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Neutron powder diffraction studies of two-dimensional magnetic oxides
Journal of Magnetism and Magnetic Materials 14 (1979) 1 5 9 - 1 6 2 © North-Holland Publishing Company
NEUTRON POWDER DIFFRACTION STUDIES OF TWO-DIMENSIONAL MAGNETIC OXIDES J.L. SOUBEYROUX Instttut Laue-Langevin, 156X, F-38042 Grenoble Cbdex, France
D. FRUCHART Laboratoire des Rayons-X, 166X, F-38042 Grenoble C~dex, France
and C. DELMAS and G. Le FLEM Laboratoire de Chtmw du Sohde du CNRS, 351 Cours de la Liberation, F-33405 Talence, France
ACrO 2 compounds have been shown to exhibit a 2D character. LiCrO 2 orders 3D at T N = 62 K; possible magnetic structures consistent w~th neutron powder diffraction measurements are discussed. The 2D ordering m this compound is ~hown by the dlffractlon patterns to persist up to 100 K. The temperature dependence o f the diffuse intensity of NaCrO 2 and KCrO 2 m&cates a N6el temperature for 3D ordering of 45 and 26 K, respectively, but no sharp peaks consequent upon this ordering are visible in diffrachon patterns taken at temperatures down to 2 K.
I. Introduction
3. Experimental results and discussion
The series of compounds ACrO2 (A = Li, Na, K) crystallize m the a-NaFeO 2 layer structure (space group R3m). This structure is well adapted to the study of the dimensionality of magnetic interactions. Previous susceptibihty measurements on LiCrO2 [1-2] and NaCrO2 [3] indicated the antiferromagnetic character of the interactions. We have investigated tlus system by means of the M6ssbauer effect [4] and susceptibility measurements [5]. We have also made neutron powder diffraction diagrams as a function of temperature.
High resolution diagrams of ACrO2 were obtained at room temperature in order to refine the crystallographic structures. Calculations were made using the
Table 1 Crystallographic parameters of ACrO 2 compounds (x = y =
0.0)
2. Experimental Our neutron measurements were carried out at the ILL on the dlffractometer D1B which is equipped with a multidetector. This enabled us to obtain high sthtistlcs which facilitated &fferences to be made between diagrams. Measurements were made at two wavelengths: 2.52 A with a high flux and 1.28 A to provide good resolution. 159
R3m
L1Cr02
NaCr02
KCrO2
a c
2 898(1) 14.423(5)
2.975(1) 15.968(5)
3.042(1) 17.888(5)
Cr
z = 0.0 B = 0.10
z = 0.0 B = 0.10
z = 0.0 B = 0.10
A
z=0.5 B = 1.00
z=0.5 B = 0.62
z=05 B = 0.30
Ox
z = 0.261(1) B = 0.27
z = 0 270(1) B = 0.24
z = 0.279(1) B = 0.25
R
2.6%
5.1%
6.0%
160
J L S o u b e y r o u x et al.
s)
012 014
016
018
I
I
/ Neutron powder dtffractlon of 2D magnetic oxides Table 2 Magnetic m t e n s m e s observed and calculated with the three models
022
hkl
<
42K 30K
40K
5o/<
~ j v , , ~ 601<
I0
0 1 0 ~~ 103.0 011 01 2 56.4 01 3 54 5 014 9.4 015 10.5 01 6 32 8 017 34 6 018 26.2
15
20
25
I c mode l 3
26.5
14.5
94.3 30.2
47 6
30.7
147
31 3
57.3 45.3 35,0 34.7 41 4 44 4 374
= 1.77u B 0 =0.0
# = 1.28~B 4 ) : 9 0 ° a)
# = 2.05ta B ~=53 °
46.5
32.8
35.1
33.0 32.2
344
23 3
33.5 20.8
65K
10
I c model 2 k] k2
57 6
70K
80/<
I c model 1 k1 k2
33,6
a) ~ Angle between m k and c-axis.
313
Fig 1. Neutron diffraction difference dmgrams of LiCrO 2.
vectors are needed, k1 = [-~,32- , 0 ] a n d k 2 = [ 3t , 2~,
2'-].
