Phonon Dispersion of Transverse Basal Plane Modes in Alkali-Graphite Compounds
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W. A. KAMITAKAHARA*, H. ZABEL AND R. M. NICKLOWt *Ames Laboratory-USDOE and Dept. of Physics, Iowa State University,
Ames,
Iowa 50011, USA; **Dept. of Physics, University of Illinois, Urbana, Illinois 61801, USA; tSolid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA
ABSTRACT Dispersion curves for low-frequency transverse modes propagating in the basal plane have been measured in the compounds KC8 , KC24 and RbC 2 4 by means of neutron inelastic scattering. The acoustic branches show at low q a behavior 2 2 4 W = Aq +Bq characteristic of layered materials. The optical branches are derived qualitatively from graphite-like optical branches hybridized with new alkali-like branches. In stage 2 compounds, the shear constant C4 4, which can be obtained by extrapolating the acoustic branch towards q=0, is appreciably smaller than in stage-1 compounds or in pure graphite. At low temperatures, it was noted in KC 2 4 that a 1 frequency gap in the acoustic branch opens up near q=0.6 A- , corresponding to the Brillouin zone boundary of the low temperature alkali superstructure.
INTRODUCTION If a layered compound has interlayer forces that are very much weaker than the intralayer forces, some unusual phonon characteristics result. One of these is the occurrence of modes with an almost quadratic dispersion relation 2 2 4 U at small q; i.e., w =Aq +Bq , where A vanishes in the absence of interlayer forces. These modes have wave vectors q in-plane (parallel to the layers), but displacements out-of-plane, and can be described as bending or rippling modes of the layers. It is convenient to designate these modes as TAj, and similar optic modes as TOIL, or both as TI. The tendency for uq2 behavior is most pronounced in graphite [1] and MOS212], compounds for which the layered character is especially strong. In this paper, we examine T1 modes for alkali-graphite intercalation compounds (AGIC's), in which the strongly-bound C layers are interleaved with alkali layers for which both the intralayer and interlayer forces are relatively weak. Several new effects associated with the alkali intercalate are observed, including (i) a dramatic lowering of the layer shearing force constant C4 4 , which can be estimated from the q+0 slope of the TAj dispersion curve, and (ii) the appearance of new alkali-like optical branches which strongly couple to the layer bending modes of the graphite host. The increasing amount of new experimental information becoming available on the dynamical properties of AGIC's, together with associated theory and calculations [3,4], should lead to a comprehensive understanding of the lattice modes in the neor future. It is hoped that the process will be facilitated by the present work, which is a natural extension of recent neutron studies of [001]L modes [51 and intercalate modes [6,7]. A fuller account of our experiment on layer bending modes, including a more thorough analysis, will soon appear [8] elsewhere.
Mat. Res.
Soc. Symp. Proc. Vol.
20 (1983) Published by Elsevier Science
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