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Ferromagnetism of LaCrOS2
~
Solid State Communications, Printed in Great Britain.
Vol.53,No.3,
pp.227-230,
FERROMAGNETISM Wintenberger,
1985.
OF
0038-1098/85 $3.00 + .00 Pergamon Press Ltd.
LaCrOS2
M., Vovan T. and Guittard,
M.
Laboratoire de Chimie Min4rale Structurale Laboratoire Associ4 au C.N.R.S. n ° 200 Facultd des Sciences Pharmaceutiques et Biologiques de Paris V 4, Avenue de l'Observatoire - 75270 Paris, Cedex 06 - France (Received
18 october
1984
by E.F. Bertaut)
Magnetic measurements and neutron diffraction show that LaCrOS2 is a ferromagnet. Tc = 35 K . The magnetic space group is P b' n m'.
Introduction The preparation of LaCrOS2 and its crystal structure determination have been described recently 1,2. i In addition to the methods of ref. , the compound can be prepared by heating a carefully homogenized mixture of finely ground La202S and Cr2S3at 950 ° C with a small amount of iodine. The samples used for the present work were prepared by this method. LaCrOS2 crystallizes with space group Pbnm (D2hlS). The refinement of the lattice parameters using X-ray powder data gives a = l].23
b = 8.43
c = 3.7[
The structure is drawn on fig. I. All atoms are located on the mirror planes (4c sites). Cr atoms are surrounded by SsO octahedra, which share edges so as to form double chains running parallel to the c axis. The structure resembles that of CeCrSe33, which crystallizes in the same space group. However they are not isomorphous : the positions of the metalloids and of the lanthanide can be taken as homologous, but the chromium position is then shifted by c/2 on going from one structure to the other. In LaCrOS2 some La-D distances are remarkably short (2.33 and 2.47 A). As a consequence are seen (LaO) n n+ ribbons, formed by two La-O chains linked by La-O bonds and running parallel to the c axis. On the other hand the distances between metalloids are normal. In CeCrSe3 also there are double chains of Crcontaining octahedra, but in that case the short distances which can be noticed are some Se - Se distances (3.28 and 3.45 ~, whereas the Se z- radius is usually near 1.9 ~). We are interested here in the structure of CeCrSe3 because the compound LaCrSe3, which is known to be antiferromagnetic 4 is thought to be isomorphous to it. For LaCrSe3 T N = 160 K and the asymptotic Curie temperature @ = 99 K. The magnetic structure shows ferromagnetic coupling between Cr 3+ ions in one chain. The moments of Cr 3+ ions belonging to two linked chains are antiparallel. The antiferromagnetism implies also some negative interchain interactions. In ref. 4 it is assumed that the anti-
La S
Cr
0
Fig l - Projection of the crystal LaCrOS2 on a, b plane.
structure of
ferromagnetic interactions are of type Cr-SeSe-Cr, favoured by the short Se - Se distances. The chemical bonds being different in LaCrOS2 we could expect differences in the magnetic structure. Their determination was-the aim of this work.
227
228
FERROMAGNETISM Magnetic measurements
It is rather difficult to achieve a complete chemical reaction when preparing the compound. Thus our first susceptibility measurements, at T > 77 K with a translation balance, were performed on a few milligrams of single crystals extracted from a preparation under a binocular microscope. The susceptibility follows a Curie-Weiss Law X = C (T - e) -I for T > ]00 K (fig. 2 ). The effective moment p = 3.8 ~B is in good agreement with S = 3/2. The temperature @ = 55 K indicates that ferromagnetic interactions are dominant. After neutron diffraction had revealed a ferromagnetic structure, magnetization measurements up to 60 KOe and down to 4.2 K were done at Laboratoire Louis Neel of Grenoble by the axial extraction method, on a microcrystalline sample which had been checked to have a susceptibility similar to that of the single crystals above 77 K. These measurements gave the Curie temperature Tc = 35 K (fig. 2 ) , and the saturation magnetization at 4.2 K o = 2.6 PB/Cr 3~ (fig. 3). (the magnetization at 4.2. K varies linearly with H -I for H > 7 KOe). Neutron diffraction Powder diagrams (fig. 4 ) were recorded on the multidetector apparatus at Reactor Siloe of nuclear research center of Grenoble, at temperatures 79, 22 and 2.2 K with wave length = 2.508 X. On these diagrams are seen a few lines due to unindentified impurities (hatched lines). For the LaCrOS2 lines at 79 K one gets a satisfactory agreement between observed intensities and those calculated for purely nuclear scattering (table I). On the diagrams recorded at 22 and 2.2. K magnetic scattering appears on lines allowed by the crystallographic space group P b n m. This indicates ferromagnetic ordering (F mode) of the moments of Cr 3+ ions. Comparison of calculated and observed intensities shows that the moments are directed along b axis (tablell). For Cr 3+ we used the form factor of Watson and Freeman ~. With help of the scalefactor deduced from the intensities of nuclear lines, the scaling of magnetic intensities of II0, 200 and ]I] leads to a moment value ~ = 2.7 ~B/Cr 3+ at 2.2 K. The precision of this determination is rather poor because the magnetic intensi-
OF LaCrOS 2
Vol.