profile refinement technique [6]. The results are summarized in table 1. Diagrams with good statimcs were obtained at various temperatures m the range 2 - 1 0 0 K. At 4.2 K, LICrO2 is ordered three-dimensionally (fig. 1). The &agram appears as a series of diffraction peaks. As the temperature is increased the intensity of peaks decreases until at 65 K, only a broad diffuse bump can be observed. 2D behaviour: in the range 6 5 - 1 0 0 K, the 2D contribution can be observed from our measurements. It can be explained by the diffraction of magnetically ordered planes. 3D ordering in real space gives a point in reciprocal space and a peak m the powder diagram. 2D ordering in real space gives a line in reciprocal space perpendmular to the plane of ordering and hence broad bumps in the powder pattern. In our case, magnetic intensity appears at all scattering angles above a lower angle corresponding to the 010-ridge. We can deduce that the 001 planes are ordered. 3D structure: m the range 2 - 6 5 K, the 3D antiferromagnetic ordering sets in. In our measurements it results in a series of peaks which can be indexed as 01l and 02l reflections using two magnetic cells: ax/3, ax/3, c and ax/~, ax/-3, 2c. In th~.s case two propagation
Tile theoretical calculation of the magnetic moment leads to several models model I m k is parallel to the c-axis; thas involves for the moments to be m a plane with a modulated intensity [ (arb ur~t)
i'
L1CrO~ '~N :62 K
•
,
,
L
• 5'0 . . . .
I
~ Experimental poml wtth E G D _.. Brdloum curve For- s:3/2
~-
Ido TiK)
I
'~
NaCr02
KCr'O2 h=26K
....
, ....
~(K)
,
,®
P
o
T (K)
50
F~g. 2. Integrated intensity versus temperature o f ACrO2 compounds
161
J L. Soubeyroux et aL / Neutron powder diffraction of 2D magnetic oxzdes model 2: m k is perpen&cular to the c-axis, the mo-
In all cases, phase angles and reflection intensities have been calculated using the AMS program of the ILL [7]. None of these models gives reasonable values for the observed intensities of 014 and 015 reflections. A more complex arrangement may be envisaged by taking account of short range defects. A single crystal investigation is under way. In fig. 2 the total integrated intens]ty above the background, in the range theta = 1 6 - 2 8 o, has been plotted versus temperature. A good fit with the Brillouin-function for s = 3 and the N6el temperature ( T N = 62 K) as determined by the M~Sssbauer effect is in agreement with 3D ordering. For NaCrO2 the minimum of the reciprocal susceptibility occurs at 46 K [3] but the 3D peaks remain very weak down to 2 K (fig. 3). As for LiCrO2, a plot of the integrated intensity of the diffuse peak versus temperature is in excellent agreement with the Brillouin-function giving T N = (45 + 2) K (fig. 2). For KCrO2 the reciprocal susceptibility does not extublt any minimum. As for NaCrO2, there is no evidence of 3D order (fig. 4). However, the integrated intensity of the diffuse peak versus temperature is in good agreement with the Brillouin-functlon gixang TN = (26 + 2) K (fig. 2).
ments in the plane are in a triangular arrangement w]th an angle of -23n. model 3: a third type of structure wtuch is in agreement with the rules of extinction of the experimental peaks and requires only a single propagation vector consists of a triangular arrangement of the moments in a zOx or zOy plane. In this case all the magnetic reflections can be calculated using the magnetic cell" ax/~, ax/~, 2c. The magnetic Bragg intensities using these three models have been calculated and the results are given in table 2. The directions of the propagation vectors kl and k2 fix the phase angles for the ferromagnetic and ant]ferromagnetic order respectively.
E E3' 8
2K
<
10K
30K t-
..........
.......
2K
35K 6K 1OK 14K
....
45K 20K
~/~3~~
50K
25K .............
~
6
0
-
~ 8
70K , 18
30K
~
K
f 28
;
Fig. 3. Neutron diffraction difference dlagrams of NaCrO2
-
-
~
35K
--_
. . . . . . . . . . . . . . . . .
is
2o
40K
23
Fig. 4. Neutron diffraction difference diagrams of KCrO2.
162
J L Soubeyroux et a l / Neutron powder dtffraction of 2D magnetw oxides
4. Conclusion
References
LiCrO2 1s ordered 2D over a wide range of temperature but the 3D order is not perfect and cannot be described by a simple model. Even in the absence of Bragg peaks associated with 3D order for NaCrO2 and KCrO2 we can deduce that these compounds are ordered but the number of short range defects is so high that no definite magnetic periodicity is estabhshed along the c-axis. Tlus fact together with the ridge-like character of the diffuse magnetic scattering around TN gives support for the two-dimensional nature of magnetism m ACrO2 compounds.
[1 ] P.F. Bongers, Dissertation, University of Leiden, Leiden (1957) p.44. [2] A. Tauber, W.M. Moiler and E Banks, J. Sohd State Chem 4 (1972) 138 [3] P.R Elhston, F. Habbal, N Saleh, G E Watson, K.W Blazey and H. Rohrer, J. Phys. Chem. Sohds 36 (1975) 877 [4] C. Delmas, F. Menil, G. Le Flem, C Fouassler and P. Hagenmuller, J Phys. Chem. Sohds 39 (1978) 51 [5] C. Delmas, G Le Flem, C. Fouassaer and P. Hagenmuller. J Phys Chem. Solids 39 (1978) 55 [6] H M. Rletveld, J. Appl Cryst 2 (1969) 65 [7 ] P. Wolfers, Thesis, Grenoble (1970).
