PHONONS IN GRAPHITE INTERCALATION COMPOUNDS

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PHONONS IN GRAPHITE INTERCALATION COMPOUNDS

S. Solin

To cite this version:

S. Solin. PHONONS IN GRAPHITE INTERCALATION COMPOUNDS. Journal de Physique Col-

loques, 1981, 42 (C6), pp.C6-283-C6-288. �10.1051/jphyscol:1981682�. �jpa-00221618�

Department o f P h y s i c s , Michigan S t a t e U n i v e r s i t y , E a s t Lansing, Michigan 4882'4, U . S . A.

Abstract.

-

Vibrational excitations in p r i s t i n e graphite and in graphite intercalation compounds a r e discussed. Modes associated with carbon atom i n t r a l a y e r motions and with the internal excitations of guest molecular species are emphasized.

1. Introduction.

-

The study of phonons in graphite intercalation compounds (GIC1s) i s by now a r e l a t i v e l y mature ~ u b j e c t . " ~ ' ~ The road towards t h i s maturity i s char- acterized by recent spurts in the growth of our knowledge of GIC1s i n general and of phonons in GIC's

i n

p a r t i c u l a r . Not surprisingly, our understanding of phonons in-GIC's i s intimately coupled t o an understanding of phonons i n p r i s t i n e graphite i t s e ~ f . ~ ' ~

D u r i n g

the past several years research e f f o r t s on vibrations in GIC's have focused on f i v e sub-areas: phonons i n p r i s t i n e graphiteY5 phonons associated with the i n t r a l a y e r motions of carbon atoms i n G I C ' s , ' - ~ internal molecular modes of guest species in G I C ' s , ~ ' ~ i n t e r l a y e r c-axis modes involving both guest and host layers ,8'9 and i n t e r c a l a t e i n t r a l a y e r modes.1° In t h i s brief manuscript, I would l i k e t o address the f i r s t three of the above mentioned areas. Thus, t h i s paper c o n s t i t u t e s an introductory s e t t i n g f o r the l a t t e r two topics which a r e under active investigation and a r e considered in other papers i n t h i s proceedings. Space limi- t a t i o n s also preclude the presentation of introductory material on GIC1s, staging, sample preparation techniques, e t c . Readers unfamiliar with those topics a r e re- ferred t o recent review a r t i c l e s . 11,12

2. P r i s t i n e Graphite.

-

Hexagonal graphite c r y s t a l l i z e s in the D6h space group with

4

four atoms in the primitive c e l l .I3 The zone center o p t i c modescan be group theo- r e t i c a l l y decomposed i n t o the following irreducible representation 14

r

= 2

E~~ + E~~

+ + 2

B~~

opt

of which the E modes and the EIU and

A2,

modes a r e respectively Raman and infrared active i n f i r s t order. All of the o p t i c a l l y a c t i v e modes have now been observed ex- 29 p e r i m e n t a l l ~ . ~ " They a r e shown in Figs. 1 and 2 which a l s o show the eigenvectors

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981682

C6-284 JOURNAL DE PHYSIQUE

Fig. 1: The Raman spectra obtained from a cleaved surface of graphite and the atomic displacements of the Raman active

EZg

modes.

1 0 (Dl

0 8 -

-

^{E C}

w 0 6 - t

/ - E C 0 2 -

Fig. 2: The

E 1

C and E I l C reflectance of graphite and the atomic displacements of

O O 1000 " ' zooo ~ 3000 ' ~ m a ' t h e IR active modes. The

E

Ilc spectrum

FREQUENCY ( C ~ - I I has been normalized f o r surface damage.

THI 5 0 40

3 0 2 0 1 0 0

M K I-

WAVE VECTOR Fig. 3 : Phonon dispersion curves f o r gra-

phite in the [OOl],

[loo]

and [ l l O ] direc- t i o n . From r e f . 16.

associated with each mode.

Of p a r t i c u l a r importance i s the AZu mode a t 848 c i l . Prior t o i t s observation, the available Rarnan, IR and neutron s c a t t e r i n g data ( t h e l a t t e r of which were limited t o < 500 cm- 1 ) had been used t o t e s t several models f o r the phonon dispersion curves of graphite.15 Although each of t h e available theoretical models " f i t " the avail- able data, t h e i r calculated values f o r t h e

A2,

frequency ranged from 800 cm-' t o 1400 cm-l. Armed with the crucial AZu frequency, Horie and coworkers16 constructed an a x i a l l y asymmetric Born-von Kaman model which provides phonon dispersion curves t h a t a r e in excellent agreement

w i t h

a l l the available experimental data. The re- s u l t s of t h e i r calculations a r e shown

i n

Fig. 3. Note,

i n

p a r t i c u l a r , t h a t the max- imum phonon energy i s not a t t h e

r

point, but occurs along the l i n e between

r

and

M.

