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LixZrS2 intercalation compounds
Synthetic Metals, 5 (1983) 141 - 146
141
Li~ ZrS2 I N T E R C A L A T I O N COMPOUNDS
PHILIPPE DENIARD, PATRICK CHEVALIER, LUC TRICHET and JEAN ROUXEL
Laboratoire de Chimie des Solides, L.A. 279, 2, rue de la Houssini~re, 44072 Nantes Cddex (France) (Received May 19, 1982)
Summary The disulfide ZrS2 has been intercalated with lithium by means of the butyllithium method. Two phases have been characterized. The first (0 ~< x < 0.20), of the NiAs type, presents no parameter variation. The second (0.30 < x ~< 1) is rhombohedral at room temperature but undergoes a phase transition to a spinel structure in the 0.30 < x < 0.50 range at 250 °C. Electrical and magnetic measurements have shown that the first phase is semiconducting, the second being of a metallic type. Comparisons are made with the Li~ZrSe2 system.
Introduction A semiconducting-metal transition was recently observed in the LixZrSe2 intercalation system 0 < x ~< 1 [1]. For x < 0.40 the phase is a semiconductor, for x ~ 0.40 it is metallic. It is remarkable that the unit cell parameters do not change in the semiconducting region. The presence of lithium was established by chemical analysis and its intercalation through the evolution of the physical properties. Previous work on the lithium-ZrS2 system was published in 1974 [2, 3]. Intercalation was performed by means of lithium solutions in liquid ammonia. A 3R rhombohedral phase was observed for 0 . 2 5 ~ x <~ 1. Lithium occupies octahedral sites in a structure of the ~-NaFeO2 type, which is also the structural t y p e of NaTiS2 (Fig. 1). The (AB)n hexagonal close stacking of the anions in ZrS2 has switched to an (ABC), face centered stacking. In the case of the Lix ZrSe2 c o m p o u n d s the hexagonal stacking is preserved, as was the case for LixTiS2 compounds. For x ~ 0.25 no parameter evolution was observed in LixZrS2. This can be considered as reflecting the presence of a two phase region between ZrS2 and Li0.2s ZrS2. However, on the basis of the physical studies on the Lix ZrS½ phase, a single phase domain for Lio.2sZrS 2 down to ZrS2 itself must not be excluded, implying then an unobservable change in the parameters of the cell. A physical, chemical, and structural study of the Lix ZrS2 system over the whole 0 ~< x ~< 1 composition range has been undertaken in the present 0379-6779/83/0000-0000/$03.00
© Elsevier Sequoia/Printed in The Netherlands
142 m
e 0
O S o
0
o
Zr
OLi
° 0
0 e
c, aV'~
ZrS 2
NiAs
~t N a F e O 2
Fig. 1. Structural models for the LixZrS2 compounds.
work. The n-butyllithium method, which is a mild and convenient way of intercalating, is preferred to the NH3 technique.
Preparation and characterization Zirconium disulfide is prepared by direct combination of the elements as previously described [4]. Zirconium powder (3N Ventron) and resublimated sulfur are heated at 550 °C in an evacuated, sealed tube for a week and then at 850 °C for 24 h. A slight excess of sulfur is used. The method gives a well crystallized stoichiometric ZrS2 phase (direct transposition of this m e t h o d to ZrSe2 leads to non-stoichiometric samples only). Intercalation is carried out at room temperature by using a solution of n-butyllithium in hexane x ZrS2 + xn.BuLi -~ LixZrS2 + ~ CsH,s.
The reaction products are washed with hexane and dried. All operations are done in a dry box under nitrogen atmosphere. According to the lithium content, X-ray spectra and chemical analysis show the formation of two phases. -- An LixZrS2 phase is observed with 0 ~< x < 0.20. In this composition range and within experimental error, the ZrS2 spectra are n o t modified by intercalation. The samples progressively darken from the initially brownish ZrS2. Chemical analysis was undertaken systematically over the entire composition range. -- The X-ray data indicate a two-phase region for the 0.20 < x ~< 0.30 composition range.
