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Studies of the superconducting behaviour of TlBa2−xLaxCuO5−y
PHYSICA
PhysicaC 178 (1991) 193-196 North-Holland
Studies of the superconducting behaviour of T1Ba2_ LaxCuOs _y A. S u n d a r e s a n a, A.K. R a j a r a j a n b, L.C. G u p t a b, M. Sharon a a n d R. V i j a y a r a g h a v a n b a Department of Chemistry, Indian Institute of Technology, Bombay 400 076, India b SolidState Physics Group, Tata Institute of Fundamental Research, Bombay 400 005, India Received 9 April 1991
The superconducting behaviour of the system T1Ba2_xLaxCuOs_~ has been investigated. Samples prepared by the procedure described in this paper show a decrease in Tc with increasing lanthanum concentration. This is in contrast to the trend reported by other authors [ 15,19 ]. This difference in behaviour was explained in terms of the oxygen deficiency y in the parent compound T1Ba2CuO5 y.
I. Introduction There are two general classes of Tl-based superconducting compounds, namely, T12Ba2Can_iCunO2n+4(n = 1, 2, 3 and 4) [ 1-7 ] and TlA2Can_lCunO2,+3 ( n = 1, 2, 3, 4 and 5; A=Sr, Ba) [ 6-11 ] systems. There are double T1-O layers in the former class of compounds just as there are double Bi-O layers in the Bi-based high-T~ materials Bi2Sr2Ca._jCu~O2,+4 ( n = 1, 2, 3 and 4) [ 12-14]. The latter class of compounds has single T1-O layers, but no Bi-based monolayer high-To systems have been reported so far. The n = 1 member T1Ba2CuO5 of the class of single T I - O layer compounds has been of particular interest and has studied extensively. In this system copper seems to be in the trivalent state. This material has been reported by several workers to be either insulating or superconducting with low Tc ( < 10 K) [ 15,16]. Superconductivity has been induced/enhanced in this system by substituting Ba by La in varied amounts [ 17,18 ]. In the La-doped materials T1Ba2_xLaxCuOs, Tc has been reported to increase with an increase of x with maximum Tc=52 K [15,19] with x=0.8. In our samples, which have been prepared by a slightly different heating procedure (see below), we have observed the opposite trend of the variation of Tc with x, namely, Tc decreases with increase of x. Our 'results suggest that the parent compound T1BaECuO5 itself could possibly be made superconducting, with
considerably high T~ ( > 50 K), by a careful adjustment of its oxygen stoichiometry and without introducing any lanthanum in the structure. We report the results of the measurements and discuss them in terms of the oxygen-stoichiometry of the parent material.
2. Experimental Samples of T1Ba2_xLaxCuOs_~ (0.2_
0921-4534/91/$03.50 © 1991 Elsevier Science Publishers B.V. All fights reserved
194
.4.
Sundaresan et al.
rials was studied by recording the p o w d e r X-ray diffraction pattern in each case. Superconductivity was studied by AC susceptibility technique. In these measurements, the t e m p e r a t u r e o f the samples was controlled to an accuracy o f within 1 K.
/
TlBa2 ,La,('uO~
,
5.855 ...... a
gcD L~E
~< 3.845
{
U 3. Results and discussions
5
Figure 1 shows the X-ray diffraction pattern o f the material TlBa~.zLao.sCuO~ ,.. All the lines o f the diffraction pattern, except the one m a r k e d *, can be indexed in terms o f the p r i m i t i v e tetragonal 1201 phase. The line m a r k e d as • seems to indicate the presence o f traces o f BaCO3. Similar X-ray diffraction patterns have been observed in the case o f samples with various other values o f x. It is i m p o r t a n t to emphasize that the X-ray diffraction patterns do not indicate the presence o f 2201 phase. The lattice p a r a m e t e r s a and c were o b t a i n e d using a least squares fitting procedure. Figures 2 ( a ) and ( b ) show that both lattice parameters, a and c, decrease with increase o f the l a n t h a n u m concentration x. It should be noted that the decrease o f c is more p r o n o u n c e d than that o f a. T e m p e r a t u r e d e p e n d e n c e o f AC susceptibility o f a n u m b e r of these materials is shown in fig. 3. It is clear from these plots that the d i a m a g n e t i c response o f the samples sets in at a t e m p e r a t u r e that decreases sys-
TIBa 2 _ x L a x C u O 5 _ y 3.835 0.0
0.4
0.6
0.8
1.0
X 9.40
"5" 9.30 "5
9.20 < o_
~
9.10
TIBa 2_x La x C U O 5 _ y
•
• •
L r
9.00 0.0
0.2
0.4
0.6
0.8
1.0
x Fig. 2. ( a ) V a r i a t i o n o f lattice parameter TIBa2_xLaxCuOs_y, ( b ) and o f c w i t h x.
