From in situ Raman measurements during potassium doping of pure ${\mathrm{C}}_{60}$ films to a structure ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$ in the range of 0≤x≤6 we found a continuous change of line intensities for the ${\mathit{A}}_{\mathit{g}}$ modes of the stable phases in this material. No shift of these modes with doping was observed, which proves a strict phase separation. The Raman line at 1467 ${\mathrm{cm}}^{\mathrm{-}1}$ for pristine ${\mathrm{C}}_{60}$ is due to oxygen contamination and is assigned to positively charged fullerene states. The intrinsic pinch mode of the pristine material is at 1458 ${\mathrm{cm}}^{\mathrm{-}1}$ and has a linewidth of 4 ${\mathrm{cm}}^{\mathrm{-}1}$.

• Received 20 April 1992

DOI:https://doi.org/10.1103/PhysRevB.45.13841