143
--
T h e s e c o n d LixZrS2 p h a s e c o r r e s p o n d s t o x values ranging f r o m 0 . 3 0 t o 1. T h e u n i t cell is r h o m b o h e d r a l , s p a c e g r o u p R 3m. This is t h e ~ - N a F e O 2 - t y p e p h a s e p r e v i o u s l y m e n t i o n e d , w h i c h c o r r e s p o n d s t o an (ABC)n c u b i c s t a c k i n g o f t h e anions. Figure 2 s h o w s t h e a a n d c p a r a m e t e r e v o l u t i o n vs. x. c(h), 6.2
__~_~~
6,1
6.0
t ¢
6.9 5.8 J a (h) 3.65
3.60
0.50
1
x
Fig. 2. Variations of the a and c/n parameters in the LixZrS2 system (n = 1 for 0 <~x < 0.20 and n = 3 for 0.30 < x < 1). I'I without annealing, ~ after annealing at 250 °C. T h e a b o v e results h a v e b e e n o b t a i n e d at r o o m t e m p e r a t u r e , b u t w h e n a n n e a l e d at 2 5 0 °C f o r 24 h, t h e LixZrS2 s a m p l e s o f t h e s e c o n d p h a s e corres p o n d i n g to 0 . 3 0 < x < 0.50 s h o w a p h a s e t r a n s i t i o n . This t r a n s i t i o n first a p p e a r e d as a significant c o n t r a c t i o n o f t h e c p a r a m e t e r a n d a slight increase in a. T h e c/ a r a t i o t a k e s t h e c h a r a c t e r i s t i c value o f 2x/6, c o r r e s p o n d i n g t o a r h o m b o h e d r o n c o n t a i n e d w i t h i n a c u b e , w i t h t h e f o l l o w i n g r e l a t i o n s between the hexagonal and the cubic parameters: CH = a¢~/3
~/2 a H = a c -4
I n d e x i n g t h e p o w d e r d i a g r a m w i t h t h e c u b i c p a r a m e t e r a l l o w e d recogn i t i o n o f t h e F d 3 m spinel space g r o u p . T a b l e 1 s h o w s t h e s p e c t r a o f Li0.3sZrS2 p r e p a r e d at 20 °C (c~-NaFeO2 s t r u c t u r e with aH = 3 . 6 4 9 ( 5 ) a n d cn = 1 8 . 3 0 ( 4 ) •) a n d a f t e r a n n e a l i n g (a c = 1 0 . 3 6 9 ( 6 ) A c o r r e s p o n d i n g to a H 3 . 6 6 6 and cH = 1 7 . 9 6 A). S t r u c t u r a l c a l c u l a t i o n s agree w i t h a d i r e c t spinel s t r u c t u r e ( l i t h i u m in t e t r a h e d r a l sites) a n d lead t o an R f a c t o r e q u a l t o 0 . 0 8 in t h a t case, instead o f 0 . 3 5 in t h e case o f t h e inverse spinel s t r u c t u r e . F o r 0.50 < x ~< 1 n o p a r a m e t e r m o d i f i c a t i o n is o b s e r v e d a f t e r a n n e a l i n g a n d the r o o m t e m p e r a t u r e r h o m b o h e d r a l p h a s e is o b t a i n e d , as f o r t h e 0 ~< x < 0.20 d o m a i n f o r w h i c h o n l y t h e t r i g o n a l s t r u c t u r e can be f o u n d . =
Physical m e a s u r e m e n t s Electrical m e a s u r e m e n t s w e r e p e r f o r m e d o n p o w d e r s using t h e c o m p l e x i m p e d a n c e m e t h o d b e t w e e n 20 a n d 250 °C at f r e q u e n c i e s v a r y i n g f r o m 20
144 TABLE 1 X-ray d i a g r a m for Li0.3sZrS 2 Room temperature
A f t e r a n n e a l i n g at 250 °C
a H = 3.649(5) A c H = 1 8 . 3 0 ( 4 ) /~
ac = 10.369(6) A
h k l
dobs.
dcalc"
h k l
dobs.