<
•
LLI
70
x =0-3
a with
T{Bo2-xLoxCu
x
in
05-y
6O
Lf) Z 50 0 [3-. 40 O0
T I B a ] . 2 Lao.8Cu 05-y
0.2
x :0-5
Iii
0/
3o
('P 20 I-LI.J Z 10
>._
u? Z
I \ z
~
g
L
~:~
J
x =0"8 <
A 2'z %2 4~ 2 0 (degrees)
'
~'2
'
Fig. 1. X-ray powder diffraction pattern of T1BaL2Lao.sCuOs_ ~. The line marked ( , ) indicates the presence of traces of BaCO3. All the other lines can be indexed in terms of the 1201-phase (space group P 4 / m m m ).
o
1~
5'o
6'0
TEMPERATURE
(K)
2'o
3~,
go
I
70
80
Fig. 3. Variation of diamagnetic response of T1Ba2_~La,CuOs_ ,. (x = 0.3-0.8 ) with temperature.
tematically with increase o f the l a n t h a n u m concentration. As m e n t i o n e d above, the most striking feature o f the present work is the fact that in our samples, T,.
195
A. Sundaresan et al. / TIBa2_ xLaxCuOs_ y
decreases with the increase of lanthanum concentration which is opposite to what has been reported in the literature [ 15,17,19 ]. This apparent contradiction can be resolved if one considers that the superconducting properties of the parent material T1Ba2CuOs_ v (no lanthanum ) can also be modified by varying the oxygen deficiency parameter y and that there is an optimum value (Ym) o f y which leads to the highest Tc. In turn, Ym corresponds to an optimum value, Vm, of the copper valence. Simple electron counting considerations suggest that the effective valence state of copper is Cu (III) for y = 0, and Cu (II) for y = 0.5. In either of the two situations, one does not observe superconductivity and Ymhas to lie in the range 0 < Y m < 0 . 5 . In YBa2Cu3OT, for instance, the average valence of copper is 2.33. A similar value of the copper valency in the present case will correspond to a value of y = 0.17. Other authors [ 17-19 ] start with the material with y ~ 0 (Cu (III) ). Substituting divalent barium by trivalent lanthanum effectively decreases the copper valency and introduces carriers in the system inducing superconductivity at a certain temperature. As the lanthanum concentration increases, the copper valency moves systematically from Cu (III) towards Vm enhancing the superconducting behaviour. In contrast to this, due to the second heating employed in the present work as described in our experimental procedure, our samples are oxygen deficient and therefore copper valency moves away from vm towards Cu(II) as the lanthanum concentration increases. This results in the lowering of T~ of the material. In order to verify the validity of this interpretation, we annealed, in a stream of oxygen, our samples which contained a high lanthanum concentration (for example, a sample with x = 0 . 8 ) that exhibited no superconductivity at T > 2 0 K. The sample after annealing in oxygen became superconducting. Figure 4 shows the AC Z(T) curve for the x = 0 . 8 sample before and after annealing. This is what one would expect on the basis of the explanation given above.
4. Conclusion The process of the synthesis of the material
~-" 3o
g
UJ
25
O9 Z © 20 09 LO
T t Bo 1.2Lao.8Cu 05_y
15 ~0
F- 5 LO
Z (_9
C:3
o
o
I
I
I
I0
20
30
I 40
I
50
60
TEMPERATURE (K) Fig. 4. Variation of diamagnetic response of T1Bat.2Lao.sCuOs_y before (dotted line) and after (continuous line) annealing the material in oxygen.
T1Ba2_xLaxCuOs_y that we have followed leads to oxygen deficiency. In such samples, La-substitution in place of barium tends to suppress superconductivity. The reported increase of Tc with increasing doping concentration of La for Ba in T1Ba2_xLaxCuO5 compounds is consistent with our observations if we postulate that there is an optimum value of the oxygen deficiency parameter y in T1Ba2CuOs_y which should result in a high superconducting transition temperature Tc > 50 K.
