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2128 P. TARTEbut most spectra were obtained with the following spectrometers : 1600-400 m-1region : Un icam SP 100 spectr omet er with N aCl and KBr prism s ; 420-300 or420-240 m-l: Perkin-Elmer 112 spectr omet er with either a CsBr or a CaI prism .Sam ples were run as KBr or KI disks.

SOMESPECTROSCOPIC PROBLEMS INHERENT TO THE STUDY OFINORGANIC SOLIDS1. S im pli$cation of the vibrational treatm ent

Apart from rather simple cases, a rigorous vibrational treatment of inorganicsolids is genera lly int ra ctable. Str ictly spea king, th e differen t vibrat iona l m odes ar eth ose of the wh ole un it cell of the cryst al ; th is implies th at , in most cases, th e num berof fun da men ta l frequen cies is fair ly high an d, as a consequ ence, a deta iled assignm entof the observed frequen cies to th e differen t vibra tional modes is nea rly impossible.

In such cases, simplified met hods mu st be used. For simp le salts (sulpha tes,phosphates, nitrates, . . .), it is genera lly ad mit ted, explicitly or implicitly, th at th ean ion (SO,+, Pods-, NO,- . . .) is vibra tin g in depen den tly from t he oth er co-ordina ted groups presen t in th e stru ctu re. This simplificat ion ha s been discussed an dextended by the author to double oxides and intermediate cases. A detailed dis-cussion ha s been present ed elsewhere [3,5] nd the conclusions may be summarizedas follows: if a solid A,B,O, is const itu ted of AO, a nd BO, co-ord ina ted groups,two extr eme oases mu st be considered:

(1)

(2)

AO, an d BO, groups have ra th er differen t vibra tional frequ encies: in th iscase, vibrational interactions between these groups are weak or negligible,an d th e groups may be cons idered as vibra ting indepen dent ly. This conceptof sepa ra te or indepen den t vibrat ions h as been pu t forwa rd a nd dis-cussed by the author a few years ago [3]. rom the vibrational point ofview, such compounds ma y be tr eat ed as salts. This is th e case for a lar geser ies of orth o-silicates an d ort ho-germa na tes [S].AO, an d BO, groups ha ve simila r vibra tional frequ encies; in th is case, th evibrat iona l intera ctions between th e groups m ay be ao large th at th e conceptof sepa ra te vibrat ions is mea nin gless : the vibrations are those of severalgroups, or of th e un it cell, ta ken as a whole. Good exam ples of such inter -actions are foun d in II-III spinels with norm al str uctu re (bivalent cat ion ontetrahedral si tes [6]).Between th ese extremes, a nu mber of int ermediat e cases ar e cha ra cterized by

weak or moderat e int eractions. Such oases ar e easily observed in th e i.r. spectra ofsome lithium compounds, where they appear as abnormally low or erratic 6Li-7Liisotopic shifts [5].It is quite evident th at th e assignm ent of an absorpt ion band to a vibrat iona lmode of a given co-ordin at ed group is mea nin gfu l only if th e concept of sepa ra t evibrations is a good approximation for the group under consideration.[B] . TARTE,Mh. Acud. Roy. Be@. 3!5,4(a)md 4(b) 1966).[S] .TAR~ and J.PREUD HoM~~E,Ac~~ q&. 16, 27 (1963).

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I n f r a - r e d s p e c t r a o f in o r g a n i c hminates 21292. Factors injluencing the vibra%nul frequencies of a co-ordinat ed group AO,.

It is actually well known th at , apa rt from th e chemical na tu re a nd th e valencysta te of th e cat ion A, th e value of Dhe co-ordin at ion nu mber n is th e prin cipal factordeter minin g th e vibra tional frequ encies of s, co-o&n&ted group AO,: the smaller theco-ordinat ion num ber, th e shorter th e bond length an d hence th e higher the vibra-tional frequencies.The relat ion between vibra tional frequencies an d co-ordin at ion nu mber is onlyan approximat e one, first becau se th ere is always a more or less import an t influenceof th e neighbour ing groups (in oth er words , the concept of separ at e vibra tions isalways an appr oximctt ion), second because th e vibra tional frequ encies ar e affected byan y distort ion or deforma tion of th e co-ordin at ed groups (th ese deforma tions ar efrequent in solids), a nd th ird because of an addit iona l effect, na mely th e isolat edor conden sed sta te of th e co-ordina ted groups un der considerat ion.