53, No. 3
ties are obtained by substraetion and also the estimation of the background level is difficult on each diagram. The magnetic space group is P h' n m'. In this group the mode Fy could be associated with a mode C (++ --, the atoms being numbered in the order of the International Tables) along a axis 6. One does not observe any contribution of this Cx mode. Discussion The existence of ferromagnetic intrachain interactions results from a predominance, often observed in other Cr 3+ - containing compounds, of positive Cr - (O or S) - Cr interactions at angles close to 90 °, over negative direct Cr-Cr interactions. The overall ferromagnetism, somewhat unexpected, implies that some of the interchain interactions are also positive. For these interchain interactions numerous bonding paths can be considered. As there are no short S - S bonds interactions of type Cr-S-S-Cr should not be large. One is then left with paths involving La atoms. They are of three kinds. a) Cr - O - La - O - Cr b) Cr - S - La - S - Cr c) Cr - S - La - O - Cr All the bond angles in these paths have values between 70 and 130 ° . The Cr - S bond is more covalent than the Cr - 0 one, but here the La-O bond is strongly enhanced with respect to the La-S one and one can assume that as a whole path (a) is more active than (b), path (c) being intermediate between the other two. On fig. (I) it is seen that for the interaction between sites I and 2 there are several possible paths, among which two are of type (a). This, added to intrachain interactions can propagate the magnetic order in (100) planes. Coupling between these planes can only occur via type (c) paths, as can be seen on fig (;) for interactions between sites I and 3 or 2 and 3. This discussion remains highly speculative. However it can be remarked that the relatively large difference betweeen Te and 0 is in agreement with the above model, which has a slightly bidimensionnal character. Acknaledgement - The authors thank E. Bar~ thelemy, J. Barlet and E. Roudaut for their contribution to the experiments.
3T=4.2K T
_,= ,oo 50
E
b
~0 2 E
f
x
0
I00
200
300
TK Fig 2 - Magnetization and inverse susceptibility versus temperature.
I0
20 30 KOe (H)
Fig 3 - Magnetization
4o
versus field at 4.2 K.
Vol. 53, No. 3
FERROMAGNETISM OF LaCrOS 2
6olI
229
311 23O
110
221 9 ]IS
79 K 121
200 I
4~
101 JlI 310 II ~ 111
12o
• 0.20 ~
II
400 o~ 320 211
I
I10
I
94,4
2,2K
6C
4(
I
10
t
I 20
Fig 4 - Neutron diffraction diagrams at 79 K
,
and 2.2 K.
I
30
O
230
Vol. 53, No. 3
FERROMAGNETISM OF LaCrOS 2 Table - I
calculated and o b s e r v e d nuclear intensities 2
I ffi pF N L(O) h k Z !10 200 210 020 120 101 310 220 111 211 f O320 00 21 130 121 410 301 311 230 221
f
I calc.
Iobs.
263 95 17,5 30 56,6 75,6 55 5,6 33,8 36
233 92 16,6 39 61,2 90,7 63 seen 46 seen
43
seen
31,2 225 14,6 20
seen 246 seen seen
698
648
the lines indicated as "seen" cannot be measured because of the ill-defined background level.
Table - II
calculated and observed magnetic intensities
I = 0.272 ~2 PFM 2 f2 L(0)min2a k I
I calc.
Iobs.
110 200 210 120 101 310 220 1ll 211 400 320
39,5 141 12,7 0,4 3,2 14 7,5 100 0
46,5 138 22 0 % 4 13 ~ 6 97 0
20
seen
021 130 121 410 301 311 230 21
20,7 3 ~ 0 12 I0
seen 0 0 seen seen
46
~52
REFERENCES
I. 2. 3.
VOVAN, T., DUGU~, J. and GUITTARD, M., Mat. Res. Bull. 13, I163 (1978). DUGU~, J., V-OVAN, T. and VILLERS, J., Acta Cryst. B 36, 1291 (1980). NGUYEN HUY DUNG, ETIENNE, J. and LARUELLE p., Bull. Soc. Chim. Ft. 2433 (1971).
4. 5. 6.
PLUMIER, R. and GOROCHOV, O., J. de Physique, Lettres 35, L265 (1974). WATSON, R.E. and FREEMAN, A.J., Acta Cryst. 14, 27 ( 1 9 6 1 ) . B--ERTAUT, E.F., Acta Crystallogr., A24, 217 (1968).
