VIBRATIONAL DYNAMICS IN IONIC LIQUIDS

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VIBRATIONAL DYNAMICS IN IONIC LIQUIDS

F. Aliotta, M. Fontana, G. Maisano, P. Migliardo, C. Vasi, F. Wanderlingh

To cite this version:

F. Aliotta, M. Fontana, G. Maisano, P. Migliardo, C. Vasi, et al.. VIBRATIONAL DY- NAMICS IN IONIC LIQUIDS. Journal de Physique Colloques, 1981, 42 (C6), pp.C6-57-C6-59.

�10.1051/jphyscol:1981617�. �jpa-00221248�

JOURNAL DE PHYSIQUE

CoZZoque C6, supple'ment au n012, Tome 42, de'cembre 1981 page C6-57

VIBRATIONAL DYNAMICS I N IONIC LIQUIDS

F. A l i o t t a , M.P. ~ o n t a n a * , G. Maisano, P. M i g l i a r d o , C. V a s i and F. Wanderlingh I s t i t u t o d i Fisica and G.N.S.M. deZ CNR, Universita' d i Messina, I t a l y

" ~ s t i t u t o d i Fisica mzd G.N.S.M. d e Z CNR, Universita' d i Pama, I t a l y

I n r e c e n t y e a r s , t h e c h e m i s t r y a n d t h e p h y s i c s o f i o n i c l i q u i d s a s e l e c t r o l y t i c g o l u t i o n s and m o l t e n s a l t s , have b e e n w i d e l y s t u d i e d b e c a u s e of t h e s t r u c t u r a l p e c u l f a r i t y o f t h e s e m a t e r f a l s . I n p a r t i c u l a r , a s f a r a s aqueous s t r o n g e l e c t r o l y t e s s o l u t i o n s o f 2-1 t r a n s i t i o n m e t a l s ( e.g. NiCl ,CuC12,CuBr ,NiBr2,ZnC12,ZnBr2) a r e c o n c e r n e d , t h e r n ~ o d ~ n a m i c a l

2 2

c a l c u l a t i o n s , t r a n s p o r t and a c o u s t i c measurements, EXAFS, X-rays a n d n e u t r o n s c a t t e r i n g , Raman s c a t t e r i n g seem t o i n d i c a t e t h e p o s s i b i l i t y of a new p o i n t o f view i n d e s c r i b i n g t h e p h y s i c a l p r o p e r t i e s o f s u c h systems. The l a t t e r o n e s a p p e a r t o b e " s t r u c t u r e dominated

"

r a t h e r t h a n '+ c o l l t s i o n dominated". The above mentioned r e s u l t s c a n b e summarized i n t h e f o l l o w i n g p o i n t s : ( 1 ) At v e r y h i g h c o n c e n t r a t i o n s t h e s o l u t e t e n d s t o form complex i o n s o f t h e ~ e ~ n 2 - n k i n d . Such a p r o c e s s i n v o l v e d + 5V/. o f t h e c a t i o n s , w h i l e t h e r e m a i n i n g o n e s d i f f u s e t o g e t h e r w i t h t h e i r h y d r a t i o n s h e l l s . ( i i ) I o n - i o n and i o n - w a t e r complexes b e g i n t o i n t e r a c t and t h e s t r u c t u r e o f t h e h y d r a t e d c r y s t a l s i s l o c a l l y r e p r o d u c e d

.

A c t u a l l y , t h e phonon d e n s i t y o f s t a t e s b u i l t up i n t h i s way becomes an e f f e c t i v e drnsity of s t a t e s which is t h e c o n v o l u t i o n o f t h e t r u e v i b r a t i o n a l spectrum w i t h t h e mode d e p r n d i n g m a t r l x e l e m e n t s . F o r d i s o r d e r e d m a t e r i a l s , a s amorphous s e m i c o n d u c t o r s , g l ~ s s e s , s u p e r i r m t c s and a s s p c i a t e d l i q u i d s a s B203,SbC13,B20 i n which t h e m e l t i n g p r o c e s s g i v e s r l s e t o t h e breakdown o f o n l y a s m a l l p e r c e n t a g e o f t h e s o l i d p h a s e bonds, t h e v i b r a t i o n a l modes seem t o b e c o l l e c c i v e i n c h a r a c t e r . I n o u r s y s t c m s , i n e f f e c t , a t h i g h c o n c e n t r a t i o n s ,

t h e s t r u c t t t r e r e s e m t ~ l e s t h a t o f t h e c o r r e s p o n d i n g c r y s t a l l i n e s t a t e ; t h i s e v i d e n c e i m p l i e s t h e e x i s t r n c c o f i n t e r a c t i o ~ ~ between t h e i o h i c complexes and t h e o b t a l r l e d d e n s i t y oE s t a t e s is a c o n v o l ~ ~ t i o n o f " a c o u s t i c a l " and

''

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981617

JOURNAL DE PHYSIQUE

m i d d l e r a n g e o r d e r . I n e f f e c t t h e Metal-Halogen s t r e t c h i n g b a n d s a r e e v i d e n t and l i e i n t h e same f r e q u e n c y r e g i o n o f t h e c o r r e s p o n d i n g modes o f t h e h y d r a t e d c r y s t a l s . ( s e e

Fig. 1 (from ref. 1 ) LOW rrequcncy RanIan spectra of CuCIl a ~ l u t ~ o n for different Ilnes: a: 5145 A; ^b: 4880 A;c: 4765 A. J. 4579 A.

