RAMAN SCATTERING STUDIES OF FOLDED SHEARING PHONONS IN KC12n (n=2 ≈ 6)

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RAMAN SCATTERING STUDIES OF FOLDED

SHEARING PHONONS IN KC12n (n=2

≈ 6)

N. Wada, M. Klein, H. Zabel

To cite this version:

JOURNAL DE PHYSIQUE

CoZZoque C6, suppl6ment au n o 12, Tome 42, de'cedre 1981 page C6-350

RAMAN SCATTERING STUDIES OF FOLDED SHEARING PHONONS IN

4-

N. ~ada*, M.V. Kleit?+)and H. Zabel

Coordinated Science ~aboratory*, Department of Physics + and Materials Research Laboratory, University of I l l i n o i s a t Urbmza-Champaign, Urbana, I l l i n o i s 61801, U.S.A.

Abstract.- Raman measurements were performed on KCl2, (n=2%6) in the very low frequency region (10~50 cm-1) at room temperature. We assign the observed Raman peaks to the folded k s phonons from the c-axis transverse acoustic branch of pristine graphite. A simple linear chain model was used to deduce the shearing force constants between the layers.

1. Introduction.- The dynamics of inter- and intra-layer interactions in graphite intercalation compounds (GIC's) have recently attracted much research interest, especially because of its important role in understanding the various dimensionality- related structural phase transitions and the staging mechanism (a stage -n compound consists of a periodic layer sequence of an intercalant layer and n graphite layers). Frequently, the concept of Brillouin zone-folding generated by the new periodicity upon intercalation has been employed to interpret the lattice dynamics of GIC1s. Neutron inelastic measurements [1,2} revealed the zone folding effects on the c-axis longitudinal acoustic phonon branches in alkali GIC's (AGIC1s). Raman measurements [ 3 , 4 1 on stage-1 AGIC's showed some disputable evidence for the in-plane zone- folding

.

In this paper we present Raman results on KCl2, (1~22.6) which demonstrate the zone-folding effects on the [OOR] TA branch. The interlayer shearing interaction will be discussed. More detailed discussion is given elsewhere [5].

2. Experimental.- Samples were prepared by the vapor transport method [6] from highly oriented pyrolytic graphite (HOPG). The HOPG samples were cleaved by Scotch tape, providing an atomically smooth surface before intercalation. Each sample was characterized by [OOR] x-ray reflection measurements before taking the Raman spectra.

Raman spectra were taken in the pseudo-Brewster angle configuration at room temperature with a 1 meter focal length home-made double-grating monochromator

+

equipped with concave holographic gratings, using a 5145 A Ar laser beam focused into a line image 50 y wide by 1 mm long. The power was limited to 300 mW to avoid local deterioration of the sample.

3. Results and Discussion.- Figure 1 shows Raman spectra of KC (n= 22.6) taken in 12n

the cross-polarized configuration at room temperature. No apparent polarization dependence of the Raman peaks was detected in these experiments. If Fig. 1, we

Fig. 1 : Raman spectra of KC1zn (n=2%6) taken in the depolarized configuration at room temperature. The spectra (a) % (e) were taken from stage 2-6 samples, respectively. The background levels are not absolute. The signal-to-background ratios were about 112 in (a), 114 in (b), 116 in (c) and 1/10 in the otherspectra. The arrows indicate the calculated values of the phonon frequencies (see text).

RAMAN SHIFT (CM-')

-1

positively identify two peaks at 19 and 23 cm in stage 2, two peaks at 23 and 34

-1

.

cm m stage 3, three peaks at 16, 19, and 39 cm in stage 4, five peaks at 17, 26, 30, 34, and 41 cm-I in stage 5, and three peaks at 24, 29, and 33 cm-' in stage 6. These observed peaks are attributed to the folded [OOR] TA phonons because of their extremely low excitation energies and their stage dependence. Note that a stage-n compound presumably posesses n zone-center shearing optical modes since a unit cell contains (n+l) layers. With use of a nearest-neighbor linear chain model, the shearing force constants (SFC1s) were deduced, as is shown in Table 1. Here,

dl,

Table 1 : The shearing force constants potassium(K)-bounding carbon(BC), BC- in KC12n (n= 2%5).

interior carbon(IC), IC-IC, and pristine $ 1 d2/dG $3/$G graphite C-C force constants, respec-

0.10 0.58 tively. The calculated phonon frequen-

Stage 2

0.18 0.84 cies using these SFC1s are indicated

Stage 3

0.14 1.03 1.13 with arrows in Fig. 1. We did not Stage 4

0.22 1.07 1.13 attempt to extract the SFC1s of stage 6, Stage 5

C6-352 JOURNAL DE PHYSIQUE

BC-BC layers is surprisingly small in stage 2 (one might expect stronger interaction because of the excess electrons donated by K atoms), and the SFC of BC-IC layers approaches that of C-C layers in pristine graphite with increasing stage. This observation of the anamalous weak interlayer interaction is difficult to understand. Note that x-ray measurements of the compressibility at high pressure also showed an extremely soft interlayer interaction in KC24 [9]. Possible explanations are (1) the electrons in the n orbits mainly form anti-bonding orbitals along the c-axis,

(2) because IT orbits are more electron-filled in GIC's, and thus the Pauli exclusion principle repulsive force becomes stronger, or (3) because of the Coulombic inter-

+

action, the majority of .rr electrons are localized on the K ion side, leaving the n

orbits on the other side relatively empty [lo]. This leads to a weak BC-C inter- layer interaction. Finally, we mention that stiffening of the BC-IC interlayer interaction with increasing stage suggests an extended charge delocalization in higher stage compounds.

We thank W. A. Kamitakahara, D. M. Hwang and P. C. Eklund for valuable discus- sions. We also thank N. Caswell for useful information about preparing high stage samples, and A. W. Moore for providing the HOPG used in these experiments. This work was supported in part by NSF 80-20550, DOE DE-AC02-76ER01198 and ONR N00014-79-C-0424.

References

1. W. D. Ellenson, D. Semmingsen, D. Grdrard, D. G. Onn, J. E. Fischer, Mater. Sci. Eng.

31,

2. A. Magerl, H. Zabel, Phys. Rev. Lett.

3,

3. M. S. Dresselhaus, G. Dresselhaus in Intercalated Layered Materials, edited by F. A. Levy (Reidel, Dordrecht, 1979) p. 423.

4.

e,

5. N. Wada, M. V. Klein, H. Zabel, to be published in Proceedings of the Interna- tional Conference on "Physics of Intercalation Compounds," Trieste, Italy,1981. 6. H. H&rold, Bull. Soc. Chim. Fr.

187,

7. D. E. Nixon, G. S. Parry, Brit. J. Appl. Phys. Ser. 2,

1,

8.

3,

9. N. Wada, R. Clarke, S. A. Solin, Solid State Cormnun.

35,