
diffraction
2. Experimental Powdered Cr,Si,Te, (sample 1) was prepared by heating the elements Cr’, Si” and Tea in the desired formula ratio in a sealed evacuated Pyrex tube at 500°C for 10 days, followed by a 10 h slow cooling. Cr,Si,T% platelets (sample 2) can be grown in a tellurium melt in the same conditions. Between 4.2 and 300 K, the magnetic susceptibility was measured by the Faraday method. Magnetization measurements were performed on a “homemade” vibrating sample magnetometer in the temperature range 4.2-300 K. Neutron diffraction patterns were collected on the DlB powder diffractometer of the high-flux
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
of Cr,Si,Te,
V. Carteaux et al. / Magnetic stnrctwe
128
C) C1
a
1 1
( (
0 0
-0-o
0
Fig. 1. (a) Projection along c of Cr,Si,T.q showing one slab, in which $ of the octahedral sites are occupied by the Si, pairs (open circles). The other octahedral sites (at the center of the represented octahedra) are occupied by chromium atoms. (b) Perspective view of the structure of Cr,Si,Tq, pointing out its 2D-dimensionality.
reactor of the Institut Laue-Langevin at Grenoble, using a wavelength of 2.525 A. The diffractometer is equipped with a position-sensitive detector (PSD) recording simultaneously 80” in
X”
28 of the powder diffraction pattern. The sample was contained in a cylindrical vanadium can (diameter 10 mm) held in a vanadium tailed liquid helium cryostat. The absence of diffraction peaks
X
(emu /mole)
(emu/mole) H=0.3T
H=0.3T
I
a /,
50
100 T
(K)
150
2
0
50
100
150 T
200
250
3
(K)
Fig. 2. (a) evolution of the inverse susceptibiliiy of a powder sample of Cr,Si,Te, (sample 1) versus temperature. (b) Evolution of the direct susceptibility. One can notice the presence of a magnetic impurity which presents an ordering temperature of about 240 K.
129
V. Carteaux et al. / Magnetic structure of Cr,Si,Te,
H=O.lT
3.0I-
2.5
20
I \
1.5
1.0
0.5 -7 0
T (W
Fig. 3. Thermal evolution of direct susceptibility of some Cr,Si,Tee platelets (sample 2). Notice the presence of a magnetic impurity which orders at about 170 K.
at very low angles was first checked during a preliminary run at 1.4 K in the range 5O < 28 < 85 O. The PSD was then positioned to record the diffraction pattern in the range 18” < 28 < 98”. The patterns were collected at various tempera-
---z-l cycle of Cr,Si,Ter,.
30
40
1
T (K) Fig. curve tion. could
5. Thermal evolution of the magnetization. The dashed would correspond to a “classical” behavior of magnetizaThe presence of a significant magnetization above T, be due either to ZD-short range magnetic ordering or to the magnetic impurity shown in fig. 2b.
tures between 1.2 K and the Curie temperature, in order to detect possible anomalies in the thermal evolution of the magnetic moment. Finally, longer
I
800
600
ho
zoo B
Fig. 4. Hysteresis
20
10
The remanent
magnetization
is q = 0.3079(5)pa
I 0
2&
(IO-~
T)
400
and the coercive
600
field H,=183(2)~10-~
T.
V. Carteaux et al. / Magnetic structure of Cr,Si,Te,
130
Table 1 Irreductible representations of R-3 (k = 0). w = exp 2ni/3 E
C3
c3”
I-,
1
I-, r, r, r,
1 1 1 1
1 1 w w* w
1 1 w* w w*
r,
1
w*
w
I
ss5
s6
1
1
1 -1
-1
-1
w* w -w*
w w* -w
-1
-w
-w*
-1 1 1
Table 2 Bertaut’s modes for Cr,Si,Tq+ The 6(c) cation position is taken from the International Table of Crystallography (vol. 1): 0 0 .r (I), f f f + z (2), f + f + .r (3) 00-z(4),;++-2(5),$ff-z(6)
r,
xOvplane
L
-
s,,
+ -
r2
_
s,,
r3 r4
_ s,,
+ -
r5 r6
s2, %,
+ -
s3x -
s4x
_
s6x
_
_ s,,
+ +
s2, %x
+ +
s3,
+
&x _
s6x
1.6
s2, %,
+ +
2ooo-( Intensity
+ -
s2, %
-
s4z
~
: 3 Fig. 7. Thermal evolution of the integrated intensities of some magnetic peaks. The intensity variation indicates that the Curie temperature is 33(l) K.
