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Submitted on 1 Jan 1981
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PHONONS IN GRAPHITE INTERCALATED WITH
BROMINE
F. Batallan, I. Rosenman, C. Simon, G. Furdin, H. Lauter
To cite this version:
JOURNAL DE PHYSIQUE
CoZZoque 66, suppZ6ment au n o 12, Tome
42,de'cembre 1981
page
C6-344PHONONS IN GRAPHITE INTERCALATED WITH BROMINE
F. B a t a l l a n , I. Rosenman, C. Simon, G. ~ u r d i n * and H . J . Lauter*"
Groupe
ZePhysique des SoZides de Z'EcoZe NormaZe Supe'rieure, Universitd Paris
7, Tour 23
-
2 PZace Jussieu, 75251 Paris Cedex 05, France.
*Universite' de Nancy
I ,Nancy, France.
**Institut Laue-Langevin, Grenob Ze, Frazce.
A b s t r a c t .
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We have i n v e s t i g a t e d t h e phonon spectrum i n s t a g e 2 g r a p h i t e i n t e r c a l a t e w i t h bromine by u s i n g i n e l a s t i c n e u t r o n s c a t t e r i n g . We have observed l o n g i t u d i n a l a n d t r a n s v e r s e phonons along t h e c d i r e c t i o n a s w e l l a s i n t h e l a y e r p l a n e . R e s u l t s a r e p r e s e n t e d f o r e n e r g i e s up t o 15 THz a t 300 K. A l i n e a r c h a i n model e x p l a i n s most o f our r e s u l t s .1. I n t r o d u c t i o n . - G r a p h i t e forms i n t e r c a l a t i o n compounds w i t h a l a r g e v a r i e t y o f atoms o r molecules. Among them t h e a c c e p t o r compounds made by i n t e r c a l a t i n g halogens o r a c i d molecules p r e s e n t t h e most i n t e r e s t i n g p r o p e r t i e s : a m e t a l l i c t y p e o f c o n d u c t i v i t y w i t h a v e r y l a r g e a n i s o t r o p y , s o t h a t t h e y can b e c o n s i d e r e d a s 2-D metals. T h e i r e l e c t r o n i c s t r u c t u r e and p r o p e r t i e s a r e now w e l l known"). But very few r e s u l t s on t h e i r l a t t i c e dynamics have y e t been p u b l i s h e d ( 2 ) and even t h e s e concern mostly Raman a c t i v e modes. A few neutron s t u d i e s i n donor compounds a r e a ~ a i l a b l e ( ~ - ~ ) . We p r e s e n t h e r e t h e f i r s t almost complete i n v e s t i g a t i o n of t h e phonon spectrum i n a g r a p h i t e a c c e p t o r i n t e r c a l a t i o n compound (GAC) : t h e s t a g e 2 compound w i t h bromine.
2. Sample and experiment.
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The sample (0.8 x 1.5 x 3 cm3) was made by d i r e c t i n t e r -c a l a t i o n o f bromine i n h i g h l y o r i e n t e d p y r o l i t i c g r a p h i t e (HOPG). It should be mentioned h e r e t h a t it i s very d i f f i c u l t t o p r e p a r e l a r g e and homogeneous samples by i n t e r c a l a t i o n . The composition a s checked by e l a s t i c n e u t r o n s c a t t e r i n g showed t h e p r e s e n c e o f a non n e g l i g i b l e amount o f s t a g e 3. However a c a r e f u l a n a l y s i s of t h e i n e l a s t i c r e s u l t s had allowed u s t o s e p a r a t e t h e phonons belonging t o s t a g e s 2 and 3 . The experiments were made on IN8 s p e c t r o m e t e r a t t h e h i g h f l u x r e a c t o r of t h e
I n s t i t u t e Laue-Langevin i n Grenoble, France. We have used e n e r g i e s up t o 15 THz and momentum r e s o l u t i o n o f 0.01 Z-I. We have made experiments a t 300 K and 10 K. A s o u r sample i s o b t a i n e d from HOPG, it can b e c o n s i d e r e d a s a 2-D powder. I n t h e r e c i p r o - c a l space we have t h e Bragg r e f l e x i o n s ( 0 , 0 , 1 ) corresponding t o t h e i d e n t i t y p e r i o d
Ic and t h e c i r c l e s (h,k) i n t h e b a s a l p l a n e o f g r a p h i t e o r bromine. We p r e s e n t h e r e o n l y r e s u l t s on s t a g e 2 o b t a i n e d a t 300 K.
3. R e s u l t s and I n t e r p r e t a t i o n .
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We have observed phonons i n f o u r d i f f e r e n t configu- r a t i o n s corresponding t o : [ 0 0 1 ] ~ , [ 0 0 1 ] ~ , [01Q1L and [ 0 1 0 1 ~ . A s t h e sample i sa powder i n t h e b a s a l p l a n e o n l y t h e phonons i n t h e c o n f i g u r a t i o n [ O o l ] ~ a r e
unambigously d e f i n e d . A l l o u r r e s u l t s i n t h e f o u r c o n f i g u r a t i o n s f o r e n e r g i e s l e s s t h a n 15 THz a r e r e p o r t e d i n F i g . 1.
