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We present inelastic neutron-scattering results for AC 60 (AϭK,Rb,Cs͒ compounds. The spectra of the high-temperature fcc phases strongly resemble the ones of pristine C 60 in the plastic phase. At equal temperatures we find identical rotational diffusion constants for pristine C 60 and Rb 1 C 60 (D r ϭ2.4 10 10 s Ϫ1 at 400 K͒. The changes taking place in the inelastic part of the spectra on cooling AC 60 indicate the formation of strong intermolecular bonds. The buildup of intensities in the gap region separating internal and external vibrations in pure C 60 is the most prominent signature of this transition. The spectra of the low-temperature phases depend on their thermal history. The differences can be explained by the formation of a polymer phase ͑upon slow cooling from the fcc phase͒ and a dimer phase ͑upon fast cooling͒, respectively. The experimental data are analyzed on the basis of lattice dynamical calculations. The density-of-states are well modeled assuming a ͓2ϩ2͔ bond for the polymer and a single intercage bond for the dimer. Indications for different intercage bonding are also found in the internal mode spectra, which, on the other hand, react only weakly to the charge transfer. The dimer phase is metastable and converts into the polymer phase with a strongly temperaturedependent time constant. The transition from the polymer to the fcc phase is accompanied by inelastic precursor effects which are interpreted as the signature of inhomogeneities arising from plastic monomer regions embedded in the polymer phase. In the polymer phase AC 60 compounds show strong anharmonic behavior in the low-temperature region. The possible connection with the metal-to-insulator transition is discussed.

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