s6z +
+
s3,
_I
s3z + s6z
s.4,
data acquisitions were performed above T, (50, 100, 200 and 268 K) to record data in the paramagnetic state. The diffraction patterns were analyzed by the Rietveld method [3] as modified by Hewat [4]. The nuclear scattering lengths and magnetic form fac-
.
Fig. 6. Belov and Kouvel curves for a powder sample of Cr,Si,Te, deduced from magnetization versus magnetic field measurements. A linear interpolation gives an ordering temperature of 32.9(5) K.
V. Carteaux et al. / Magnetic structure of Cr,Si,Te,
tors were taken from Koester and Rauch [5] and Watson and Freeman [6], respectively. Bertaut’s representation theory [7] was used to identify the possible models of magnetic structure.
131
3. Magnetic study Magnetic measurements on sample 1 were first performed by the Faraday method. Fig. 2 presents
l-
a
b
26.0
36.0
48.0
50.0
66.0
76.0
66.0
Fig. 8. Comparison between observed and calculated intensities of diffraction in the paramagnetic (50 K (a)) and ordered magnetic state (1.2 K (b)). The bottom line is the difference pattern at the same scale. Magnetic peaks are identified by stars. For sake of clarity, hkl have w omitted for 26 z- 55 O.
V. Carteaux et al. / Magnetic structure of Cr,Si,Te,
132
respectively the Cr,Si,T% inverse and direct susceptibility versus temperature. The first curve indicates a ferromagnetic behavior at about 30 K. The latter shows another ferromagnetic behavior with an ordering temperature of about 240 K. Our first problem was to determine whether the Curie temperature of Cr,Si,T% was 30 or 240 K. Susceptibility measurements performed on sample 2 were very helpfull (fig. 3). From figs. 2 and 3, we can conclude that the Cr,Si,T% Curie temperature is 30 K, and that the higher ordering temperatures correspond to magnetic impurities. They are chromium telluride Cr,Te, in sample 2, whereas the study of the nature of the impurity existing in sample 1, and which orders ferromagnetically at about 240 K, is currently in progress. The magnetization versus magnetic field curve (fig. 4) shows the hysteresis cycle of Cr,Si,T%. It presents a rather low remanent magnetization a, = 0.3079(5)~, and an important coercitive field H, = 183(2) x 1O-4 T. The magnetization versus temperature curve (fig. 5) reveals a “non-classical” behavior: we can observe a significant magnetization above the ordering temperature of about 30 K. This phenomenon could be due either to short range 2D magnetic ordering or to the presence of the magnetic impurity cited above. In order to determine accurately the Curie temwe used the Belov and perature of Cr,Si,Te,, Kouvel method [8,9]. This method is based on the
I
I
1.0
2.0
3’0
J
T(K)
evolution of the refined magnetic moments of cr’+.
- Fig. 9. Thermal
calculations of isothermal curves H/a versus u2. The measurements were performed with high magnetic fields (up to 2 T) for different temperatures. With this method, the ordering temperature is determined univocally by the isothermal curve which passes through the origin. Considering the curves reported on fig. 6, and with a sample interpolation, we find an ordering temperature of 32.9(5) K. The neutron diffraction study below shows that the magnetic peaks (corresponding to the magnetic ordering) disappear at 33(l) K, in excellent agreement with the temperature deduced from the Belov and Kouvel method.
