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The lithium and molecular intercalates of FeOCl
Mat. Res. Bul. Vol. 13, pp. 221-227, 1978. P e r g a m o n P r e s s , Inc. Printed in the United States.
THE LITHIUM AND MOLECULAR INTERCALATES OF FeOC1 P. Palvadeau, L. Coic and J. Rouxel Laboratoire de Physicochlmie des Solides, Faculte des Sciences Chemin de la Houssiniere, 44072 Nantes Cedex, F r a n c e and J. P o r t i e r Laboratoire de Chimie des Solides du CNRS Universite de Bordeaux I, 351 cours de la Liberation 33405 Talence, F r a n c e (Received J a n u a r y 11, 1978; Communicated by P. Hagenmuller)
ABSTRACT Lithium intercalation compounds of the l a m e l l a r iron oxychloride have been obtained by means of the butyl lithium technique. Chemical and g e o m e t r i c a l considerations suggest that the Li + ions a r e localized in octahedral sites. Molecular i n t e r c a l a t e s a r e formed with amines. Single c r y s t a l studies on FeOC1 (C5H5N)0.33 show the existence of a s u p e r s t r u c t u r e along the a axis. It m a y be related to an a l t e r n a t e a r r a n g e m e n t of the pyridine molecules between the chlorine l a y e r s of the host structure. Univalent groups such as OH-, NH~, (CH3NH)- can also be substituted for the chlorine ions in the FeOC1 s t r u c t u r e .
Introduction Iron oxychloride FeOCI has a l a y e r s t r u c t u r e which is an orthorhombic distortion of the tetragonal PbFC1 type (1, 2). It may be considered as constituted by iron and oxygen l a y e r s bound t o g e t h e r along the b axis by a double l a y e r of chlorine atoms (Fig. 1). It was shown in the e a r l y sixties that the chlorine ions in FeOC1 and A1OC1, which has a s i m i l a r structure, can be exchanged with univalent groups such as OH" and NH~ (3, 4, 5, 5, 7) in the scope of topochemical reactions. At the s a m e time, the f i r s t m o l e c u l a r i n t e r c a l a t e s have been prepared by reaction with amines (5, 6, 7, 8, 9). Similar r e s u l t s have been reported r e c e n t l y in this field (10, 11, 12). No t r u e alkali metal intercalation compound was known until the possibility of intercalation was r e c e n t l y established by an e l e c t r o c h e m i c a l way (13). The present paper is concerned with chemical p r e p a r a t i o n and c h a r a c t e r i z a t i o n of lithium i n t e r calates of FeOC1. New r e s u l t s about m o l e c u l a r i n t e r c a l a t e s a r e also described. 221
222
P. PALVADEAU, et al.
Vol. 13, No. 3
Fe
O
3+
O
w
FIG. 1 Structure of FeOC1
Z
The LixFeOC1 intercalation compounds FeOC1 is prepared by heating Fe203 with FeC13 in slight e x c e s s . The reaction is c a r r i e d out at 4b0°C for a few days in a pyrex tube sealed under vacuum: F e 2 0 3 + FeC13.--~ 3FeOC1
Vol. 13, No. 3
INTERCALATES OF FeOCI
223
The excess of FeCl~ is remove either by sublimation under vacuum at 300°C or by dissolution Tn pure ethanol. The parameters of the orthorhombic cell are : o
o
a = 3,781 (6) A, b = 7,893 (8) A, good agreement with previous results (1,2).
c = 3,293 (3) A in
The preparation of single crystals, more than 3 mm long and about 5.10-3mm thick, was achieved by a chemical transport technique using FeCl 3 i t s e l f as a transport reagent.
