Phonon Softening in a Mixed Layered System,K1−xRbxC8

By means of inelastic neutron scattering, the dispersion of the longitudinal [$00q$] phonon modes has been measured for the first time in the mixed layered compound ${\mathrm{K}}_{1-x}{\mathrm{Rb}}_x{\mathrm{C}}_8$ over the whole composition range $0<~x<~1$. From the optic and acoustic phonon branches interlayer force constants are derived which are strongly composition dependent. At $x=0.65\mathrm{}$ the elastic constant $C_{33}$ shows an anomalous softening of more than 20%, which may be due to a composition-dependent charge transfer between the intercalant and graphite layers.