Acknowledgement One of us (AS) would like to thank PMB/DST for providing a SRF fellowship and the grant necessary to carry out this work.
References [ 1 ] R.M. Hazen, L.W. Finger, R.J. Angel, C,T. Prewitt, N.L. Ross, C.G. Hadidiacos, P.J. Heaney, D.R. Veblen, Z.Z. Sheng, A. El Ali and A.M. Hermann, Phys. Rev. Lett. 60 (1988) 1657. [2]M.A. Subramanian, J.C. Calabrese, C.C. Torardi, J. Gopalakrishnan, T.R. Askew, R.B. Flippen, K.J. Morrissey, U. Chowdry and A.W. Sleight, Nature 332 (1988) 420. [3] C.C. Torardi, M.A. Subramanian, J.C. Calabrese, J. Gopalakrishnan, K.J. Morrissey, T.R. Askew, R.B. Flippen, U. Chowdhry and A.W. Sleight, Science 240 (1988) 631.
196
,-t. Sundarg~an el a t / TlBa: ~La,('uO< ,
[4] M. Kikuchi, N. Kobayashi, K. lwasaki, D. Shindo, T. Oku, A. Tokiwa, T. Kajitani. K. Hiraga, Y. Syono and Y. Muto, Jpn. J. Appl. Phys. 27 (1988) LI050. [ 5] J.B. Parise, C.C. Torardi, M.A. Subramanian, J, Gopalakrishnan and A.W. Sleight, Physica C 159 (1989) 239. [6] S.S.P. Parkin, V.Y. Lee, A.I. Nazzal, R. Savoy, T.C. Huang, G. Gorman and R. Beyers, Phys. Rev. B 38 (1988) 6531. [7] R. Beyers, S.S.P. Parkin, V.Y. Lee, A.I. Nazzal, R. Savoy, G. Gorman, T.C. Huang and S. La Placa, Appl. Phys. Left. 53 (1988) 432. [8] S.S.P. Parkin, V.Y. Lee, A.I, Nazzal, R. Savoy, R. Beyers and S.J. La Placa, Phys. Rev. Lett. 61 (1988) 750. [9]B. Morosin, D.S. Ginley, P.F. Hlava, M.J. Caw, R.J. Baughman, J.E. Schirber, E,L. Venturini and J.F. Kwak, Physica C 152 (1988) 413. [ 10] M. Hervieu, A. Maignan, C. Martin, C. Michel, J. Provost and B. Raveau, J. Solid State Chem. 75 ( 1988 ) 212. l 11 ] H. Ihara, R. Sugise, M. Hirabayashi, N. Terada, M. Jo, K. Hayashi, A. Negishi, M. Tokumoto, Y. Kimura and T. Shimomura, Nature 334 (1988) 510.
[ 12 ] H. Sawa, H. Fujiki, K. Tomimoto and J. Akimitsu, Jpn. J. Appl. Phys. 27 (1988) L830. [13] M.A, Subramanian, C.C. Torardi, J.C. Calabrese, J. Gopalakrishnan, K.J. Morrissey, T.R. Askew, R.B. Flippen, U. Chowdhry and A.W. Sleight, Science 239 ( 1988 ) 1015. [ 14] J.M. Tarascon, W.R. McKinnon, P. Barboux, D.M. Hwang, B.G. Hull, Y. Le Page, N. Stoffel and M. Giround. Phys. Rev. B 38 (1988) 8885. [15] H.C. Ku, M.F. Tai, J.B. Shi, M.J. Shieh, S.W. Hsu, G.tt. Hwang, D.C. Ling, T.J. Watson-Yang and T.Y. Lin, Jpn. J. Appl. Phys. 28 (1989) L923. [16] I.K. Gopalakrishnan, J.V. Yakhmi and R.M. lyer, Physica C 175 (1991) 183. [17]T. Manako, Y. Shimakawa, Y. Kubo, T. Satoh and H. lgarashi, Physica C 158 (1989) 143. [ 18] C. Martin, D. Bourgault, M. Hervieu, J. Provost, C. Michel and B. Raveau, Mod. Phys. Lett. B (1989), preprint, [ 19] M.A. Subramanian, G.H. Kewei, J.B. Parise, J.A. Goldstone and R.B. Von Dreele, Physica C 166 (1990) 19.
