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PHONON RAMAN SPECTROSCOPY IN GRAPHITE
INTERCALATION COMPOUNDS
D. Hwang, S. Solin
To cite this version:
JOURNAL DE PHYSIQUE
CoiZoque C6, suppZe'ment au n o 12, Tome 42, de'cembre 1981 page C6-347
PHONON RAMAN SPECTROSCOPY I N GRAPHITE INTERCALATION COMPOUNDS
D . M . Hwang and S.A. S o l i n *
Department of Physics, University of IZZinois a t Chicago CircZe, Chicago, IZZinois 60680, U.S.A.
*Department of Physics, Michigan State University, East Lansing, Michigan 48824, U.S.A.
Abstract.- The atoms or molecules intercalated in a graphite intercalation compound are correlated spatially among themselves, resulting in a static structure factor with sharp peaks. Graphite phonons with momenta corresponding to these structure factor peaks have a higher probability of being scattered into the Brillouin zone center and becoming Raman active. With this momentum selection scheme, we can interpret the observed Raman features of graphite intercalation compounds with various in- plane structures.
1. Introduction.- Graphite intercalation compounds (GIC's) exhibit various ordered and disordered in-plane structures of the intercala- tion species. It is interesting to examine how the phonon modes of the host graphite crystal are perturbed by the intercalation structures. Stage 1 donor-type GIC's have the simplest 3-dimensional ordered structures among all intercalation compounds. Nevertheless, the inter- pretation of their unusual Raman features has been a vexing problem for several years. 192
The Raman spectra for various stage 1 donor GIC's are summarized in Fig. 1. A strong frequency dependent continuum is observed in the spectra for EuC6 and MC8 (M = K, Rb and Cs). It couples with the graphite E(~) mode, resulting in a broad Fano peak at 1500&10 cm-l,
2 g
For MC a doublet or triplet Raman feature is also observed around
8i
I.580 c m
.
LiC6 only exhibits a relatively sharp peak at 1594k5 c m.
Note that at room temperature, MC8 has a (2~2)~0' superlattice in- plane structure, while that for EuC6 and LiC6 is (fiM)~30'.On the basis of phonon band structure calculations for KC8 and Rbc8,1, it is possible to associate the 580 cm-l feature with the gra- phite M point phonons, which are rendered first-order Raman active by zone folding of the (2x2) superlattice .l However, the zone folding approach can not account for the continuous background observed. Also, it has been found5 that the Raman spectrum for CsC8 in its high temperature disordered phase is hardly distinguishable from that in the room temperature ordered phase. Thus, it is suggested5 that both the 580 cm-l feature and the continuous background are disorder-
C6-348 JOURNAL DE PHYSIQUE
: Raman spectra for (a) EuC6, f??&Co and (c) LiCr. recorded at room te%?erature (fr8m Ref.
3).
The 1840 cm- peak of EuC6 is found to be sample depentent and IS attributed toEuC an impurity on the surface. The spe$i;ra for KC and RbC8 are similar to that for
c ~ c ~ .
induced scattering.
Here we present a model incorporating the static structure factor for phonon scattering and the momentum selection rules for the Raman process, and show that both the disorder-induced scattering and the zone folding effect are extreme cases of a same basic phenomenon.
2. Theory.- Due to the weak interlayer interaction, the phonon disper- sion curves of graphite intralayer modes are essentially preserved in GIC Is
.
However, the graphite phonons can be elastically scattered by the intercalation layers and exchange momentum & with the intercalation lattice with relative probability S(&). Here S(k) is the static struc- ture factor for elastic scattering and should have the same general form for X-ray, electron, neutron, and phonon scattering.For GIC's with perfect commensurate ordered structures, S(k) contains only sharp superlattice Bragg peaks. The above momentum selection scheme is identical to the zone folding mechanism. On the other hand, if the in-bercalation species were completely spatially uncorrelated, they would contribute a flat background to S(&). The resulting disorder-induced scattering would reflect the phonon density of states of the whole Brillouin zone.
In real GIC's, the intercalation species are strongly spatially correlated. They result in a structure factor with essenthally discrete peaks. Non-zone-cnter phonons with momenta corresponding to the peaks of S(k) will have a higher probability of being scattered into the zone center and becoming Raman active.
Consider CsC8 as an example. Fig. 2 shows the structure factor in its ordered and disordered phases. The structure factor for the dis-
1
Fig. 2 : The static structure factor determined by X-ray scattering for
(a) the excess sample CsCo at 700 K. (b) the non-excess- sampleo~sO.
ic8
. at 700 K and (c) CsCn at300 K (from ~ e f ; 6). O ~ h e momentum of the (100) peak in (c) corres- ponds to the I'M distance in the graphite 2-dimensional Brillouin zone shown as the insert.
excess and the non-excess samples, respectively, which correspond to 90% and 100% of the FM distance in the Brillouin zone of graphite. The F W M of these peaks are 20% of I'M. Since the phonon bands near the M points are flat,4 only a small shift in the Raman frequency is expected when the Cs layer undergoes the order-disorder phase transi- tion.
Using this structure factor momentum selection scheme, we can also explain why the 580 cm-l peak of the density of states of the graphite phonons does not show up in the Raman spectra for other dis- ordered GIC's, as well as why the strong Fano peak only exists in the spectra for M C ~ and E U C ~ Details will be given
3. Conclusion.- GIC's are a unique system of materials with a high density of "impurities" which are spatially correlated among them- selves, but only weakly perturb the phonon bands of the host crystal. A momentum selection scheme based on the static structure factor must be employed to describe the disorder-induced Raman scattering.
This research is supported by the Research Board of the Univer- sity of Illinois at Chicago Circle, the Atlantic Richfield Foundation Grant of Research Corporation 9422, and the NSF Grant DMR80-10486. References :
1. M. S. Dresselhaus, G. Dresselhaus, P. G. Eklund and D. D. L. Chung. Mater. Sci. Eng.
2,
141 (1977); M. S. Dresselhaus and G. Dressel- haus, to be published.2. 3 . A. Solin, Physica B B , 443 (1980).
3. D. M. Hwang and D. Guerard, Solid State Commu., to be published.
4. C. Horie, M. Maeda and Y. Kuramoto, Physica S B , 4.30 (1980). 5. N. Caswell and S. A. Solin, Phys. Rev. B20, 2551 (1979).
6. R.Clarke, N. Caswell and S. A. Solin, Phys. Rev. Lett.
42,
61 (1979)7. D. M. Hwang,S. A. Solin and D.Guerard, Springer Series in Solid State Sciences, to be published.