This effect is well known for some ser ies of compoun ds, such a s bora tes, phos-pha tes or silicat es: th e str etching frequencies of conden sed groups (co-ordina;tedgroups int erlinked by comm on oxygen at oms to form chains, or sheet s or tr idimen-siona l net work s) ar e significan tly higher tha n th e str etching frequencies of isola,tedgroups (as in o&o-bera tes, phospha tes or silicat es). This is explained qualita tivelyby importa nt vibra tional int era ctions between neighbour ing (identical) groups an d isconfirmed by a semi-quan tita tive ma th emat ical tr eatm ent by MATOSSI [7]: t h eobserved frequ encies ar e th ose, not, of a AO, group, but of a chain, of a sheet or of atr idimensional net work of such groups.

As a consequ ence, th e str etching frequ encies of conden sed AO, (octa hedr al)groups ma y be as high as th e str etching frequencies of isolat ed A04 (tet ra hedr al)groups, a situ at ion which m ay lead to serious err ors of inter pret at ion if th e above-men tioned effects are neglected.

When stating vibrational frequencies of co-ordinated groups AO,, it is thus of utmostim portan ce to clearly specify if th e groups under consid eration are isolated orcondensed.

Tables showing t he importa nce of th ese effects have already been published byth e au thor [3, 51.

APPLICATION TO THE VIBRATIONAL BEH AVIOUR OF Al-O BONDS(a) General considerations

The char acter istic frequencies of AlO, co-ordina ted groups proposed by differen tau th ors ar e collected in Table 1. The agreement is fairly good for tet ra hedr alAlO, groups, but significan t discrepa ncies ar ise in th e case of octah edra l AlO,groups.

No discussion will be given her e of th e higher figures of 1000 an d even 1200 cm-lp r o p o s e d i n s o m e e a r l ie r w o r k s f o r AlO, tet ra hedre. These results were obtainedwith alum inosilicat es, an d such high frequencies ar e clear ly relat ed, n ot to AlO,tet ra hedr s, but to Si-O-Al mixed vibra tions.

[7] F. MATOSSI, . Chem. Phys. 17, 679 (1949).12

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I n f m -r e d s p e c t r a o f i n o r g a n i c a h u n in ~ te s 2131

Fig. l(a,

di0a5t2;

I

908070

6050 w40

30

20IO0200 300 400 500 600 700 800 900 1000

80706050 (b)

200 300 400 500 600 700 II00

1009060

$ 7060

.p 50

.r(c)

g 40ceI- 3020IO0 300 400 500 600 700 800 900 1000

cm-b a n d c ) . I &a -r e d s p e c t r a o f c o m p o u n d e w i th o o n d e n m d t et r a h ed r a : Ca O*Al,O,; 12 Ce O-7Al,O,; @Al,O,.

AlO,

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2132 P. TARTE700-800 m-l region mu st be assigned to vibrat ional modes having t he highestpossible frequ encies, na mely th e str etching frequ encies of AlO, tet ra hedr a.

Fina lly, th e dilute solid solut ion 2CaO~(Al,.,,Fe,.,,),O, will be ta ken a s an exam pleof the last type. The spectr a of th is solid solut ion an d of th e pur e ferr ite compoun d

10090 \ , 5Al,0,.3Y,O,

ap

Fig. 2.

807060504030

IIV IIO0200 xx) 400 500 700 600 900 loo0

cm-

I&a-red spectru m of yttrium a luminate with garnet stru ctur e(isolated AlO, tet ra hedr a an d AlO, octa hed ra ).

60

?A 4ko do 7bo do

cmFig. 3. I&a-red spectr um of th e spine1 LiA.l,O, (ord er ed type).are compared in Fig. 4. The spectr um of th e solid solut ion is char acter ized by th eoccur ren ce of a weak ba nd at 784 m-l, th e relat ive in ten sity of which depends on th eam oun t of Al-Fe isomorph ic repla cement , as ha s been shown by a deta iled infra -redst udy of th e whole ser ies of solid solut ions 2CaO~(Fe,_&l,),O, with z up to 0.70 12].[12] .*TAFws, Rev. Chim. Minkrale 1, 25 (1964).

