Raman scattering from low-frequency phonons in stage-2 graphite-rubidium intercalation compounds

First-order Raman scattering from low-frequency phonons in stage-2 graphite-rubidium intercalation compounds is observed at $\omega \sim 18, 22, 35, \mathrm{and} 115$ ${\mathrm{cm}}^{-1\mathrm{}}$, and these frequencies are used to develop a lattice-dynamical model for this compound. The model is based on $k_z$-axis zone folding of the pristine graphite dynamical matrix which is consistent with the staging periodicity of the intercalation compounds. The doublet (at 18 and 22 ${\mathrm{cm}}^{-1\mathrm{}}$) is identified with transverse modes involving rigid shearing displacements of the graphite and the rubidium sheets, whereas the feature observed at 115 ${\mathrm{cm}}^{-1\mathrm{}}$ is identified with an out-of-plane mode. The 35-${\mathrm{cm}}^{-1\mathrm{}}$ feature is observed only for temperatures $T<\mathrm{}170\mathrm{}$ K, which is the temperature reported for a phase transition in this compound. We associate the $\omega \sim 35$ ${\mathrm{cm}}^{-1\mathrm{}}$ feature with zone-edge phonons which are rendered Raman active by the phase transition at 170 K. The Raman activity is ascribed to an in-plane, zone-folding interaction associated with a low-temperature commensurate rubidium-layer ordering.