Ab initio calculations of the phonon spectrum of ${\mathrm{K}}_6{\mathrm{C}}_{60}$ are presented, based on the local-density approximation of density-functional theory and on linear response theory. The effects of doping on frequencies and infrared intensities are identified and their physical origin discussed in detail for optically allowed modes. Whereas structural relaxation is primarily responsible for the frequency changes, and especially for the softening of tangential modes, the change in infrared relative intensities is a direct consequence of the electron transfer. The potassium vibrations are found to lie within $60–100{\mathrm{cm}}^{-1\mathrm{}}$ and are well decoupled from ${\mathrm{C}}_{60}$ intramolecular modes.

• Received 13 November 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.4915