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I &a -r e d s p e c t r a o f i n o r g a n i c a lu m in a t e s 2133

This ban d mu st be assigned to th e AlO, tetr ah edra isolated (as far as dilutesolid solutions ar e considered) in t he st ru ctu re.

It is evident from th e preceding r esults th at isolated AlO, tetr ah edra a bsorbat slight ly, but system at ically lower frequ encies th an cond ens ed AlO, tetr ah edra .The n ext res ult s will show th at mu ch la rger differen ces will be foun d in th e case ofoctahedral co-ordination.Y7ondensed AlO, octahedra. Represen ta tive comp oun ds of th is cat egory a rea-AlSO, (coru nd um ), th e low tem pera tu re form of LiAlO,, an d some spine1aluminates.

Fig. 4.

100

60

01 I I ! I I I 1330 400 500 600 700 800 so0 1000cm-

I &a -r e d s p e a t r u m o f p u r e d i c a l c iu m fe r r i t e 2 Ca O .F e ,O ,of th e dilu te solid solu ti on 2 CaO*(A1,. ,,Fe,,, ,) ,08 (da sh ed(filll l ine )l ine) .

Among these, corundum is the simplest case, since its structure is built up byAlO, octa hedra only. Unfortu na tely, a nu mber of experiment s car ried out in th islaboratory leads to the conclusion that the i.r. spectrum of corundum is not fullyreproducible, some details depending on the thermal history of the sample [13].Nevertheless, Fig. 5 gives what is considered by the author to be the most reliablespectr um obtained so far : th e most cha ra cterist ic feat ur e (which anyway is commonto all spectra of a-Al,O,) being the occurrence of two strong bands near 650 and600 cm-l.The spectru m of th e low-tem pera tu re form of LiAlO, is repr oduced in Fig. 6,together with th e spectr um of th e high-temper at ur e form : th is is a clear-cut exampleof th e downwa rd sh ift occur rin g in Al-O frequ encies when the co-ordina tion n um beris changed from 4 to 6. Some deta ils of these spectra will be discus sed in th e nextsection: it will be simply point ed out , here, th at t he str ong absorpt ion due to AlO,octa hedra is centr ed n ear 600 cm-l.Fina lly, th e spectr um of nat ur al MgAl,O, (Fig. 7) will be tak en a s an exam ple of anorm al II-III spinel, with s-fold co-ordin at ed a lum inu m.1131 P . TARTE,U n p u b l is h e d r e s u l t s .

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2134 P. TARTEloo9080706050403020IO0,coo 900 700 500 400

cm-IDO, I I I I I90

80

706050403020IO

0300 400 500 600 700 I800 900

Fig . 5 . I&a-red spe o-t r u m o f a-AlaOs.

00

Fig . 6 . Co mp ar ed in -fr a -r e d s p e c t r a o f t h eh ig h - (y ) an d o f t h el o w - ( a ) t e m p e r a t u r eforms of LiAIO,.

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2136 P. TAFKIKEprobably relat ed to oth er AlO, vibra tions more or less affected by OH (OD) bendingor torsional motions.

In the yttrium-aluminum garnet EiAl,Os*3Y,O,,6Al a toms (out of 10) are four -foldco-ordin at ed, th e rem ain ing 4 being six-fold co-ordin at ed. The spectru m of th iscomp ound (Fig. 2) ha s alr ead y been discussed from th e point of view of AIO, vibra -tions, which were assigned to th e thr ee str ong ban ds of th e 700-800 cm-l region. Th estr ong bands near 500 cm-l ar e in tur n a ssigned to st retching vibrat ions of isolatedAlO, octa hedr a. Oth er a pr iori p ossible ass ignmen ts would be bending vibra tions

