Phonon symmetry and Raman spectra of fullerites

Physica C 235-240 (1994) 1201- ! 202 North-Holland

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Phonon Symmetry and Raman Spectra of Fullerites Yu.S.Grushko a

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A g r o u p - t h e o r y a n a l y s i s of p h o n o n s y m m e t r i e s in C 6 0 : I 4 c r y s t a l s is p e r f o r m e d . T h e Raman s p e c t r a of v a r i o u s f u l l e r i t e s and s h u n g i t e s are i n v e s t i g a t e d . 1. G R O U P - S Y M M E T R Y ANALYSIS P H O N O N S Y M M E T R Y IN C60: l&.

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Figure 1 presents Raman spectra of fullerites C60, C70 and native materials, s h u n g i t e s . The latter c o n t a a n a s i g n i f i c a n t a m o u n t of carbon, and, p r o b a b l y , a small p e r c e n t a g e of the fullerites. The c h a r a c t e r i s t i c i n t e n s e high-frequency lines in C60 (1467 cm- ]) , and ]n C70 (1567 cm -I ) are well-known. These ]. i nes have not been o b s e r v e d in R a m a n s p e c t r a of s h u n g i t e s , but there are two intense high-frequency bands 1350-1360 cm-'l and 1590 -1600 cm -I there (Figure i). F r e q u e n c y s h i f t s of these b a n d s in d i f f e r e n t s a m p l e s of s h u n g i t e s ( a b o u t + I0 cm -I ) are due to the variation of s a m p ie composit ions. Large hal f - w i d t h s of t~ese bands (about 80 cm -i ) indicate the disordered structure of c a r b o n ir~ the shung]tes. F i g u r e 2 pi-esents the s p e c t r u m of a C6012,~ crystal .

R a m a n s p e c t r a and symmetries of p h o n o n s in C60 and C70 are s t u d i e d e l s e w h e r e . In this w o r k we p r e s e n t for the first time the r e s u l t s of the s y m m e t r y a n a l y s i s for C 6 0 : I ~. When doping C60 crystals w i t h iodine atoms, the latter occupy interstitutial places, forming layers with hexagonal B r a v e l a t t i c e [1,2]. T h e symmetry of whole crystal is d e s c r i b e d by PZ space group. Ii and I2 a t o m s fill 2d (1/3, 2/3, z) and 3f (1/2, O, 1/2) positions, correspondingly. In the B r i l l o u i n zone center, the contribution of iodine atom vibrations into vibrational s p e c t r u m of the C60I ~ crystal is the f o l l o w i n g . r'= ri+ 2r2+ r_,.o. 2Pa ÷. .r.5.+ 9v== = Ag+2Au+E 19+2E1u+E2g+2E2u. In R a m a n s p e c t r a , o n l y those Ag, E1g, and E 2 g m o d e s can be observed which I atoms in 2d p o s i t i o n contribllte in. ,

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Yu.S. Grushko et al./Physica C 235-240 (1994) 1201-1202

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Raman shift (crn-1) F i g u r e I. Raman spectra of C60, C70 crystals, and s h u n gi,tes. T = 300K. kexe = 514 nm.

T h e r e is a w i d e intense band in the low-frequency region of the s p e c t r u m . Three pron o u n c e d p e a k s at 165, 190 and 213 cm -I can be d i s t i n g u i s h e d in this band. It should be m e n t i o n e d that, in the case of u n d o p e d C 6 0 crystals, no line is o b s e r v e d ~n the s p e c tral r a n g e below 250 cm -I . It is k n o w n that the Ag vibr a t i o n f r e q u e n c y of a free 12 m o l e c u l e (in a gas phase) is equal to 213 cm -I . Thus, we can a s s i g n the l o w - f r e q u - e n c y lines in the Raman spectrum of C6oI2, ~ crystals to the Is-molecule vibrations. According to the X-ray data [1,2] four iodine atoms occupy two n o D - e q u i v a l e n t pos i t i o n s in the p r i m i t i v e cell C60I A crystals. In the case of lower iodine c o n c e n t r a t i o n (C6012.A), a n u m b e r of ='sole"

F i g u r e 2. Raman spectra C6oI2. &. T=300K. k e x c = 514

of nm.

i o d i n e m o l e c u l e s h a v i n g no 12 neighbc, rs can appear. On this a s s u m p t i o n , we c a n a t t r i b u t e the R a m a n lines at 165 and 190 cm -I to the Ag v i b r a t i o n s of i n t e r a c t i n g 12 m o l e c u l e s , q~e line at 213 cm -I can be a s c r i b e d to the A g v i b r a t i o n of a sole i o d i n e m o l e c u l e in C60Is. 4 matrix. It s h o u l d be noted that the i n s e r t i o n of I H m o l e c u l e s into C60 crystals does not influence intramolecular v i b r a t i o n s . So, the f r e q u e n c y of the most intense Raman line in C6012. ~ is 1468 cm -I REFERENCES. l . O . Z h o u and D . E . C o x , J , P h y s . Chem. Solids, 53 (1992) !373. ~ . Q . Z h u et al., N a t u r e , 355 (1992) 712.