Solid State Communications, Printed in Great Britain.
Vol.53,No.3,
pp.227-230,
FERROMAGNETISM Wintenberger,
1985.
OF
0038-1098/85 $3.00 + .00 Pergamon Press Ltd.
LaCrOS2
M., Vovan T. and Guittard,
M.
Laboratoire de Chimie Min4rale Structurale Laboratoire Associ4 au C.N.R.S. n ° 200 Facultd des Sciences Pharmaceutiques et Biologiques de Paris V 4, Avenue de l'Observatoire - 75270 Paris, Cedex 06 - France (Received
18 october
1984
by E.F. Bertaut)
Magnetic measurements and neutron diffraction show that LaCrOS2 is a ferromagnet. Tc = 35 K . The magnetic space group is P b' n m'.
Introduction The preparation of LaCrOS2 and its crystal structure determination have been described recently 1,2. i In addition to the methods of ref. , the compound can be prepared by heating a carefully homogenized mixture of finely ground La202S and Cr2S3at 950 ° C with a small amount of iodine. The samples used for the present work were prepared by this method. LaCrOS2 crystallizes with space group Pbnm (D2hlS). The refinement of the lattice parameters using X-ray powder data gives a = l].23
b = 8.43
c = 3.7[
The structure is drawn on fig. I. All atoms are located on the mirror planes (4c sites). Cr atoms are surrounded by SsO octahedra, which share edges so as to form double chains running parallel to the c axis. The structure resembles that of CeCrSe33, which crystallizes in the same space group. However they are not isomorphous : the positions of the metalloids and of the lanthanide can be taken as homologous, but the chromium position is then shifted by c/2 on going from one structure to the other. In LaCrOS2 some La-D distances are remarkably short (2.33 and 2.47 A). As a consequence are seen (LaO) n n+ ribbons, formed by two La-O chains linked by La-O bonds and running parallel to the c axis. On the other hand the distances between metalloids are normal. In CeCrSe3 also there are double chains of Crcontaining octahedra, but in that case the short distances which can be noticed are some Se - Se distances (3.28 and 3.45 ~, whereas the Se z- radius is usually near 1.9 ~). We are interested here in the structure of CeCrSe3 because the compound LaCrSe3, which is known to be antiferromagnetic 4 is thought to be isomorphous to it. For LaCrSe3 T N = 160 K and the asymptotic Curie temperature @ = 99 K. The magnetic structure shows ferromagnetic coupling between Cr 3+ ions in one chain. The moments of Cr 3+ ions belonging to two linked chains are antiparallel. The antiferromagnetism implies also some negative interchain interactions. In ref. 4 it is assumed that the anti-
La S
Cr
0
Fig l - Projection of the crystal LaCrOS2 on a, b plane.
structure of
ferromagnetic interactions are of type Cr-SeSe-Cr, favoured by the short Se - Se distances. The chemical bonds being different in LaCrOS2 we could expect differences in the magnetic structure. Their determination was-the aim of this work.
227
228
FERROMAGNETISM Magnetic measurements
It is rather difficult to achieve a complete chemical reaction when preparing the compound. Thus our first susceptibility measurements, at T > 77 K with a translation balance, were performed on a few milligrams of single crystals extracted from a preparation under a binocular microscope. The susceptibility follows a Curie-Weiss Law X = C (T - e) -I for T > ]00 K (fig. 2 ). The effective moment p = 3.8 ~B is in good agreement with S = 3/2. The temperature @ = 55 K indicates that ferromagnetic interactions are dominant. After neutron diffraction had revealed a ferromagnetic structure, magnetization measurements up to 60 KOe and down to 4.2 K were done at Laboratoire Louis Neel of Grenoble by the axial extraction method, on a microcrystalline sample which had been checked to have a susceptibility similar to that of the single crystals above 77 K. These measurements gave the Curie temperature Tc = 35 K (fig. 2 ) , and the saturation magnetization at 4.2 K o = 2.6 PB/Cr 3~ (fig. 3). (the magnetization at 4.2. K varies linearly with H -I for H > 7 KOe). Neutron diffraction Powder diagrams (fig. 4 ) were recorded on the multidetector apparatus at Reactor Siloe of nuclear research center of Grenoble, at temperatures 79, 22 and 2.2 K with wave length = 2.508 X. On these diagrams are seen a few lines due to unindentified impurities (hatched lines). For the LaCrOS2 lines at 79 K one gets a satisfactory agreement between observed intensities and those calculated for purely nuclear scattering (table I). On the diagrams recorded at 22 and 2.2. K magnetic scattering appears on lines allowed by the crystallographic space group P b n m. This indicates ferromagnetic ordering (F mode) of the moments of Cr 3+ ions. Comparison of calculated and observed intensities shows that the moments are directed along b axis (tablell). For Cr 3+ we used the form factor of Watson and Freeman ~. With help of the scalefactor deduced from the intensities of nuclear lines, the scaling of magnetic intensities of II0, 200 and ]I] leads to a moment value ~ = 2.7 ~B/Cr 3+ at 2.2 K. The precision of this determination is rather poor because the magnetic intensi-
OF LaCrOS 2
Vol.
