We report the results of inelastic neutron-scattering measurements on ${\mathrm{Rb}}_{2.6}$${\mathrm{K}}_{0.4}$${\mathrm{C}}_{60}$. Librational modes were observed as broad peaks with maxima between 4.1 and 4.7 meV, as the temperature is lowered from 300 to 12 K. As in ${\mathrm{K}}_3$${\mathrm{C}}_{60}$, no change in the width or position of the librational peak was observed when the sample was cooled through the superconducting transition. Thus any coupling of the librations to electronic states is small. The magnitude of the orientational potential barrier was estimated from the librational peak frequency. A flat background observed in the low-energy inelastic-scattering spectra is ascribed to two-phonon scattering. The density of states of intramolecular modes is similar to that of previously studied ${\mathit{M}}_3$${\mathrm{C}}_{60}$ compounds; modes at 53 and 66 meV in pure ${\mathrm{C}}_{60}$ are not observed in ${\mathrm{Rb}}_{2.6}$${\mathrm{K}}_{0.4}$${\mathrm{C}}_{60}$.

• Received 7 September 1993

DOI:https://doi.org/10.1103/PhysRevB.49.1005