The Raman spectra of pristine ${\mathrm{C}}_{70}$ films and ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{70}$ doped to the maximum-conductivity phase (nominally ${\mathrm{K}}_4$${\mathrm{C}}_{70}$) are studied. All the Raman modes for ${\mathrm{C}}_{70}$ are only moderately broadened upon K doping, distinctively different from the superconducting ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ where several ${\mathit{H}}_{\mathit{g}}$ modes are broadened to the extent that they have almost disappeared from the spectrum. Assuming that the electron-phonon interaction is responsible for the broadening, the electron-phonon coupling strength is found to be comparable to the Coulomb repulsion. These results are important for understanding the absence of superconductivity in ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{70}$.

• Received 16 August 1993

DOI:https://doi.org/10.1103/PhysRevB.48.16881