dcalc"
1 0 1 0 1 1 2 0 0
2.602 2.387 2.014 1.852 1.821 1.745 1.566 1.526 1.490
2.600 2.392 2.014 1.853 1.825 1.748 1.557 1.525 1.494
1 3 4 3 5 4 5 5 4
6.04 3.12 2.596 2.386 1.993 1.833 1.753 1.580 1.497
6.00 3.126 2.594 2.381 1.998 1.832 1.753 1.581 1.497
0 1 0 1 1 1 0 0 2
4 5 7 8 0 3 2 12 4
i 1 0 3 1 4 3 3 4
1 1 0 1 1 0 1 3 4
Hz to 40 kHz. The samples were pressed pellets sintered at 450 °C with an 85% density. The faces of these pellets were covered with a thin gold layer by sputtering. The results indicate that the conductivity is thermally activated for 0 ~< x < 0.20 (for example, an activation energy of 0.41 eV is found for x = 0.15 (Fig. 3) and is of a metallic type for x > 0.30. The magnetic measurements were performed by the Faraday m e t h o d in the 4 - 300 K temperature range. The magnetic susceptibility of the x = 0.15 sample exhibits Curie-Weiss paramagnetism (Fig. 4) whereas for x > 0.30, the susceptibility is temperature independent. These two types of behaviour are consistent with the electrical measurements. Llog ~.TT ( ~ . cm )-I.K
• -2
x = 0,15 --
,
E • =0,41 eV =
,
V
-3
-4
i
I
2
3
Fig. 3. V a r i a t i o n s o f log o T vs. 103/T.
103/T ( K -1 )
145
,\ -1. 103 ( e m u . m o l e . 1 ) 2(
10
0
i 100
i 200
i 300
T(K)
•
Fig. 4. Variations of the reciprocal magnetic susceptibility X-I X 10 a vs. T.
Discussion When annealed, the LixZrS2 samples of the rhombohedral 3R phase show a phase transition for 0.30 < x < 0.50. The anionic stacking of the cubic ABC type is n o t modified during the transition, which leads to a direct spinel structure. The main effect is that lithium, which is octahedrally surrounded in the 3R a-NaFeO2 structural type, has moved to tetrahedral sites. The m a x i m u m lithium c o n t e n t which allows the transition is x = 0.50, corresponding to an LiZr2S4 classical spinel formulation. Copper also may lead to spinels of the Ml-xZr2S4 type with ZrS2 [4]. It can also be noticed that the ionicity of the structure increases with the lithium c o n t e n t and this will favour an occupancy of octahedral sites. The anionic stacking is preserved in the spinel-a-NaFeO2 transition and this can be taken into account by the fact t h a t we were unable to characterize any two-phase region between the two systems, in spite of a significant discontinuity in the a and c variations vs. x . However, a very narrow two-phase region may exist. Such an aNaFeO2 spinel transition has already been observed in the LixY~Zrl-~S2 system [ 5]. In this system, which is closely related to our compounds, there was no discontinuity in the parameter curves and for 0.94 < x ~< 1 the spinel rhombohedral phase was followed by an NaC1 arrangement. Once more, the ABC stacking was preserved. The increase in ionicity due to the partial substitution of zirconium by y t t r i u m may explain the transformation. Such a succession of these stackings (NaC1, spinel and rhombohedral types) has been recently proposed for chlorides in the LiC1-VC12 system [6]. A s e m i c o n d u c t o r - m e t a l transition was observed in the LixZrSe2 compounds. This system is single phase for 0 < x ~< 1. The transition occurs for x = 0.40 in a phase corresponding to a simple transition CdI2(ZrSe2) -~ NiAs (LiZrSe2). In the present work the NiAs t y p e is observed up to x = 0.20 and both electrical and magnetic measurements show a semiconducting phase.
146
The higher ionicity o f the structure is probably responsible for the structural change at x = 0.30 and we have already seen that in the still more ionic situation o f LixYxZr,-xS2 a n o t h e r change occurs to an NaC1 type.
References 1 C. Berthier, Y. Chabre, P. Segransan, P. Chevalier, L. Trichet and A. Le M6haut6, Solid State Ionics, 5 (1981) 379. 2 A. Le Blanc-Soreau, M. Danot, L. Trichet and J. Rouxel, Mater. Res. Bull., 9 (1974) 191;M. S. Whittingham and F. R. Gamble, Mater. Res. Bull., 10 (1975) 363. 3 L. Trichet, D. Jerome and J. Rouxel, C. R. Acad. Sci., 280 (1975) 1025. 4 L. Trichet and J. Rouxel, C. R. Acad. Sci., 267 (1968) 1322. 5 O. Abou Ghaloun, P. Chevalier, L. Trichet and J. Rouxel, Rev. Chim. Mineral., 17 (1980) 368. 6 L. Hanebali, M. Machej, C. Cros and P. Hagenmuller, Mater. Res. Bull., 16 (1981) 887.