We have n o t performed a complete c a l c u l a t i o n o f t h e f o r c e c o n s t a n t model. However by u s i n g o n l y some simple assumptions, we were a b l e t o i n t e r p r e t most of o u r experimental r e s u l t s . We have based our approach on t h e c a l c u l a t i o n s f o r p r i s t i n e which i n t r o d u c e f o r c e c o n s t a n t p a r a m e t e r s i n t h e Born-von Karman models. We now c o n s i d e r o u r experimental r e s u l t s o f t h e phonons f o r each c o n f i g u r a t i o n :
a )
-
[ 0 0 1 ] ~-
we assume t h a t t h e t h r e e experimental branches correspond t o r i g i d modes, we t h e n c a l c u l a t e t h e phonon branches g i v e n by a l i n e a r harmonic c h a i n model w i t h i n t e r a c t i o n s l i m i t e d t o t h e n e a r e s t neighbors. We have used two f o r c e c o n s t a n t s and two masses which r e p r e s e n t an average p e r carbon atom, a s h a s been p r e v i o u s l y u s e d ( 5 ) . We f i n d a v e r y good agreement with t h e experimental r e s u l t s ( s e e F i g . 2 ) f o r t h e f o l l o w i n g v a l u e s , mC = 12.01 a.m.u, mBr = 11.42 a.u.m, = 2840 dyn/cm and (PE-Br = 1280 dyn/cm. The r a t i o mBr/mC corresponds t o t h e formula C14Br, which r e s u l t s from a n e l a s t i c scan s t u d i e s we have made.F i g . 1 : Phonons i n second s t a g e bromine F i g . 2 : Phonons i n t h e [001]L. The ver- i n t e r c a l a t i o n compound of g r a p h i t e . The t i c a l b a r s a r e t h e experimental v a l u e s . v e r t i c a l b a r s correspond t o c o n s t a n t Q The continuous l i n e s a r e t h e r e s u l t s o f s c a n s and t h e h o r i z o n t a l c o n s t a n t E s c a n s . t h e computation w i t h $s-Br/(PG-C= 0.45
C6-346 JOURNAL DE PHYSIQUE
b)
@G&~T?
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Although f o r t h i s branch resonances a r e v e r y d i f f i c u l t t o observe,we g e t an almost f l a t phonon branch a t I THz. The f l a t n e s s means t h e absence of pro- p a g a t i o n a l o n g [001] d i r e c t i o n f o r t h e t r a n s v e r s e o p t i c a l phonons. T h i s f e a t u r e can
t
be r e l a t e d t o t h e absence of t r a n s v e r s e coupling : $CBr = 0. T h i s i s confirmed by t h e v a l u e 1 THz which i s e x a c t l y
fi
l e s s t h a n t h e v a l u e a tr
p o i n t i n H O P G ' ~ ) . T h i s shows a l s o t h a t $EC i n t h i s GAC i s t h e same a s i n HOPG.C )
lc&iX1&
- I n t h e range o f energy t h a t we have i n v e s t i g a t e d , t h e branch i sl i n e a r , and g i v e s a sound v e l o c i t y o f t h e same v a l u e a s t h a t i n H O P G ' ~ ) . T h i s means t h a t we can t a k e f o r t h e r a d i a l c o n s t a n t between carbon atoms i n t h e g r a p h i t e l a y e r t h e same v a l u e s a s i n p r i s t i n e H O P G ( ~ ' ~ ) .
d ) [OPO]T
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We have observed t h r e e branches : two TO and one TA. A t t h er
p o i n t , t h e s e a r e connected t o t h e [ 0 0 1 ] ~ branches. As t h e energy r a i s e s , t h e t h r e e branches merge t o g e t h e r and g i v e a t t h e zone boundary t h e same v a l u e o f HOPG. Due t o t h e m u l t i p l i c i t y o f t h e i n t e r a c t i o n s involved i n t h e s e branches we need a more complex model i n o r d e r t o i n t e r p r e t t h e r e s u l t s .References
1. C. Simon, F. B a t a l l a n , I. Rosenman and H. F u z e l l i e r , Phys. Rev.
g,
2836 (1981) 2. S.A. S o l i n , Physicaz,
443 (1980) and t h i s c o n f e r e n c e .3. W.D. E l l e n s o n , D. Semmingsen, D. Guerard, D.G. Onn and J.E. F i s c h e r , Mat. S c i . and Eng.
31,
137 (1977)4. J. Rossat-Mignod, D. F r u c h a r t , M . J . Moran, J . W . M i l l i k e n and J . E . F i s c h e r , S y n t h e t i c Metals
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