PhysicaC 178 (1991) 193-196 North-Holland
Studies of the superconducting behaviour of T1Ba2_ LaxCuOs _y A. S u n d a r e s a n a, A.K. R a j a r a j a n b, L.C. G u p t a b, M. Sharon a a n d R. V i j a y a r a g h a v a n b a Department of Chemistry, Indian Institute of Technology, Bombay 400 076, India b SolidState Physics Group, Tata Institute of Fundamental Research, Bombay 400 005, India Received 9 April 1991
The superconducting behaviour of the system T1Ba2_xLaxCuOs_~ has been investigated. Samples prepared by the procedure described in this paper show a decrease in Tc with increasing lanthanum concentration. This is in contrast to the trend reported by other authors [ 15,19 ]. This difference in behaviour was explained in terms of the oxygen deficiency y in the parent compound T1Ba2CuO5 y.
I. Introduction There are two general classes of Tl-based superconducting compounds, namely, T12Ba2Can_iCunO2n+4(n = 1, 2, 3 and 4) [ 1-7 ] and TlA2Can_lCunO2,+3 ( n = 1, 2, 3, 4 and 5; A=Sr, Ba) [ 6-11 ] systems. There are double T1-O layers in the former class of compounds just as there are double Bi-O layers in the Bi-based high-T~ materials Bi2Sr2Ca._jCu~O2,+4 ( n = 1, 2, 3 and 4) [ 12-14]. The latter class of compounds has single T1-O layers, but no Bi-based monolayer high-To systems have been reported so far. The n = 1 member T1Ba2CuO5 of the class of single T I - O layer compounds has been of particular interest and has studied extensively. In this system copper seems to be in the trivalent state. This material has been reported by several workers to be either insulating or superconducting with low Tc ( < 10 K) [ 15,16]. Superconductivity has been induced/enhanced in this system by substituting Ba by La in varied amounts [ 17,18 ]. In the La-doped materials T1Ba2_xLaxCuOs, Tc has been reported to increase with an increase of x with maximum Tc=52 K [15,19] with x=0.8. In our samples, which have been prepared by a slightly different heating procedure (see below), we have observed the opposite trend of the variation of Tc with x, namely, Tc decreases with increase of x. Our 'results suggest that the parent compound T1BaECuO5 itself could possibly be made superconducting, with
considerably high T~ ( > 50 K), by a careful adjustment of its oxygen stoichiometry and without introducing any lanthanum in the structure. We report the results of the measurements and discuss them in terms of the oxygen-stoichiometry of the parent material.
2. Experimental Samples of T1Ba2_xLaxCuOs_~ (0.2_
0921-4534/91/$03.50 © 1991 Elsevier Science Publishers B.V. All fights reserved
194
.4.
Sundaresan et al.
rials was studied by recording the p o w d e r X-ray diffraction pattern in each case. Superconductivity was studied by AC susceptibility technique. In these measurements, the t e m p e r a t u r e o f the samples was controlled to an accuracy o f within 1 K.
/
TlBa2 ,La,('uO~
,
5.855 ...... a
gcD L~E
~< 3.845
{
U 3. Results and discussions
5
Figure 1 shows the X-ray diffraction pattern o f the material TlBa~.zLao.sCuO~ ,.. All the lines o f the diffraction pattern, except the one m a r k e d *, can be indexed in terms o f the p r i m i t i v e tetragonal 1201 phase. The line m a r k e d as • seems to indicate the presence o f traces o f BaCO3. Similar X-ray diffraction patterns have been observed in the case o f samples with various other values o f x. It is i m p o r t a n t to emphasize that the X-ray diffraction patterns do not indicate the presence o f 2201 phase. The lattice p a r a m e t e r s a and c were o b t a i n e d using a least squares fitting procedure. Figures 2 ( a ) and ( b ) show that both lattice parameters, a and c, decrease with increase o f the l a n t h a n u m concentration x. It should be noted that the decrease o f c is more p r o n o u n c e d than that o f a. T e m p e r a t u r e d e p e n d e n c e o f AC susceptibility o f a n u m b e r of these materials is shown in fig. 3. It is clear from these plots that the d i a m a g n e t i c response o f the samples sets in at a t e m p e r a t u r e that decreases sys-
TIBa 2 _ x L a x C u O 5 _ y 3.835 0.0
0.4
0.6
0.8
1.0
X 9.40
"5" 9.30 "5
9.20 < o_
~
9.10
TIBa 2_x La x C U O 5 _ y
•
• •
L r
9.00 0.0
0.2
0.4
0.6
0.8
1.0
x Fig. 2. ( a ) V a r i a t i o n o f lattice parameter TIBa2_xLaxCuOs_y, ( b ) and o f c w i t h x.