Lithium intercalation has been obtained by the butyllithium technique described by M. ARMANDin the case of graphite compounds (14). The reaction was performed under dry N2 atmosphere. N-butyllithium (1M commercial solution in hexane) was diluted ten times and a known amount allowed to react with FeOCl. Each reaction has been conducted for a few days at 60°C, the reaction times varying between two and five days. Reaction products were then f i l t e r e d , washed with hexane and dried. Lithium content was determined by atomic absorption spectroscopy in both the obtained material and the solution. Under the above conditions and for the smallest values of x (x< 0,50), no side reaction other than : x FeOCl + x C4H9Li ~ + Li x FeOCl has been observed. 2 c8H18
I f a concentrated solution of butyllithium is used (IM), a fast reaction is observable, with heat evolution and partial destruction of FeOCl. As a consequence of lithium intercalation the brown FeOCl crystals progressively darken. X-ray powder spectra show a progressive attenuation of the FeOCl pattern, suggesting a continuous intercalation of lithium with a deterioration of the c r y s t a l l i z a t i o n , but no significant parameter evolution has been observed. Therefore i t is impossible to determine the upper l i m i t for x from the X-ray powder diagrams. The value x = 1, which appears as a theoretical l i m i t on the base of structural considerations (see below), has never been reached. For values of x higher than 0.50, X-ray lines appear which can be attributed to decomposition products. I t can be assumed that the upper x l i m i t is close to 0.50.
I t is possible to discuss the sites of the Li + ions in the host structure. In the D~9 space group of FeOCI~ the iron, oxygen and chlorine atoms occupy the 2h following Wyckoff positions : Fe
:
2(b)
0,y,I/2
;
1/2, Y, 0
C1
:
2(a)
O,y,O
;
1/2, Y, 1/2 with y = 0.330
0
:
2(a)
with
with y = 0.116
y =-0.048
From chemical and geometrical considerations i t appears that the Li + ions can be localized only on the positions i l l u s t r a t e d by Fig. 2, where l i t h i u m is surrounded by f i v e chlorine and one oxygen atoms.
P. PALVADEAU, et al.
224
,.'.') Li" at Z:O .
Q Li" at Z=1/2
Vol. 13, No. 3
Four chlorine atoms are coplanar (belonging to the same chlorine layer in the host structure). The f i f t h chlorine atom, Cl 5, is located above this plane (belonging to the opposite chlorine layer), and the oxygen atom below the plane (belonging to the oxygen-iron layer). There are two sites of this type in each unit cell corresponding to the following positions : Li I : 1/2, y=0.36, 1/2 Li 2 : O,
y=0,64, 1/2
Position of Li + ions FIG. 2 Fig. 3 shows the evolution of potential according to the values of y. These calculations have been performed with lithium s t a t i s t i c a l l y distributed in the two sites and an average charge on the iron atoms. The other possibility with all the Li + ions in one E ,~-1 xlO0 site seems to be unlikely for two reasons : ( i ) i t does not show any minimum, ( i i ) i t would probably lead to an alteration of the symmetry of the host structure. The related distances ~re : 4 Li-Cll,2,3, 4 = 2.52 I Li-Cl 5 2.45 I Li-0 2.47 A,
Y x100 J
3;
45
Potential evolution versus y FIG. 3
3'0
'
Vol. 13, No. 3
INTERCALATES OF FeOC1
225
o
o
to be compared with Li - Cl = 2,56 A in lithium chloride and Li - 0 = 2.30 A in Li20 (Fig. 4)
1
3
C
I
4 J( C
0 L..]
a Li + environnement FIG. 4
Molecular intercalates The f i r s t molecular intercalates of lamellar oxyhalides were prepared in 1961 in the case of aluminium derivatives by P. Hagenmuller, J. Rouxel and al. (4,5,6,8) and in 1965 in the case of FeOCl by J. Portier (7), P. Hagenmuller and al. (9). Intercalation in aluminium thio, seleno and tellurohalides has also been performed (15,16,17). Because of the expansion of the b parameter i t has been supposed that intercalated molecules such as (CH3)3N were localized between the FeOCI (or MYX) sheets. A layer - type FeOCl - pyridine complex has recently been reported by F. Kanamaru, S. Yam~naka and M. ~oizumi (10,~1). I t has an orthorhombic unit cell with a = 3.78 A, b = 13,45 A, C = 3.30 A and Z = 2 for both formulations FeOCl (C~H~N)I/4 and FeOCI (C~H~N)I/3. Here again the b parameter expansion sugge~t~ a localization of~t~e pyridine molecule between adjacent chlorine layers. M~ssbauer resonance studies, as well as ESR spectra, are in agreement with the existence of conduction electrons resulting from partial transfer of the lone pair of the nitrogen atom to the FeOCl layers.
226
P. PALVADEAU, et al.
Vol. 13, No. 3
We found that by intercalation of pyridine in FeOCl single crystals the a parameter has actually to be doubled (table I ) . This superstructure suggests an alternate arrangement of the pyridine molecules.
