A comparative study on the Raman-active modes of solid ${\mathrm{C}}_{70}$ and alkali-metal-saturated ${\mathit{M}}_{\mathit{x}}$${\mathrm{C}}_{70}$ (x∼6;M=K,Rb,Cs) has been carried out. The effect of saturation doping of alkali metals into the ${\mathrm{C}}_{70}$ fullerene lattice leads to significant changes in the vibrational spectra that are found to be insensitive to the radius or mass of the alkali-metal species. Whereas the high-frequency tangential modes (ω>1000 ${\mathrm{cm}}^{\mathrm{-}1}$) decrease in frequency by ∼50 ${\mathrm{cm}}^{\mathrm{-}1}$ with M doping, the radial modes (ω<800 ${\mathrm{cm}}^{\mathrm{-}1}$) are found to be relatively insensitive to M doping, some of them increasing slightly in frequency. The effect on the vibrational modes of ${\mathrm{C}}_{70}$ due to electron-charge transfer from the alkali-metal dopant to the fullerene molecule is in most respects similar to previous observations on M-doped ${\mathrm{C}}_{60}$.

• Received 2 March 1993

DOI:https://doi.org/10.1103/PhysRevB.48.3501