NEUTRON POWDER DIFFRACTION STUDIES OF TWO-DIMENSIONAL MAGNETIC OXIDES J.L. SOUBEYROUX Instttut Laue-Langevin, 156X, F-38042 Grenoble Cbdex, France
D. FRUCHART Laboratoire des Rayons-X, 166X, F-38042 Grenoble C~dex, France
and C. DELMAS and G. Le FLEM Laboratoire de Chtmw du Sohde du CNRS, 351 Cours de la Liberation, F-33405 Talence, France
ACrO 2 compounds have been shown to exhibit a 2D character. LiCrO 2 orders 3D at T N = 62 K; possible magnetic structures consistent w~th neutron powder diffraction measurements are discussed. The 2D ordering m this compound is ~hown by the dlffractlon patterns to persist up to 100 K. The temperature dependence o f the diffuse intensity of NaCrO 2 and KCrO 2 m&cates a N6el temperature for 3D ordering of 45 and 26 K, respectively, but no sharp peaks consequent upon this ordering are visible in diffrachon patterns taken at temperatures down to 2 K.
I. Introduction
3. Experimental results and discussion
The series of compounds ACrO2 (A = Li, Na, K) crystallize m the a-NaFeO 2 layer structure (space group R3m). This structure is well adapted to the study of the dimensionality of magnetic interactions. Previous susceptibihty measurements on LiCrO2 [1-2] and NaCrO2 [3] indicated the antiferromagnetic character of the interactions. We have investigated tlus system by means of the M6ssbauer effect [4] and susceptibility measurements [5]. We have also made neutron powder diffraction diagrams as a function of temperature.
High resolution diagrams of ACrO2 were obtained at room temperature in order to refine the crystallographic structures. Calculations were made using the
Table 1 Crystallographic parameters of ACrO 2 compounds (x = y =
0.0)
2. Experimental Our neutron measurements were carried out at the ILL on the dlffractometer D1B which is equipped with a multidetector. This enabled us to obtain high sthtistlcs which facilitated &fferences to be made between diagrams. Measurements were made at two wavelengths: 2.52 A with a high flux and 1.28 A to provide good resolution. 159
R3m
L1Cr02
NaCr02
KCrO2
a c
2 898(1) 14.423(5)
2.975(1) 15.968(5)
3.042(1) 17.888(5)
Cr
z = 0.0 B = 0.10
z = 0.0 B = 0.10
z = 0.0 B = 0.10
A
z=0.5 B = 1.00
z=0.5 B = 0.62
z=05 B = 0.30
Ox
z = 0.261(1) B = 0.27
z = 0 270(1) B = 0.24
z = 0.279(1) B = 0.25
R
2.6%
5.1%
6.0%
160
J L S o u b e y r o u x et al.
s)
012 014
016
018
I
I
/ Neutron powder dtffractlon of 2D magnetic oxides Table 2 Magnetic m t e n s m e s observed and calculated with the three models
022
hkl
<
42K 30K
40K
5o/<
~ j v , , ~ 601<
I0
0 1 0 ~~ 103.0 011 01 2 56.4 01 3 54 5 014 9.4 015 10.5 01 6 32 8 017 34 6 018 26.2
15
20
25
I c mode l 3
26.5
14.5
94.3 30.2
47 6
30.7
147
31 3
57.3 45.3 35,0 34.7 41 4 44 4 374
= 1.77u B 0 =0.0
# = 1.28~B 4 ) : 9 0 ° a)
# = 2.05ta B ~=53 °
46.5
32.8
35.1
33.0 32.2
344
23 3
33.5 20.8
65K
10
I c model 2 k] k2
57 6
70K
80/<
I c model 1 k1 k2
33,6
a) ~ Angle between m k and c-axis.
313
Fig 1. Neutron diffraction difference dmgrams of LiCrO 2.
vectors are needed, k1 = [-~,32- , 0 ] a n d k 2 = [ 3t , 2~,
2'-].
profile refinement technique [6]. The results are summarized in table 1. Diagrams with good statimcs were obtained at various temperatures m the range 2 - 1 0 0 K. At 4.2 K, LICrO2 is ordered three-dimensionally (fig. 1). The &agram appears as a series of diffraction peaks. As the temperature is increased the intensity of peaks decreases until at 65 K, only a broad diffuse bump can be observed. 2D behaviour: in the range 6 5 - 1 0 0 K, the 2D contribution can be observed from our measurements. It can be explained by the diffraction of magnetically ordered planes. 3D ordering in real space gives a point in reciprocal space and a peak m the powder diagram. 2D ordering in real space gives a line in reciprocal space perpendmular to the plane of ordering and hence broad bumps in the powder pattern. In our case, magnetic intensity appears at all scattering angles above a lower angle corresponding to the 010-ridge. We can deduce that the 001 planes are ordered. 3D structure: m the range 2 - 6 5 K, the 3D antiferromagnetic ordering sets in. In our measurements it results in a series of peaks which can be indexed as 01l and 02l reflections using two magnetic cells: ax/3, ax/3, c and ax/~, ax/-3, 2c. In th~.s case two propagation
Tile theoretical calculation of the magnetic moment leads to several models model I m k is parallel to the c-axis; thas involves for the moments to be m a plane with a modulated intensity [ (arb ur~t)
i'
L1CrO~ '~N :62 K
•
,
,
L
• 5'0 . . . .
