Raman scattering study of the high-frequency graphitic intralayer modes in Li-graphite and the stage dependence of the mode frequency in donor graphite intercalation compounds

Raman spectra of the high-frequency (∼1600 ${\mathrm{cm}}^{\mathrm{-}1}$) graphite interlayer modes have been studied in several Li-graphite intercalation compounds (GIC’s) ${\mathrm{LiC}}_{\mathrm{p}}$=6,12,17,18. In stage-2 ${\mathrm{LiC}}_{17}$ we have also used these modes to study the T=250 K reversible structural phase transition from high-T disordered to low-T ordered Li intercalate layers and found that a Raman singlet at room temperature evolves into an unresolved Raman doublet (or triplet) below the phase transition temperature. A careful line-shape study of the low-T spectrum could not clearly distinguish between these two possibilities. A triplet is consistent, however, with previous x-ray diffraction studies of the phase transition in the bulk. Raman spectra of ${\mathrm{LiC}}_6$ were obtained and found to be in good agreement with previous studies. Spectra of pure stage-2 ${\mathrm{LiC}}_{12}$ were recorded for the first time. The high-frequency graphitic intralayer mode frequencies obtained in this work, together with other previously published data, indicate that a ‘‘universal’’ linear relation between mode frequency and reciprocal stage index does not exist in donor-type GIC’s. That is, well outside of experimental error, the Li results show clearly that several factors (i.e., intercalate layer order, charge transfer, etc.) all make significant contributions to the small fractional change (∼0.5%) observed in the graphitic mode frequencies. The net effect of all factors in the graphitic intralayer mode frequency is not well parametrized by the stage index alone.