IS0 r I90 ii

0200 300 400 500 600 700 900 rcm0

cm-Fig . 8 . In f r e -re d epec t r um o f t h e h y d r o g a r n e t S r & l& OH ) ,, a n d o f t h e d e u t e r a t e dc o m p o u n d .of AlO, tet ra hedr a, an d str etching vibra tions of YO, dodecah edra . Exper iment aldata on the bending frequencies of isolated AlO, tetrahedra are completely missing ;but from t he 700-800 cm-l value corr esponding to the str etching motions , the bendingfrequ encies ma y be estima ted to lie in the 300-400 cm-l region. Now, as far as Y-Ofrequencies are concerned, it must be noticed that the corresponding isotypic ironcompoun d GFe,Oa-3Y,O, ha s no absorpt ion ban d in th e 500 cm-l region, a nd t ha tY-O st ret ching frequen cies corr espond ing to &fold co-ordinat ion of ytt riu m havebeen foun d in the 300-400 cm-l region in th e spectr um of ytt riu m o&o-bora teYBOs [14].It is th us very h ighly probable th at at least some of the str ong bands n ear500 cm-1 in th e spectru m of 5Al,O,-3Y,O, mu st be a ssigned to th e str etching vibra -tions of AIOs octa hedr a. This is in full agreement with th e un equivocal assignmentalr ead y discus sed for Sr ,A12(0H),,.Fina lly, th e cas e of na tu ra l gar net s X,Al,Si,0,2, with XII = Mg, Fe, Mn and Ca,will be briefly report ed. A deta iled assignm ent of the spectr a of th ese compoun dsis not easy, because of the occurrence of both SiO, bending and AlO, stretching[lb] J. P . LAPERCECEB a n d P . T AR T E ,~S@ctr och im. cta 22, 1201 (1966).

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I n f r e -r e d s p e c t r a o f i n o r g a n i c a l u m i n a t e s 2137

frequencies in the same spectral region; however, a systematic discussion, whichhas been reported elsewhere [5] leads to the conclusion that, in these compounds,th e AlO, st ret chin g frequ encies a re repr esent ed by t wo bands located, th e first onein th e region 509-460 cm-l, an d th e second one nea r 390 cm-l.

In conclusion, isolat ed AlO, octa hedr a a re cha ra cter ized by str ong ban ds inthe 539-400 cm-l region. It may be objected that one type of structure only (thegarn et type) ha s been investigat ed h ere ; th e chem ical comp ositions, however, ar e sowidely differen t, th at t he proposed frequen cy r an ge ma y be considered as repr esent -ative of AlO, isolsted octahedra.(b) Disczlssion of some speci& cases

We sha ll discuss here the spectra of some par ticular compounds, either becau seth ey r epres ent good exam ples of the gener al pr inciples discus sed in previous sections ,

- - CoA1204600 700 800 900 1000

Fig . 9 . I n f r a -r ed spec t r a o f t he sp in e l s CoAl ,O ,( no r ma l ) an d NiAl ,O , ( e ssen t i a l l y i nve r se ) .

or because our results or interpretations are at variance with those presented bySCHROEDERnd LYONS l] .

(1) II-III Aluminate spinels. Th e spect ra of CoAl,O, an d NiAJ .0, (Fig. 9) aretaken as an example of a normal (CoAI,O,) and an essentially inverse (NiAI,O,)spinel. Although th e co-ordina tion of Al is octah edra l in the first compoun d an d inpa rt tet ra hed ra l in th e second, th e differen ce in position of th e high-frequ ency ban dis ra th er sm all (683 an d 728 cm-l for Co an d NiAl,O, resp ectively). This is due, inpar t, t o th e fact th at we ar e dealing here with conden sed octa hedra in the first com-pound, and isohted tetrahedra in the second: here is a good example of a pointdiscussed pr eviously, na mely th e stret chin g frequ encies of conden sed AO, octa hedr ama y be a s high as the str etching frequencies of isolated AO, tetr ah edra. Moreover,it should be rem embered tha t t he vibrat iona l m odes of II-III spinels ar e not rea llyindependent vibrat ions, th is leading to some avera ging of th e vibra tionalfrequencies.In conclusion, we are dealing her e with a most un favoura ble cas e, an d it would

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2138 P.Tmmbe quite uncaut ious to conclude, from infra-red dat a alone, the normal or inversestr uctu re of CoAl,O, an d NiAl,O, spinels.

