Phonon Spectrum of Second Stage Graphite-Bromine Intercalation Compounds
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CH. SIMON, F. BATALLAN AND I. ROSENMAN Groupe de Physique des Solides de l'Ecole Normale Superieure - Universitd Paris 7-2, Place Jussieu, 75251 Paris Cedex 05, FRANCE H. LAUTER Institut Laue-Langevin,
156 X, 38042 Grenoble Cedex, FRANCE
ABSTRACT Coherent inelastic neutron scattering results on the second-stage graphite intercalation compound C8 Br are presented. Phonons polarized along the c-axis and in the basal plane were observed. A phenomenological Born-von Karmdn force constant model with non-axial symmetry is used with parameters adjusted in agreement with the results. Several specific points of the phonon spectrum which are outside of the limits of the model are discussed.
INTRODUCTION Many investigators have studied the lattice dynamics of graphite intercalation compounds (GIC). The first studies were made by Raman scattering and concern the high energy lattice mode near 47 THz [1]. Infrared absorption was also observed in the same range of energies [1]. At lower energies, some results of inelastic neutron scattering in GIC's have been published. The first studies 12] were limited to the longitudinal modes propagating along the cdirection (perpendicular to the layers). In C3 6 K energy gaps have been observed in these branches and explained with a rigid-layer model [3] and later with a shell model [4]. Phonons of this polarization were also observed in the layer plane in C 8 Rb [5]. We have previously published an almost complete lowenergy spectrum of phonons in second-stage C 1 4 Br with phonons polarized longitudinal and transverse along the c-axis and in the layer plane [6-8]. In these works we developed a phenomenological Born-von Kdrmdn model which explained our experimental results. We present here the results of inelastic neutron scattering in C 8 Br, the most concentrated bromine second state GIC. We have observed phonons polarized longitudinal and transverse along the c-axis and in the layer plane. We have used the phenomenological Born-von Kgrman model previously developed to interpret the C1 4 Br results. A comparison of the model with these new and very precise experiments show clear differences however. In particular along the c-direction, characteristic features of the phonon spectrum related to the charge transfer and staging phenomenon are discussed. EXPERIMENTAL RESULTS The starting HOPG sample is held in a quartz container which after the intercalation process, is sealed in presence of excess bromine. The stage is verified by (001) neutron diffraction. In contrast to our previous study [81 the present results are for a very pure second-stage sample. The (hkO) neutron diffraction results are indexed in a unit cell which is /3 x 7 times the twodimensional (2-d) unit cell of graphite. These results are in agreement with The chemical formula of the compound is the work of Eeles and Turnbull [9].
Mat. Res. Soc. Symp. Proc. Vol.
20 (1983) OElsevier Science Publishing Co.,
Inc.
336
C8 Br as obtained by thermogravimetric studies [10]. This compound is the GIC which contains the most brom
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