141
Li~ ZrS2 I N T E R C A L A T I O N COMPOUNDS
PHILIPPE DENIARD, PATRICK CHEVALIER, LUC TRICHET and JEAN ROUXEL
Laboratoire de Chimie des Solides, L.A. 279, 2, rue de la Houssini~re, 44072 Nantes Cddex (France) (Received May 19, 1982)
Summary The disulfide ZrS2 has been intercalated with lithium by means of the butyllithium method. Two phases have been characterized. The first (0 ~< x < 0.20), of the NiAs type, presents no parameter variation. The second (0.30 < x ~< 1) is rhombohedral at room temperature but undergoes a phase transition to a spinel structure in the 0.30 < x < 0.50 range at 250 °C. Electrical and magnetic measurements have shown that the first phase is semiconducting, the second being of a metallic type. Comparisons are made with the Li~ZrSe2 system.
Introduction A semiconducting-metal transition was recently observed in the LixZrSe2 intercalation system 0 < x ~< 1 [1]. For x < 0.40 the phase is a semiconductor, for x ~ 0.40 it is metallic. It is remarkable that the unit cell parameters do not change in the semiconducting region. The presence of lithium was established by chemical analysis and its intercalation through the evolution of the physical properties. Previous work on the lithium-ZrS2 system was published in 1974 [2, 3]. Intercalation was performed by means of lithium solutions in liquid ammonia. A 3R rhombohedral phase was observed for 0 . 2 5 ~ x <~ 1. Lithium occupies octahedral sites in a structure of the ~-NaFeO2 type, which is also the structural t y p e of NaTiS2 (Fig. 1). The (AB)n hexagonal close stacking of the anions in ZrS2 has switched to an (ABC), face centered stacking. In the case of the Lix ZrSe2 c o m p o u n d s the hexagonal stacking is preserved, as was the case for LixTiS2 compounds. For x ~ 0.25 no parameter evolution was observed in LixZrS2. This can be considered as reflecting the presence of a two phase region between ZrS2 and Li0.2s ZrS2. However, on the basis of the physical studies on the Lix ZrS½ phase, a single phase domain for Lio.2sZrS 2 down to ZrS2 itself must not be excluded, implying then an unobservable change in the parameters of the cell. A physical, chemical, and structural study of the Lix ZrS2 system over the whole 0 ~< x ~< 1 composition range has been undertaken in the present 0379-6779/83/0000-0000/$03.00
© Elsevier Sequoia/Printed in The Netherlands
142 m
e 0
O S o
0
o
Zr
OLi
° 0
0 e
c, aV'~
ZrS 2
NiAs
~t N a F e O 2
Fig. 1. Structural models for the LixZrS2 compounds.
work. The n-butyllithium method, which is a mild and convenient way of intercalating, is preferred to the NH3 technique.
Preparation and characterization Zirconium disulfide is prepared by direct combination of the elements as previously described [4]. Zirconium powder (3N Ventron) and resublimated sulfur are heated at 550 °C in an evacuated, sealed tube for a week and then at 850 °C for 24 h. A slight excess of sulfur is used. The method gives a well crystallized stoichiometric ZrS2 phase (direct transposition of this m e t h o d to ZrSe2 leads to non-stoichiometric samples only). Intercalation is carried out at room temperature by using a solution of n-butyllithium in hexane x ZrS2 + xn.BuLi -~ LixZrS2 + ~ CsH,s.
The reaction products are washed with hexane and dried. All operations are done in a dry box under nitrogen atmosphere. According to the lithium content, X-ray spectra and chemical analysis show the formation of two phases. -- An LixZrS2 phase is observed with 0 ~< x < 0.20. In this composition range and within experimental error, the ZrS2 spectra are n o t modified by intercalation. The samples progressively darken from the initially brownish ZrS2. Chemical analysis was undertaken systematically over the entire composition range. -- The X-ray data indicate a two-phase region for the 0.20 < x ~< 0.30 composition range.