TABLE A X-Ray Powder Diffraction Data. for FeOCl (C5H5N)o,33 h k l
d.obs
d. cal
I
h k l
d. obs
d. cal
I
O2 2 2 1 4 0 2 0 3 0 4 O6 1 4 O5 3 3
6.80 3.486 3.094 2.952 2.663 2.362 2.261
6.78 3.478 3.118 2.960 2.661 2.362 2.262 2.267 2.094 1.8951
F F f f F F m
0 6 1 4 3 0 2 7 0 0 8 O 0 1 2 0 2 2 2 7 1 O4 2 2 3 2 t f = vw
1.8708 1.8363 1.7491 1.6966 1.6373 1.5994 1.5433 1.4842 1.4455 t t f = vvw
1.8643 1 8440 1 7492 1 6951 1 6383 1 6004 1 5436 1.4841 1.4478
tf ttf tf ttf m tf tf f tf
0 0 0 1 1 1 O 1 1 1
2.098 1.8955
f f = w
An upper l i m i t FeOCl (C5H5N)1/2 can be attained by using an excess of pyridine, but FeOCl (C5H5N)1/3 is formed again under vacuum. Apparently the use of lithium solutions in l i q u i d ammoniac does not lead to a lithium intercalation under usual conditions. This seems to be a major difference as compared with TiS 2 (18). A new compound is obtained, with the formulation FeO(NH) Li (19), This emphasizes the substitution of chlorine layers by other groups, by specific topochemical reactions, as described by Hagenmuller, Rouxel and Portier (5,8,9). For instance, the following reactions have been observed : MYX +
2 NH3
)
MYNH2 +
NH4X
MYX +
2 CH3NH2
> MYNHCH3+ CH3NH3X
Conclusion Lithium ions and lewis bases have been intercalated in FeOCl as previouly done in layer - type chalcogenides. Neither structural alteration nor parameter evolution is observed and this is probably related to the size of the available sites in FeOCl. During intercalation a drastic modification of the properties of the host structure occurs, as suggested by the physical measurements now under progress. FeOCl can be used as an intercalation cathod in secondary batteries, for i t has been shown electrochemically that the lithium - FeOCl system has a good r e v e r s i b i l i t y (20). Numerous moleculars intercalates of FeOCl have now been isolated, but the chlorine layers can also be substituted by various univalent groups.
Vol. 13, No. 3
INTERCALATES OF FeOC1
227
References
I°
S Goldsztaub, C.R., 198, 667 (1934)
2.
M D. Lind, Acta Cryst., B 26, 1058 (1970)
3.
P Hagenmuller and J. Rouxel, C.R., 250, 1859 (1960)
4.
P Hagenmuller, J. Rouxel and B. Le Neindre, C.R., 252, 282 (1961)
5.
J
6.
P Hagenmuller, J. Rouxel and J. Portier, C.R., 254, 2000 (1962)
7.
J
8.
P Hagenmuller, J. Rouxel, J. David, A. Colin and B.Le Neindre Z. anorg, allg. Chem. 323, 1 1963.
.
Rouxel, Thesis Bordeaux 1961. Portier, Thesis Bordeaux 1966
P. Hagenmuller, J. Portier, B. Barbe and P. BoucIier Z. anorg, allg. Chem. 355, 209 (1967)
10. F. Kanamaru, S. Yamanaka and M. Koizumi, Chem. L e t t . , 373 (1974) 11. F. Kanamaru, M. Shimada, M. Koizumi, M.Takano and M. Takada J. Sol. State Chem. 7, 297 (1973) 12. Y. Shoji, N. Toshikazu, T. Masami Thermochim. Acta 1_99(2),236:9 (1977) 13. M. Armand, L. CoTc, P. Palvadeau and J. Rouxel J. Power Sources in press 14. M. Armand in "Fast ion transport in solids" W. Van Gool Ed. North Holland 1973. 15. P. Hagenmuller, J. Rouxel, J. David and A. Colin C.R. 253 ,667, (1961) 16. J. Rouxel and P. Palvadeau Bull. Soc. Chim. Fr 82698 (1967) 17. P. Palvadeau Thesis Nantes 1967 18. J. Rouxel, M. Danot and J. Bichon C.R. Acad. Sc. Paris 276 (1973) 19. A. Lecerf, M. Rault, J. Portier and G. Villers Bull, Soc. Chim. 4j1208-111965 20. P. Palvadeau, J. Rouxel, M. Armand French Patent (1976)
THE LITHIUM AND MOLECULAR INTERCALATES OF FeOC1 P. Palvadeau, L. Coic and J. Rouxel Laboratoire de Physicochlmie des Solides, Faculte des Sciences Chemin de la Houssiniere, 44072 Nantes Cedex, F r a n c e and J. P o r t i e r Laboratoire de Chimie des Solides du CNRS Universite de Bordeaux I, 351 cours de la Liberation 33405 Talence, F r a n c e (Received J a n u a r y 11, 1978; Communicated by P. Hagenmuller)