<
•
LLI
70
x =0-3
a with
T{Bo2-xLoxCu
x
in
05-y
6O
Lf) Z 50 0 [3-. 40 O0
T I B a ] . 2 Lao.8Cu 05-y
0.2
x :0-5
Iii
0/
3o
('P 20 I-LI.J Z 10
>._
u? Z
I \ z
~
g
L
~:~
J
x =0"8 <
A 2'z %2 4~ 2 0 (degrees)
'
~'2
'
Fig. 1. X-ray powder diffraction pattern of T1BaL2Lao.sCuOs_ ~. The line marked ( , ) indicates the presence of traces of BaCO3. All the other lines can be indexed in terms of the 1201-phase (space group P 4 / m m m ).
o
1~
5'o
6'0
TEMPERATURE
(K)
2'o
3~,
go
I
70
80
Fig. 3. Variation of diamagnetic response of T1Ba2_~La,CuOs_ ,. (x = 0.3-0.8 ) with temperature.
tematically with increase o f the l a n t h a n u m concentration. As m e n t i o n e d above, the most striking feature o f the present work is the fact that in our samples, T,.
195
A. Sundaresan et al. / TIBa2_ xLaxCuOs_ y
decreases with the increase of lanthanum concentration which is opposite to what has been reported in the literature [ 15,17,19 ]. This apparent contradiction can be resolved if one considers that the superconducting properties of the parent material T1Ba2CuOs_ v (no lanthanum ) can also be modified by varying the oxygen deficiency parameter y and that there is an optimum value (Ym) o f y which leads to the highest Tc. In turn, Ym corresponds to an optimum value, Vm, of the copper valence. Simple electron counting considerations suggest that the effective valence state of copper is Cu (III) for y = 0, and Cu (II) for y = 0.5. In either of the two situations, one does not observe superconductivity and Ymhas to lie in the range 0 < Y m < 0 . 5 . In YBa2Cu3OT, for instance, the average valence of copper is 2.33. A similar value of the copper valency in the present case will correspond to a value of y = 0.17. Other authors [ 17-19 ] start with the material with y ~ 0 (Cu (III) ). Substituting divalent barium by trivalent lanthanum effectively decreases the copper valency and introduces carriers in the system inducing superconductivity at a certain temperature. As the lanthanum concentration increases, the copper valency moves systematically from Cu (III) towards Vm enhancing the superconducting behaviour. In contrast to this, due to the second heating employed in the present work as described in our experimental procedure, our samples are oxygen deficient and therefore copper valency moves away from vm towards Cu(II) as the lanthanum concentration increases. This results in the lowering of T~ of the material. In order to verify the validity of this interpretation, we annealed, in a stream of oxygen, our samples which contained a high lanthanum concentration (for example, a sample with x = 0 . 8 ) that exhibited no superconductivity at T > 2 0 K. The sample after annealing in oxygen became superconducting. Figure 4 shows the AC Z(T) curve for the x = 0 . 8 sample before and after annealing. This is what one would expect on the basis of the explanation given above.
4. Conclusion The process of the synthesis of the material
~-" 3o
g
UJ
25
O9 Z © 20 09 LO
T t Bo 1.2Lao.8Cu 05_y
15 ~0
F- 5 LO
Z (_9
C:3
o
o
I
I
I
I0
20
30
I 40
I
50
60
TEMPERATURE (K) Fig. 4. Variation of diamagnetic response of T1Bat.2Lao.sCuOs_y before (dotted line) and after (continuous line) annealing the material in oxygen.
T1Ba2_xLaxCuOs_y that we have followed leads to oxygen deficiency. In such samples, La-substitution in place of barium tends to suppress superconductivity. The reported increase of Tc with increasing doping concentration of La for Ba in T1Ba2_xLaxCuO5 compounds is consistent with our observations if we postulate that there is an optimum value of the oxygen deficiency parameter y in T1Ba2CuOs_y which should result in a high superconducting transition temperature Tc > 50 K.