143
--
T h e s e c o n d LixZrS2 p h a s e c o r r e s p o n d s t o x values ranging f r o m 0 . 3 0 t o 1. T h e u n i t cell is r h o m b o h e d r a l , s p a c e g r o u p R 3m. This is t h e ~ - N a F e O 2 - t y p e p h a s e p r e v i o u s l y m e n t i o n e d , w h i c h c o r r e s p o n d s t o an (ABC)n c u b i c s t a c k i n g o f t h e anions. Figure 2 s h o w s t h e a a n d c p a r a m e t e r e v o l u t i o n vs. x. c(h), 6.2
__~_~~
6,1
6.0
t ¢
6.9 5.8 J a (h) 3.65
3.60
0.50
1
x
Fig. 2. Variations of the a and c/n parameters in the LixZrS2 system (n = 1 for 0 <~x < 0.20 and n = 3 for 0.30 < x < 1). I'I without annealing, ~ after annealing at 250 °C. T h e a b o v e results h a v e b e e n o b t a i n e d at r o o m t e m p e r a t u r e , b u t w h e n a n n e a l e d at 2 5 0 °C f o r 24 h, t h e LixZrS2 s a m p l e s o f t h e s e c o n d p h a s e corres p o n d i n g to 0 . 3 0 < x < 0.50 s h o w a p h a s e t r a n s i t i o n . This t r a n s i t i o n first a p p e a r e d as a significant c o n t r a c t i o n o f t h e c p a r a m e t e r a n d a slight increase in a. T h e c/ a r a t i o t a k e s t h e c h a r a c t e r i s t i c value o f 2x/6, c o r r e s p o n d i n g t o a r h o m b o h e d r o n c o n t a i n e d w i t h i n a c u b e , w i t h t h e f o l l o w i n g r e l a t i o n s between the hexagonal and the cubic parameters: CH = a¢~/3
~/2 a H = a c -4
I n d e x i n g t h e p o w d e r d i a g r a m w i t h t h e c u b i c p a r a m e t e r a l l o w e d recogn i t i o n o f t h e F d 3 m spinel space g r o u p . T a b l e 1 s h o w s t h e s p e c t r a o f Li0.3sZrS2 p r e p a r e d at 20 °C (c~-NaFeO2 s t r u c t u r e with aH = 3 . 6 4 9 ( 5 ) a n d cn = 1 8 . 3 0 ( 4 ) •) a n d a f t e r a n n e a l i n g (a c = 1 0 . 3 6 9 ( 6 ) A c o r r e s p o n d i n g to a H 3 . 6 6 6 and cH = 1 7 . 9 6 A). S t r u c t u r a l c a l c u l a t i o n s agree w i t h a d i r e c t spinel s t r u c t u r e ( l i t h i u m in t e t r a h e d r a l sites) a n d lead t o an R f a c t o r e q u a l t o 0 . 0 8 in t h a t case, instead o f 0 . 3 5 in t h e case o f t h e inverse spinel s t r u c t u r e . F o r 0.50 < x ~< 1 n o p a r a m e t e r m o d i f i c a t i o n is o b s e r v e d a f t e r a n n e a l i n g a n d the r o o m t e m p e r a t u r e r h o m b o h e d r a l p h a s e is o b t a i n e d , as f o r t h e 0 ~< x < 0.20 d o m a i n f o r w h i c h o n l y t h e t r i g o n a l s t r u c t u r e can be f o u n d . =
Physical m e a s u r e m e n t s Electrical m e a s u r e m e n t s w e r e p e r f o r m e d o n p o w d e r s using t h e c o m p l e x i m p e d a n c e m e t h o d b e t w e e n 20 a n d 250 °C at f r e q u e n c i e s v a r y i n g f r o m 20
144 TABLE 1 X-ray d i a g r a m for Li0.3sZrS 2 Room temperature
A f t e r a n n e a l i n g at 250 °C
a H = 3.649(5) A c H = 1 8 . 3 0 ( 4 ) /~
ac = 10.369(6) A
h k l
dobs.
dcalc"
h k l
dobs.