C
20.88-
b I_( 100
Fig. 10. Magnetostriction
I 200
T(K)
I
100
I
I 200
I
J T(K)
for the cell parameters of Cr,Si,Tq+ Figs. 10a and b show respectively the thermal evolution of the parameters a and c.
V. Carteaux et al. / Magnetic structure of Cr,Si,Te,
4. Neutron diffraction study The neutron diffraction patterns were collected at various temperatures in the range 1.2-50 K, and the thermal evolution of the intensities of some magnetic peaks appears in fig. 7. This indicates that the Curie temperature corresponds to the minimum of X-‘(T) observed at 33 K. Below 33 K, new purely magnetic peaks appear which can be indexed in the nuclear cell. Furthermore, the absence of any magnetic contribution on the (0 0 1) reflections gives strong evidence of moments lying along the c axis. The magnetic structure has been solved using the representation analysis method of Bertaut [7]. For the space group R-3 (k = 0), the irreducible representations given in table 1 lead to the modes summarized in table 2. r, and r, representations are ruled out because they lead to a star configuration of spins in the (a, 6) plane for Cr3+. Only the r, and r, representations are in agreement with moments lying along the c axis: (1) the I’, representation corresponds to an antiferromagnetic behavior. (2) for the r, representation, the ferromagnetic behavior is due to the collinear configuration of spins lying along the c axis. Atomic coordinates and the spin component were refined simultaneously at each temperature. The best fit (Rmag = 0.095 at T = 1.2 K) corresponds to the I” mode as expected. The component of the magnetic moment on the c axis of the cell is listed table 3 with atomic coordinates. The comparison between observed and calculated profiles is given in fig. 8. The spins of Cr3+ exhibit a value 1_1=2.73(5)pa at T = 1.2 K, which is in excellent agreement with the results of the magnetic study. The thermal evolution of the magnetic moments, refined between 1.2 and 32 K, is presented on fig. 9. From fig. 10, which gives the thermal
133
Table 3 Refined cell parameters and atomic coordinates of Cr,SirT% at 50 K and 1.2 K. (Values at 1.2 K correspond to the second line.) a = 6.763(2) A, c = 20.582(3) A, (a = 6.773(2) A, c = 20.528(2) A) Atom
position
x
Y
z
&
0.3302(8) 0.3313(6) 0.0569(6) 0.0566(6) 0.2488(4) 0.2491(4)
0.30 0.15 0.30 0.15 0.30 0.15
Cr
6(c)
0 0
Si
6(c)
Te
18(f)
0 0 0.666(2) 0.666(2)
0 0 0 0 0.977(l) 0.977(l)
Rt
R Prof
R WProf
R N”d
R Mat?
4.20% 4.96%
8.63% 8.98%
7.83% 7.97%
4.20% 4.21%
9.52%
Refined magnetic moment (in pa) at T =1.2
K: S, = 2.73(5).
evolution of the cell parameters, it is clear that the occurrence of magnetic ordering induces a strong magnetostriction for the cell.
Acknowledgments The authors are very indebted to Professors R. Brec and G. Ferey for helpful discussions and a critical reading of the manuscript, and to Dr J. Pannetier (ILL Grenoble) for the collection of neutron diffraction data.
References (11 G. Ouvrard, E. Sandre and R. Brec, J. Solid State Chem. 73 (1988) 27. [2] R.E. Marsh, J. Solid State Chem. 77 (1988) 190. [3] H.M. Rietveld, J. Appl. Cryst. 2 (1969) 65. [4] A.W. Hewat, Harwell Report AERE, R 7350 (1973). [5] L. Koester and H. Rauch, IAEA Contract 2517/RB (1981). [6] R.E. Watson and J. Freeman, Acta Cryst. 14 (1961) 27. [7] E.F. Bertaut, Acta Cryst. 24 (1968) 217. [S] K.P. Belov and A.N. Goryaga, Fiz. Met. Metall. 2 (1956) 3. [9] J.S. Kouvel, Gen. El. Res. Lab., Report 59 R.L. 1799 (1957).