ABSTRACT Lithium intercalation compounds of the l a m e l l a r iron oxychloride have been obtained by means of the butyl lithium technique. Chemical and g e o m e t r i c a l considerations suggest that the Li + ions a r e localized in octahedral sites. Molecular i n t e r c a l a t e s a r e formed with amines. Single c r y s t a l studies on FeOC1 (C5H5N)0.33 show the existence of a s u p e r s t r u c t u r e along the a axis. It m a y be related to an a l t e r n a t e a r r a n g e m e n t of the pyridine molecules between the chlorine l a y e r s of the host structure. Univalent groups such as OH-, NH~, (CH3NH)- can also be substituted for the chlorine ions in the FeOC1 s t r u c t u r e .
Introduction Iron oxychloride FeOCI has a l a y e r s t r u c t u r e which is an orthorhombic distortion of the tetragonal PbFC1 type (1, 2). It may be considered as constituted by iron and oxygen l a y e r s bound t o g e t h e r along the b axis by a double l a y e r of chlorine atoms (Fig. 1). It was shown in the e a r l y sixties that the chlorine ions in FeOC1 and A1OC1, which has a s i m i l a r structure, can be exchanged with univalent groups such as OH" and NH~ (3, 4, 5, 5, 7) in the scope of topochemical reactions. At the s a m e time, the f i r s t m o l e c u l a r i n t e r c a l a t e s have been prepared by reaction with amines (5, 6, 7, 8, 9). Similar r e s u l t s have been reported r e c e n t l y in this field (10, 11, 12). No t r u e alkali metal intercalation compound was known until the possibility of intercalation was r e c e n t l y established by an e l e c t r o c h e m i c a l way (13). The present paper is concerned with chemical p r e p a r a t i o n and c h a r a c t e r i z a t i o n of lithium i n t e r calates of FeOC1. New r e s u l t s about m o l e c u l a r i n t e r c a l a t e s a r e also described. 221
222
P. PALVADEAU, et al.
Vol. 13, No. 3
Fe
O
3+
O
w
FIG. 1 Structure of FeOC1
Z
The LixFeOC1 intercalation compounds FeOC1 is prepared by heating Fe203 with FeC13 in slight e x c e s s . The reaction is c a r r i e d out at 4b0°C for a few days in a pyrex tube sealed under vacuum: F e 2 0 3 + FeC13.--~ 3FeOC1
Vol. 13, No. 3
INTERCALATES OF FeOCI
223
The excess of FeCl~ is remove either by sublimation under vacuum at 300°C or by dissolution Tn pure ethanol. The parameters of the orthorhombic cell are : o
o
a = 3,781 (6) A, b = 7,893 (8) A, good agreement with previous results (1,2).
c = 3,293 (3) A in
The preparation of single crystals, more than 3 mm long and about 5.10-3mm thick, was achieved by a chemical transport technique using FeCl 3 i t s e l f as a transport reagent.
Lithium intercalation has been obtained by the butyllithium technique described by M. ARMANDin the case of graphite compounds (14). The reaction was performed under dry N2 atmosphere. N-butyllithium (1M commercial solution in hexane) was diluted ten times and a known amount allowed to react with FeOCl. Each reaction has been conducted for a few days at 60°C, the reaction times varying between two and five days. Reaction products were then f i l t e r e d , washed with hexane and dried. Lithium content was determined by atomic absorption spectroscopy in both the obtained material and the solution. Under the above conditions and for the smallest values of x (x< 0,50), no side reaction other than : x FeOCl + x C4H9Li ~ + Li x FeOCl has been observed. 2 c8H18
I f a concentrated solution of butyllithium is used (IM), a fast reaction is observable, with heat evolution and partial destruction of FeOCl. As a consequence of lithium intercalation the brown FeOCl crystals progressively darken. X-ray powder spectra show a progressive attenuation of the FeOCl pattern, suggesting a continuous intercalation of lithium with a deterioration of the c r y s t a l l i z a t i o n , but no significant parameter evolution has been observed. Therefore i t is impossible to determine the upper l i m i t for x from the X-ray powder diagrams. The value x = 1, which appears as a theoretical l i m i t on the base of structural considerations (see below), has never been reached. For values of x higher than 0.50, X-ray lines appear which can be attributed to decomposition products. I t can be assumed that the upper x l i m i t is close to 0.50.