(2) LiAl,O* s@neZ. The spect ru m of this compoun d is extr emely complicated andth e au th or ha s shown tha t th is complexity, which is highly abnorma l for a spinel,may be explained-at least in par t-by two factors, namely import an t vibrat iona lint era ctions, an d th e occur ren ce of both cations (Li an d Al) on both types of cristallo-graphic sites (tetr ah edral an d octa hedra l) [a]. However, th e distr ibut ion of th ecat ions on tetr ah edral an d octa hedra l sites is not ra ndom, as adm itted by SCHROEDERand LYONS [l] fter EBRAY and HARDY [ 51, but on the cont ra ry ordered as faras th e sam ple has been obta ined at 125OC or below. An order-disorder tr an sitionta kes place nea r 1270 only; th is tr an sition brings out import an t cha nges inth e i.r. spectr um , but these cha nges have already been discussed elsewhere [4].

Tab le 2 . Vib ra t iona l fr e qu enc ies , 6Li-7Li i so top ic sh i f t s an d b an da s s i g n m e n t sfo r th e o rd ere d form o f L iAl ,O ,

Vi b r a t i o n a l f r e q u e n c i e s As s i g n m e n t s*Li Li Av T h i s p a p e r S ch r o e d e r a n d L y on s87077012868 264 9

869 (--I)77 2 (+2)729 (+I)681 (-1)64 7 (-2)

IAIO,

s t r e t c hAI-O(Al% 7)s t r e t c h

AlO,s t r e t c h

62 568 866 7

54 163 249 4

01 466 1

63 649 0

-11

-4

MixedAl-O

Li-0v i b r a t i o n s

ILiO,s t r e t c h

447 446 (-1)423 420 -3

40 1400 390365 (4) 361 -4 ?338 337 f-1)

I i xedAl-OLi-0v i b r a t i o n sWe shall her e limit ourselves to th e discussion of some ban d as signmen ts which d onot agree with those of S C H R O E D E R and LYONS [l]. Our observed frequencies forth e 6Li an d th e Li compoun d ar e collected in Table 2*, together with t he contr over-sial assignments. If Schroeder an d Lyons assignm ent , is accepted, only th e weakband at 869 cm-l is left for AlO, tet ra hedra , an un tena ble position if it is rema rkedth at th is band disappear s in th e disordered form , t he str uctur e of which still cont ainsAIO, tet ra hedra . Moreover, th is assignment is in disagreement with th e resultsdiscussed in th e previous section. Thus, th e assignment of th e two str ong high-frequency ban ds to AlO, st ret ch is mu ch more reliable. The two next, ban ds (681 and617 cm -l) exh ibit no significan t sLi-7Li isotopic shift and thu s ar e essent ially AI-O

* T h e r e i s n e a r ly a s y st e m a t i c d i ffe r e n c e b e tw e e n o u r fr e q u e n c i e s a n d t h o s e o f S c h r o e d e ra n d L y on s . T h e r e a s o n fo r t h i s d i sc r e p a n c y i s n o t e v id e n t b u t , s i n c e o u r s p e c t r o m e te r w a sc a r e f u l ly ca l i b r a t e d a g a in s t fr e q u e n c y s t a n d a r d s , w e a s s u m e o u r v a lu e s t o b e e s s e n t i a l l yc o r r e c t .[15] L. DEBRAY a n d A. HARDY, Compt. Rend. Acad. Sci. Paris 251, 725 (1960).

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2 1 4 0 P.TARTEvalue, an d second , becau se th e frequ ency itself is mu ch t oo high for a LiO, vibra tion(which in fact is observed below 300 cm-l).

On th e oth er ha nd, th e occur ren ce of mixed LiO-A10 vibrat ions is fair lyimpr obable her e, since th e vibra tional frequencies of LiO, and AIO, octah edra ar e sodifferent (

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I &s -r e d s p e c t r a o f i n o r g a n i c a l u m i n a t e s 2141

(5) Tricalcium alum inate 3CaO*Al,O,. The. spectr um of Schroeder an d Lyonsis in essential agreement with ours (which ha s been published elsewhere [IS]).