Acknowledgement One of us (AS) would like to thank PMB/DST for providing a SRF fellowship and the grant necessary to carry out this work.
References [ 1 ] R.M. Hazen, L.W. Finger, R.J. Angel, C,T. Prewitt, N.L. Ross, C.G. Hadidiacos, P.J. Heaney, D.R. Veblen, Z.Z. Sheng, A. El Ali and A.M. Hermann, Phys. Rev. Lett. 60 (1988) 1657. [2]M.A. Subramanian, J.C. Calabrese, C.C. Torardi, J. Gopalakrishnan, T.R. Askew, R.B. Flippen, K.J. Morrissey, U. Chowdry and A.W. Sleight, Nature 332 (1988) 420. [3] C.C. Torardi, M.A. Subramanian, J.C. Calabrese, J. Gopalakrishnan, K.J. Morrissey, T.R. Askew, R.B. Flippen, U. Chowdhry and A.W. Sleight, Science 240 (1988) 631.
196
,-t. Sundarg~an el a t / TlBa: ~La,('uO< ,
[4] M. Kikuchi, N. Kobayashi, K. lwasaki, D. Shindo, T. Oku, A. Tokiwa, T. Kajitani. K. Hiraga, Y. Syono and Y. Muto, Jpn. J. Appl. Phys. 27 (1988) LI050. [ 5] J.B. Parise, C.C. Torardi, M.A. Subramanian, J, Gopalakrishnan and A.W. Sleight, Physica C 159 (1989) 239. [6] S.S.P. Parkin, V.Y. Lee, A.I. Nazzal, R. Savoy, T.C. Huang, G. Gorman and R. Beyers, Phys. Rev. B 38 (1988) 6531. [7] R. Beyers, S.S.P. Parkin, V.Y. Lee, A.I. Nazzal, R. Savoy, G. Gorman, T.C. Huang and S. La Placa, Appl. Phys. Left. 53 (1988) 432. [8] S.S.P. Parkin, V.Y. Lee, A.I, Nazzal, R. Savoy, R. Beyers and S.J. La Placa, Phys. Rev. Lett. 61 (1988) 750. [9]B. Morosin, D.S. Ginley, P.F. Hlava, M.J. Caw, R.J. Baughman, J.E. Schirber, E,L. Venturini and J.F. Kwak, Physica C 152 (1988) 413. [ 10] M. Hervieu, A. Maignan, C. Martin, C. Michel, J. Provost and B. Raveau, J. Solid State Chem. 75 ( 1988 ) 212. l 11 ] H. Ihara, R. Sugise, M. Hirabayashi, N. Terada, M. Jo, K. Hayashi, A. Negishi, M. Tokumoto, Y. Kimura and T. Shimomura, Nature 334 (1988) 510.
[ 12 ] H. Sawa, H. Fujiki, K. Tomimoto and J. Akimitsu, Jpn. J. Appl. Phys. 27 (1988) L830. [13] M.A, Subramanian, C.C. Torardi, J.C. Calabrese, J. Gopalakrishnan, K.J. Morrissey, T.R. Askew, R.B. Flippen, U. Chowdhry and A.W. Sleight, Science 239 ( 1988 ) 1015. [ 14] J.M. Tarascon, W.R. McKinnon, P. Barboux, D.M. Hwang, B.G. Hull, Y. Le Page, N. Stoffel and M. Giround. Phys. Rev. B 38 (1988) 8885. [15] H.C. Ku, M.F. Tai, J.B. Shi, M.J. Shieh, S.W. Hsu, G.tt. Hwang, D.C. Ling, T.J. Watson-Yang and T.Y. Lin, Jpn. J. Appl. Phys. 28 (1989) L923. [16] I.K. Gopalakrishnan, J.V. Yakhmi and R.M. lyer, Physica C 175 (1991) 183. [17]T. Manako, Y. Shimakawa, Y. Kubo, T. Satoh and H. lgarashi, Physica C 158 (1989) 143. [ 18] C. Martin, D. Bourgault, M. Hervieu, J. Provost, C. Michel and B. Raveau, Mod. Phys. Lett. B (1989), preprint, [ 19] M.A. Subramanian, G.H. Kewei, J.B. Parise, J.A. Goldstone and R.B. Von Dreele, Physica C 166 (1990) 19.