I t is possible to discuss the sites of the Li + ions in the host structure. In the D~9 space group of FeOCI~ the iron, oxygen and chlorine atoms occupy the 2h following Wyckoff positions : Fe
:
2(b)
0,y,I/2
;
1/2, Y, 0
C1
:
2(a)
O,y,O
;
1/2, Y, 1/2 with y = 0.330
0
:
2(a)
with
with y = 0.116
y =-0.048
From chemical and geometrical considerations i t appears that the Li + ions can be localized only on the positions i l l u s t r a t e d by Fig. 2, where l i t h i u m is surrounded by f i v e chlorine and one oxygen atoms.
P. PALVADEAU, et al.
224
,.'.') Li" at Z:O .
Q Li" at Z=1/2
Vol. 13, No. 3
Four chlorine atoms are coplanar (belonging to the same chlorine layer in the host structure). The f i f t h chlorine atom, Cl 5, is located above this plane (belonging to the opposite chlorine layer), and the oxygen atom below the plane (belonging to the oxygen-iron layer). There are two sites of this type in each unit cell corresponding to the following positions : Li I : 1/2, y=0.36, 1/2 Li 2 : O,
y=0,64, 1/2
Position of Li + ions FIG. 2 Fig. 3 shows the evolution of potential according to the values of y. These calculations have been performed with lithium s t a t i s t i c a l l y distributed in the two sites and an average charge on the iron atoms. The other possibility with all the Li + ions in one E ,~-1 xlO0 site seems to be unlikely for two reasons : ( i ) i t does not show any minimum, ( i i ) i t would probably lead to an alteration of the symmetry of the host structure. The related distances ~re : 4 Li-Cll,2,3, 4 = 2.52 I Li-Cl 5 2.45 I Li-0 2.47 A,
Y x100 J
3;
45
Potential evolution versus y FIG. 3
3'0
'
Vol. 13, No. 3
INTERCALATES OF FeOC1
225
o
o
to be compared with Li - Cl = 2,56 A in lithium chloride and Li - 0 = 2.30 A in Li20 (Fig. 4)
1
3
C
I
4 J( C
0 L..]
a Li + environnement FIG. 4
Molecular intercalates The f i r s t molecular intercalates of lamellar oxyhalides were prepared in 1961 in the case of aluminium derivatives by P. Hagenmuller, J. Rouxel and al. (4,5,6,8) and in 1965 in the case of FeOCl by J. Portier (7), P. Hagenmuller and al. (9). Intercalation in aluminium thio, seleno and tellurohalides has also been performed (15,16,17). Because of the expansion of the b parameter i t has been supposed that intercalated molecules such as (CH3)3N were localized between the FeOCI (or MYX) sheets. A layer - type FeOCl - pyridine complex has recently been reported by F. Kanamaru, S. Yam~naka and M. ~oizumi (10,~1). I t has an orthorhombic unit cell with a = 3.78 A, b = 13,45 A, C = 3.30 A and Z = 2 for both formulations FeOCl (C~H~N)I/4 and FeOCI (C~H~N)I/3. Here again the b parameter expansion sugge~t~ a localization of~t~e pyridine molecule between adjacent chlorine layers. M~ssbauer resonance studies, as well as ESR spectra, are in agreement with the existence of conduction electrons resulting from partial transfer of the lone pair of the nitrogen atom to the FeOCl layers.
226
P. PALVADEAU, et al.
Vol. 13, No. 3
We found that by intercalation of pyridine in FeOCl single crystals the a parameter has actually to be doubled (table I ) . This superstructure suggests an alternate arrangement of the pyridine molecules.