53, No. 3
ties are obtained by substraetion and also the estimation of the background level is difficult on each diagram. The magnetic space group is P h' n m'. In this group the mode Fy could be associated with a mode C (++ --, the atoms being numbered in the order of the International Tables) along a axis 6. One does not observe any contribution of this Cx mode. Discussion The existence of ferromagnetic intrachain interactions results from a predominance, often observed in other Cr 3+ - containing compounds, of positive Cr - (O or S) - Cr interactions at angles close to 90 °, over negative direct Cr-Cr interactions. The overall ferromagnetism, somewhat unexpected, implies that some of the interchain interactions are also positive. For these interchain interactions numerous bonding paths can be considered. As there are no short S - S bonds interactions of type Cr-S-S-Cr should not be large. One is then left with paths involving La atoms. They are of three kinds. a) Cr - O - La - O - Cr b) Cr - S - La - S - Cr c) Cr - S - La - O - Cr All the bond angles in these paths have values between 70 and 130 ° . The Cr - S bond is more covalent than the Cr - 0 one, but here the La-O bond is strongly enhanced with respect to the La-S one and one can assume that as a whole path (a) is more active than (b), path (c) being intermediate between the other two. On fig. (I) it is seen that for the interaction between sites I and 2 there are several possible paths, among which two are of type (a). This, added to intrachain interactions can propagate the magnetic order in (100) planes. Coupling between these planes can only occur via type (c) paths, as can be seen on fig (;) for interactions between sites I and 3 or 2 and 3. This discussion remains highly speculative. However it can be remarked that the relatively large difference betweeen Te and 0 is in agreement with the above model, which has a slightly bidimensionnal character. Acknaledgement - The authors thank E. Bar~ thelemy, J. Barlet and E. Roudaut for their contribution to the experiments.
3T=4.2K T
_,= ,oo 50
E
b
~0 2 E
f
x
0
I00
200
300
TK Fig 2 - Magnetization and inverse susceptibility versus temperature.
I0
20 30 KOe (H)
Fig 3 - Magnetization
4o
versus field at 4.2 K.
Vol. 53, No. 3
FERROMAGNETISM OF LaCrOS 2
6olI
229
311 23O
110
221 9 ]IS
79 K 121
200 I
4~
101 JlI 310 II ~ 111
12o
• 0.20 ~
II
400 o~ 320 211
I
I10
I
94,4
2,2K
6C
4(
I
10
t
I 20
Fig 4 - Neutron diffraction diagrams at 79 K
,
and 2.2 K.
I
30
O
230
Vol. 53, No. 3
FERROMAGNETISM OF LaCrOS 2 Table - I
calculated and o b s e r v e d nuclear intensities 2
I ffi pF N L(O) h k Z !10 200 210 020 120 101 310 220 111 211 f O320 00 21 130 121 410 301 311 230 221
f
I calc.
Iobs.
263 95 17,5 30 56,6 75,6 55 5,6 33,8 36
233 92 16,6 39 61,2 90,7 63 seen 46 seen
43
seen
31,2 225 14,6 20
seen 246 seen seen
698
648
the lines indicated as "seen" cannot be measured because of the ill-defined background level.
Table - II
calculated and observed magnetic intensities
I = 0.272 ~2 PFM 2 f2 L(0)min2a k I
I calc.
Iobs.
110 200 210 120 101 310 220 1ll 211 400 320
39,5 141 12,7 0,4 3,2 14 7,5 100 0
46,5 138 22 0 % 4 13 ~ 6 97 0
20
seen
021 130 121 410 301 311 230 21
20,7 3 ~ 0 12 I0
seen 0 0 seen seen
46
~52
REFERENCES
I. 2. 3.
VOVAN, T., DUGU~, J. and GUITTARD, M., Mat. Res. Bull. 13, I163 (1978). DUGU~, J., V-OVAN, T. and VILLERS, J., Acta Cryst. B 36, 1291 (1980). NGUYEN HUY DUNG, ETIENNE, J. and LARUELLE p., Bull. Soc. Chim. Ft. 2433 (1971).
4. 5. 6.
PLUMIER, R. and GOROCHOV, O., J. de Physique, Lettres 35, L265 (1974). WATSON, R.E. and FREEMAN, A.J., Acta Cryst. 14, 27 ( 1 9 6 1 ) . B--ERTAUT, E.F., Acta Crystallogr., A24, 217 (1968).