I
~ Experimental poml wtth E G D _.. Brdloum curve For- s:3/2
~-
Ido TiK)
I
'~
NaCr02
KCr'O2 h=26K
....
, ....
~(K)
,
,®
P
o
T (K)
50
F~g. 2. Integrated intensity versus temperature o f ACrO2 compounds
161
J L. Soubeyroux et aL / Neutron powder diffraction of 2D magnetic oxzdes model 2: m k is perpen&cular to the c-axis, the mo-
In all cases, phase angles and reflection intensities have been calculated using the AMS program of the ILL [7]. None of these models gives reasonable values for the observed intensities of 014 and 015 reflections. A more complex arrangement may be envisaged by taking account of short range defects. A single crystal investigation is under way. In fig. 2 the total integrated intens]ty above the background, in the range theta = 1 6 - 2 8 o, has been plotted versus temperature. A good fit with the Brillouin-function for s = 3 and the N6el temperature ( T N = 62 K) as determined by the M~Sssbauer effect is in agreement with 3D ordering. For NaCrO2 the minimum of the reciprocal susceptibility occurs at 46 K [3] but the 3D peaks remain very weak down to 2 K (fig. 3). As for LiCrO2, a plot of the integrated intensity of the diffuse peak versus temperature is in excellent agreement with the Brillouin-function giving T N = (45 + 2) K (fig. 2). For KCrO2 the reciprocal susceptibility does not extublt any minimum. As for NaCrO2, there is no evidence of 3D order (fig. 4). However, the integrated intensity of the diffuse peak versus temperature is in good agreement with the Brillouin-functlon gixang TN = (26 + 2) K (fig. 2).
ments in the plane are in a triangular arrangement w]th an angle of -23n. model 3: a third type of structure wtuch is in agreement with the rules of extinction of the experimental peaks and requires only a single propagation vector consists of a triangular arrangement of the moments in a zOx or zOy plane. In this case all the magnetic reflections can be calculated using the magnetic cell" ax/~, ax/~, 2c. The magnetic Bragg intensities using these three models have been calculated and the results are given in table 2. The directions of the propagation vectors kl and k2 fix the phase angles for the ferromagnetic and ant]ferromagnetic order respectively.
E E3' 8
2K
<
10K
30K t-
..........
.......
2K
35K 6K 1OK 14K
....
45K 20K
~/~3~~
50K
25K .............
~
6
0
-
~ 8
70K , 18
30K
~
K
f 28
;
Fig. 3. Neutron diffraction difference dlagrams of NaCrO2
-
-
~
35K
--_
. . . . . . . . . . . . . . . . .
is
2o
40K
23
Fig. 4. Neutron diffraction difference diagrams of KCrO2.
162
J L Soubeyroux et a l / Neutron powder dtffraction of 2D magnetw oxides
4. Conclusion
References
LiCrO2 1s ordered 2D over a wide range of temperature but the 3D order is not perfect and cannot be described by a simple model. Even in the absence of Bragg peaks associated with 3D order for NaCrO2 and KCrO2 we can deduce that these compounds are ordered but the number of short range defects is so high that no definite magnetic periodicity is estabhshed along the c-axis. Tlus fact together with the ridge-like character of the diffuse magnetic scattering around TN gives support for the two-dimensional nature of magnetism m ACrO2 compounds.
[1 ] P.F. Bongers, Dissertation, University of Leiden, Leiden (1957) p.44. [2] A. Tauber, W.M. Moiler and E Banks, J. Sohd State Chem 4 (1972) 138 [3] P.R Elhston, F. Habbal, N Saleh, G E Watson, K.W Blazey and H. Rohrer, J. Phys. Chem. Sohds 36 (1975) 877 [4] C. Delmas, F. Menil, G. Le Flem, C Fouassler and P. Hagenmuller, J Phys. Chem. Sohds 39 (1978) 51 [5] C. Delmas, G Le Flem, C. Fouassaer and P. Hagenmuller. J Phys Chem. Solids 39 (1978) 55 [6] H M. Rletveld, J. Appl Cryst 2 (1969) 65 [7 ] P. Wolfers, Thesis, Grenoble (1970).