dcalc"
1 0 1 0 1 1 2 0 0
2.602 2.387 2.014 1.852 1.821 1.745 1.566 1.526 1.490
2.600 2.392 2.014 1.853 1.825 1.748 1.557 1.525 1.494
1 3 4 3 5 4 5 5 4
6.04 3.12 2.596 2.386 1.993 1.833 1.753 1.580 1.497
6.00 3.126 2.594 2.381 1.998 1.832 1.753 1.581 1.497
0 1 0 1 1 1 0 0 2
4 5 7 8 0 3 2 12 4
i 1 0 3 1 4 3 3 4
1 1 0 1 1 0 1 3 4
Hz to 40 kHz. The samples were pressed pellets sintered at 450 °C with an 85% density. The faces of these pellets were covered with a thin gold layer by sputtering. The results indicate that the conductivity is thermally activated for 0 ~< x < 0.20 (for example, an activation energy of 0.41 eV is found for x = 0.15 (Fig. 3) and is of a metallic type for x > 0.30. The magnetic measurements were performed by the Faraday m e t h o d in the 4 - 300 K temperature range. The magnetic susceptibility of the x = 0.15 sample exhibits Curie-Weiss paramagnetism (Fig. 4) whereas for x > 0.30, the susceptibility is temperature independent. These two types of behaviour are consistent with the electrical measurements. Llog ~.TT ( ~ . cm )-I.K
• -2
x = 0,15 --
,
E • =0,41 eV =
,
V
-3
-4
i
I
2
3
Fig. 3. V a r i a t i o n s o f log o T vs. 103/T.
103/T ( K -1 )
145
,\ -1. 103 ( e m u . m o l e . 1 ) 2(
10
0
i 100
i 200
i 300
T(K)
•
Fig. 4. Variations of the reciprocal magnetic susceptibility X-I X 10 a vs. T.
Discussion When annealed, the LixZrS2 samples of the rhombohedral 3R phase show a phase transition for 0.30 < x < 0.50. The anionic stacking of the cubic ABC type is n o t modified during the transition, which leads to a direct spinel structure. The main effect is that lithium, which is octahedrally surrounded in the 3R a-NaFeO2 structural type, has moved to tetrahedral sites. The m a x i m u m lithium c o n t e n t which allows the transition is x = 0.50, corresponding to an LiZr2S4 classical spinel formulation. Copper also may lead to spinels of the Ml-xZr2S4 type with ZrS2 [4]. It can also be noticed that the ionicity of the structure increases with the lithium c o n t e n t and this will favour an occupancy of octahedral sites. The anionic stacking is preserved in the spinel-a-NaFeO2 transition and this can be taken into account by the fact t h a t we were unable to characterize any two-phase region between the two systems, in spite of a significant discontinuity in the a and c variations vs. x . However, a very narrow two-phase region may exist. Such an aNaFeO2 spinel transition has already been observed in the LixY~Zrl-~S2 system [ 5]. In this system, which is closely related to our compounds, there was no discontinuity in the parameter curves and for 0.94 < x ~< 1 the spinel rhombohedral phase was followed by an NaC1 arrangement. Once more, the ABC stacking was preserved. The increase in ionicity due to the partial substitution of zirconium by y t t r i u m may explain the transformation. Such a succession of these stackings (NaC1, spinel and rhombohedral types) has been recently proposed for chlorides in the LiC1-VC12 system [6]. A s e m i c o n d u c t o r - m e t a l transition was observed in the LixZrSe2 compounds. This system is single phase for 0 < x ~< 1. The transition occurs for x = 0.40 in a phase corresponding to a simple transition CdI2(ZrSe2) -~ NiAs (LiZrSe2). In the present work the NiAs t y p e is observed up to x = 0.20 and both electrical and magnetic measurements show a semiconducting phase.
146
The higher ionicity o f the structure is probably responsible for the structural change at x = 0.30 and we have already seen that in the still more ionic situation o f LixYxZr,-xS2 a n o t h e r change occurs to an NaC1 type.
References 1 C. Berthier, Y. Chabre, P. Segransan, P. Chevalier, L. Trichet and A. Le M6haut6, Solid State Ionics, 5 (1981) 379. 2 A. Le Blanc-Soreau, M. Danot, L. Trichet and J. Rouxel, Mater. Res. Bull., 9 (1974) 191;M. S. Whittingham and F. R. Gamble, Mater. Res. Bull., 10 (1975) 363. 3 L. Trichet, D. Jerome and J. Rouxel, C. R. Acad. Sci., 280 (1975) 1025. 4 L. Trichet and J. Rouxel, C. R. Acad. Sci., 267 (1968) 1322. 5 O. Abou Ghaloun, P. Chevalier, L. Trichet and J. Rouxel, Rev. Chim. Mineral., 17 (1980) 368. 6 L. Hanebali, M. Machej, C. Cros and P. Hagenmuller, Mater. Res. Bull., 16 (1981) 887.