Phonon-Raman Anomaly in the Ternary Graphite Intercalation CompoundK1−xRbxC8

We report the first measurements of the Raman spectra of an alkali ternary-solid-solution graphite-intercalation compound ${\mathrm{K}}_{1-x}{\mathrm{Rb}}_x{\mathrm{C}}_8$, $0<~x<~1$. A broad asymmetric softening of the graphite $M$-point phonon modes is observed at $x\approx 0.67$. This anomaly is attributed to electronic band-structure modifications which alter the electron-phonon contribution to the phonon self-energies and which result from the formation of K${\mathrm{Rb}}_2$-like clusters.