In the inscrt tPe absurptlon is shown.

F u r t h e r m o r e from t h e b c l i a v i o u r o f t h e d e p o l a - Fig. 2 (from ref. . 3 )

r i z a t i o n r a t i o w i t h t h e c o n c e n t r a t i o n ( s e e f i g . 7 ) , e v i d e n c e h a s b e e n found f o r a l o c a l s t r u c t u r e t r a n s f o r m a t i o n a t w 2 H o l a r , where a s a f t ~ t e dominated s t r u c t u r e t o o k o v e r t h e w a t e r d o i n i n a t i d s t r u c t u r e . Another i n t e r e s t i n g s t r u c -

'

t u r i t l p c r u l f a r l t y a p p e a r s when t h e a n h y d r o u s m e l t P '-

and g l a s s y o f t h e same s a l t s a r e i n v e s t i g a t e d . 0

---i--

--- -,

f i g . 1 ) .

I n p a r t i c u l a r ( s e e f i g . 2 ) w i t h a J e t - f l o w t e c h - n i q u e , i t h a s b e e n p o s s i b l e t o d e t e c t , by c h a n g i n g t h e Xinc., a p r e s o n a n c e Raman e f f e c t r e l a t e d t o t h e A & g band o f t h e complex c u ~ l ~ ~ - , c e n t r e d a t 290cm-I

U d

0 2 4 6 8 1 0 1 2

C (molarity 1 o f S t o k e s s h i f t .

- -

U) z

,,,

z

z a

5

a

5 0 150 250 350 450

RAMAN SHIFT (cm-1) Expcrinientnf Rnninn spectra for ;lie saturated solutiolls t b l (13) ('dC'l2. (c) CiCIl. (d) SrC'f, atld 1i)r (e) pure watcr. k'wcitalioil power: IOOnlW, counting tinlet cl1annel0.6sec; scat1 speed 100c1n"/src. (?he t1,p spec- lrurll is fro111 a sin& cryst:tl of CdC1,,0.2 St1,O (a).

Conccntrntlun dcprlalcncu of tho dcpolarlzntlon rut p for CdCll u~il ZIICI, solutions n t 230 and 285 con-' Il~unnll shllta, rcspctlvcly.

Fig. 3 (from ref. 2 )

-

- -

. . . . .

.

.,\' .

p:

1 . 1

I .. : 3 s 450 550A (n,l

I

.

I ;. ,.. . '. ..;5.:. ,

. . ^"'

. *, '. .L

. 1 3

, I \: ?.-. ,..*: ^,,

. , \<, ^a.:'. .,. ..-,: ...:, z.::.

': ' I

. '. %... .

:;,

: . l i > : .

--: -J ..

'..-.-.

; : *:

(Gfi,

:. *;\,, I ;

,,I\ '-- ^...,.

.

; , +..+I;

..,

. $ +

.,,...

. I --.__. -.-.

,, _. ' _I

':,. ^.%+..%-

(&..i

-'-..--

- _

I I I

20 150 300 450 600

STOKES SHIFT bw(cm-')

Fig.4, in fact, shows that for glassy and molten ZnCIZ

,

,

an s

both t o each other ind to that o f the saturated aqueous solution. Furthermore, in the case of molten ZnC12, our

2:[(

-

quite well and strongly supports our initial hypo- thesis that the vibrational dynamics o f the system 2nCLI mlten

w are collective in character. As far a s the high

w frequency region is concerned, the studied of the

changes in the spectral shape of the 0-11 stretching ba~rd as a function of the solute concentration, confirm our interpretation that a structure imposed by the so!lute does exist, in which tire water mole- cules are strongly polarized.

-

- 1

Anisotropic reduced Raman intensity

I

of molten ZuC12

Fig. 4a (from ref. 5)

Stokes h ~ l t w(~m*')

Anisotropic reduced Raman intensity for glassy ZnC12

Fig. 4b (from ref. 5)

Thc! rest~ltinp, effect is a very large change of the 0- II stretching region.

Rcl,r~.scntation of ":tvcr:v:c" dispersion cul-vcs (Ruf.

1G) on thc r~ght s l d u nnd P:~, (4 o n the Icft sl&. for lnoltcn ZnC12.

Fig. 5 (from ref. 5)

References

1.- El.P.Fantana,C.E1afsano,P.Migliardo and F.Wanderlingh, Solid State C o m n . ~ , 4 8 9 ( 1977).

2

.-

3.- F.Allorta,M.P.Fontana,C.Flaisano,P.Migliardo and F.Wanderlingh, Optica Acta,x,971 (1980).

4.- F.A~iotta,C.Maisano,P.Migliardo and C.Vasi, Opt.Connn.2, 274 ( 1980).

5.- F.A~iotta,C.PlaIsano,P.Migllardo,C.Vasi ,F.Wanderlingh ,G.P.Smith and R.Triolo, J.Chem.

Phys. July 15 (1981).