GIC1s was the nearest layer model.19 In t h i s model i t was noted t h a t in an nth stage GIC n +

3,

the EZg 1580 c i l i n t r a l a y e r mode would develop a s a t e l l i t e caused by the small perturbation of the bounding i n t e r c a l a t e layers. Thus f o r n

3

t h e Raman spectra of GICts will contain a pair of l i n e s a t

5

1580 cm- 1 . One of these i s associated with i n t e r i o r carbon layers which have a nearest layer environment identical t o t h a t in p r i s t i n e graphite. One i s associated with bounding carbon layers perturbed by the intercalated layer. In stages

1

and 2 GIC's there e x i s t s only one type of nearest layer environment and thus only a single l i n e should be ob- served in the 1580 cm-I region. The above statements a r e verified by the Raman spectra of potassium graphite G I C ' s ~ ~ which a r e shown in Fig. 4. These spectra a r e c h a r a c t e r i s t i c of the stage dependence of t h e Raman spectra of a l l GIC1s studied t o date.' Moreover, the stage dependence of t h e r e l a t i v e i n t e n s i t i e s of the members of the 1580 cm-' doublet can be quantitatively accounted f o r

by

t h e nearest layer model. 1

Ramon rhlft (cm-')

Fig.

4:

Raman spectra of stage n , na ²

^K-

GICts with compositions ClZnK.

Stage 1 a l k a l i GICts e x h i b i t a Raman spectrum which i s c h a r a c t e r i s t i c a l l y d i f -

f e r e n t from those of Fig.

4,

but which provide an elegant v e r i f i c a t i o n of the c a l -

culated dispersion curves of Fig.

3.'

Note from the Raman spectrum of stage

l

RbC8 and C$C8 shown in Fig. 5 the polarized members of a t r i p l e t of modes in the

580 cm-' region." This t r i p l e t i s quantitatively accounted f o r by the dispersion

curves of Fig.

2

and i s associated with

M

point out of plane motions of the carbon

atoms.' These

M

point modes which a r e f i r s t order Raman inactive i n p r i s t i n e

C6-286 JOURNAL DE PHYSIQUE

Fig. 5: P o l a r i z e d Raman spectra o f t h e t r i p l e t r e g i o n f o r t h e stage 1 (a) Cs; and ( b ) Rb GIC's w i t h t h e i n c i d e n t p o l a r i z a t i o n perpendicular and t h e s c a t t e r e d p o l a r i z a - t i o n p a r a l l e l t o t h e s c a t t e r i n g plane.

P o l a r i z e d modes (A o r A1) a r e f o r b i d d e n i n t h i s c o n f i g u r a t i o n ?

STAGE I FeC13 - Graphltc

300 200 100

RAMAN SHIFT Icm-')

F i g . 6: P o l a r i z e d Raman spectra o f t h e i n t e r c a l a n t i n t r a l a y e r modes o f stage 1 FeC1 3-graphite.

g r a p h i t e are a c t i v a t e d by b o t h d i s o r d e r induced s c a t t e r i n g 2 ' and B r i l l o u i n zone f o l d i n g e f f e c t s 3 i n t h e 3-0 r e l a t i v e l y ordered s t r u c t u r e s o f stage 1 RbC8 and CsC8. 23 The stage 1 a l k a l i GIC's a l s o e x h i b i t a broad Fano resonance a t e l 5 0 0 cm-l ( n o t shown) which a r i s e s from electron-phonon coup1 i n g between Raman a c t i v e phonons i n t h e 800-1600 cm-l range and what appears t o be an e l e c t r o n i c continuum background. 24 4. I n t e r n a l Molecular Modes.

-

To d a t e t h e r e i s very l i t t l e d a t a on i n t e r n a l mole- c u l a r modes o f guest species i n GIC's. T h i s l a c k of data i s n o t f o r want o f serious e f f o r t s t o observe such modes s i n c e several groups have made serious attempts i n t h i s regard.

The f i r s t observation o f molecular modes i n a GIC was t h a t o f Br2 molecular modes i n bromine graphite.6 However, t h e Raman spectrum o f t h a t m a t e r i a l e x h i b i t s t h e m u l t i p l e i n t e n s e overtone peaks which a r e c h a r a c t e r i s t i c o f s t r o n g resonance Raman s c a t t e r i n g . The c l a s s i c example o f t h i s phenomena i s given i n t h e Raman res- onance spectrum o f

c ~ s . ' ~

To my knowledge t h e o n l y o t h e r system i n which i n t e r n a l modes o f the guest species have been observed from a GIC i s FeC13 graphite.7 The low frequency p o l a r i z e d Raman s p e c t r a o f stage 1 FeC13 g r a p h i t e i s shown i n Fig. 6.

understanding o f v i b r a t i o n a l e x c i t a t i o n s i n GIC1s. For GIC's t h e molecular modes and carbon atom i n t r a l a y e r modes can o n l y b e c o n v e n i e n t l y probed u s i n g o p t i c a l tech- niques, e.g. Raman s c a t t e r i n g and IR spectroscopy. During t h e p a s t year new and e x c i t i n g neutron s c a t t e r i n g r e s u l t s on i n t e r l a y e r guest-host modes i n GIC's and on 9 i n t r a l ayer i n t e r c a l a t e modes i n moni o n i c guest species1' have been reported. I n a d d i t i o n t h e former have a l s o been observed i n t h e Raman spectra o f several stages o f a l k a l i ~ 1 ~ ' s . ~ These new neutron and Raman r e s u l t s a r e t h e subjects o f o t h e r papers i n t h i s conference. 8,lO

6. Acknowledgements.

- ^I

g r a t e f u l l y acknowledge i m p o r t a n t c o n t r i b u t i o n s from my c o l l a b o r a t o r s N. Caswell

,

G. Lucovsky, R. J. Nemanich and G.N. Papatheodorou.

T h i s work was supported by t h e NSF under g r a n t number DMR 80-10486.

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