But the interpr etat ion proposed by Schr oeder an d Lyons rema ins open to discus-sion. This interpr etat ion relies on a assert ed peculiar str uctur e of 3CaO-Al,O,, whereAlO, gr oups ar e in th e form of very elonga ted octa hedr a with t wo widely differen tAl-O dista nces, na mely four Al-O short dista nces (l-9 8) forming a squar e, andtwo oth ers equa l to 2.6 A: th e complex series of ban ds in th e 906-800 cm-l region

Tab le 4

C o m p o u n dCo- o r d ina t ion

nu mb er Al -O d i s t a nces Ref . Al -0 f r equ en c iesCaO.Al,O,0 igh )

4 1.68 to I.82 (1) 900-650LiAlO, 4 1.755 an d 1.766 (2) 850-650a( low)LiAlO, 6 1.90 (3) 640-520a-A1203 6 1.92 (4) 650-590P y r o p o 6 1.886 (5) 460-390G e n e r a l

I4 1*77-1+x0 900-650

d a t a , 6 135-1.98 (6) 680-400(1) M. W. DOU GILL, iva tu r e 180, 292 (1963); ci te d in H. F . W. TAYLOR : t h e Chemistry of

Cements, Vol. 1, p. 153. Acad em ic P r ess (1964).(2) M. MAnEZIO, Acta Cw8t. 19, 96 1965).(3) M. M~REZIO snd J . P . REMEI KA, J . Chem . Phys. 44, 3143 (1966).(4) R. E. NE~NHAM an d Y. M. DE HAAN, 2. K?%t. 117, 235 (1962).(5) G. V. GIB BS an d J . V. SBZITH ,Am . Mineralogist 50, 2023 (1965).(6) Al-0 d is t an ces f rom International Tab les f or X-ray crystullography, Vol. 3, p . 262. Kyn ooh

Press (1962) .

is assigned t o th e squar e AlO, group with sh ort distan ces, wherea s t he strongban d n ear 740 cm-r is assign ed to th e long (2.6 A) Al-O dista nces.

Now, it is wort h while to point out th at :(1 ) Ther e is no definite proof of th e occur ren ce of elonga ted octa hedr a in th e str uc-

tu re of XJa O.Al,Oa. Afte r J EFF ERY [ 191 , th e stru cture determ inat ion of tr i-calciumalum inat e ha s still to be completed, an d at present th e evidence for a postu latedstr uctu re or pseu do-st ru ctur e is too doubtful to ma ke discussion wort h while.

(2) Even if th is type of str uctur e is funda ment ally corr ect, th e assignment of th e740 cm-l frequency to an Al-O bond with 2.6 A dista nce seems to be un ten able ifone consider s th e actu al correlat ions between co-ordin at ion nu mber , Al-O dista ncesan d Al-O str etching frequ encies (Table 4) : it is quite evident tha t the stretchingfrequency of an Al-O bond with 2.6 A dist ance should be very low, even if du eallowance is ma de for possible intera ctions or pertu rbat ions relat ed to environm enta linfluen ces. In th e au th ors opinion, all th e str ong ban ds in th e 709-900 cm-l region[18] P . TARTE , S i l icates Ind . 31, 43 (1966).[19] J . W. J EF FE RY, Th e Chem btry of Cements (Edit ed by H. F. W. TA~oR), Vol. 1, p. 160.

Academ ic P re ss (1964) .

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I &a -r e d s p e c t r a o f i n o r g a n i c a lu m in a t e s 2143number and also on environmental conditions, an important factor which has beennearly completely neglected by most authors.

Only if these condit ion8 are ful$lled is it possible to define spectral regions w hich maybe considered a8 really characterist ic of a given t ype of co-ordinat ed group.

Fu rt her examples of similar problems will be pu blished in due cour se.Ac k n o w le d g e m e n t s -T h e a u th o r g r a t e f u l ly a o h n o w le d g e s t h e l in a m ia l s u p p o r t g iv e n to hisl a b o r a to r y b y t h e F o n d a N a t io n a l d e l a Re c h e r c h e Sc i e n t i fl q u e , a n d t h e t e c h n i c a l a s s i s t a n c eof M. RICHOUX.