TABLE A X-Ray Powder Diffraction Data. for FeOCl (C5H5N)o,33 h k l
d.obs
d. cal
I
h k l
d. obs
d. cal
I
O2 2 2 1 4 0 2 0 3 0 4 O6 1 4 O5 3 3
6.80 3.486 3.094 2.952 2.663 2.362 2.261
6.78 3.478 3.118 2.960 2.661 2.362 2.262 2.267 2.094 1.8951
F F f f F F m
0 6 1 4 3 0 2 7 0 0 8 O 0 1 2 0 2 2 2 7 1 O4 2 2 3 2 t f = vw
1.8708 1.8363 1.7491 1.6966 1.6373 1.5994 1.5433 1.4842 1.4455 t t f = vvw
1.8643 1 8440 1 7492 1 6951 1 6383 1 6004 1 5436 1.4841 1.4478
tf ttf tf ttf m tf tf f tf
0 0 0 1 1 1 O 1 1 1
2.098 1.8955
f f = w
An upper l i m i t FeOCl (C5H5N)1/2 can be attained by using an excess of pyridine, but FeOCl (C5H5N)1/3 is formed again under vacuum. Apparently the use of lithium solutions in l i q u i d ammoniac does not lead to a lithium intercalation under usual conditions. This seems to be a major difference as compared with TiS 2 (18). A new compound is obtained, with the formulation FeO(NH) Li (19), This emphasizes the substitution of chlorine layers by other groups, by specific topochemical reactions, as described by Hagenmuller, Rouxel and Portier (5,8,9). For instance, the following reactions have been observed : MYX +
2 NH3
)
MYNH2 +
NH4X
MYX +
2 CH3NH2
> MYNHCH3+ CH3NH3X
Conclusion Lithium ions and lewis bases have been intercalated in FeOCl as previouly done in layer - type chalcogenides. Neither structural alteration nor parameter evolution is observed and this is probably related to the size of the available sites in FeOCl. During intercalation a drastic modification of the properties of the host structure occurs, as suggested by the physical measurements now under progress. FeOCl can be used as an intercalation cathod in secondary batteries, for i t has been shown electrochemically that the lithium - FeOCl system has a good r e v e r s i b i l i t y (20). Numerous moleculars intercalates of FeOCl have now been isolated, but the chlorine layers can also be substituted by various univalent groups.
Vol. 13, No. 3
INTERCALATES OF FeOC1
227
References
I°
S Goldsztaub, C.R., 198, 667 (1934)
2.
M D. Lind, Acta Cryst., B 26, 1058 (1970)
3.
P Hagenmuller and J. Rouxel, C.R., 250, 1859 (1960)
4.
P Hagenmuller, J. Rouxel and B. Le Neindre, C.R., 252, 282 (1961)
5.
J
6.
P Hagenmuller, J. Rouxel and J. Portier, C.R., 254, 2000 (1962)
7.
J
8.
P Hagenmuller, J. Rouxel, J. David, A. Colin and B.Le Neindre Z. anorg, allg. Chem. 323, 1 1963.
.
Rouxel, Thesis Bordeaux 1961. Portier, Thesis Bordeaux 1966
P. Hagenmuller, J. Portier, B. Barbe and P. BoucIier Z. anorg, allg. Chem. 355, 209 (1967)
10. F. Kanamaru, S. Yamanaka and M. Koizumi, Chem. L e t t . , 373 (1974) 11. F. Kanamaru, M. Shimada, M. Koizumi, M.Takano and M. Takada J. Sol. State Chem. 7, 297 (1973) 12. Y. Shoji, N. Toshikazu, T. Masami Thermochim. Acta 1_99(2),236:9 (1977) 13. M. Armand, L. CoTc, P. Palvadeau and J. Rouxel J. Power Sources in press 14. M. Armand in "Fast ion transport in solids" W. Van Gool Ed. North Holland 1973. 15. P. Hagenmuller, J. Rouxel, J. David and A. Colin C.R. 253 ,667, (1961) 16. J. Rouxel and P. Palvadeau Bull. Soc. Chim. Fr 82698 (1967) 17. P. Palvadeau Thesis Nantes 1967 18. J. Rouxel, M. Danot and J. Bichon C.R. Acad. Sc. Paris 276 (1973) 19. A. Lecerf, M. Rault, J. Portier and G. Villers Bull, Soc. Chim. 4j1208-111965 20. P. Palvadeau, J. Rouxel, M. Armand French Patent (1976)