Microscopic dynamics of A₁C₆₀ compounds

Abstract

We present the dynamics of A₁C₆₀ compounds as investigated by inelastic neutron scattering using Rb₁C₆₀ as example. Special emphasis is given to the influence of disorder on the dynamical response in the various phases. In the high-temperature fcc phases we find hardly any differences in the dynamics between Rb₁C₆₀ compounds and pristine C₆₀, i.e. we observe the typical signatures of a plastic crystal phase. In particular, it turns out that at equal temperatures the rotational diffusion constants for pristine C₆₀ and Rb₁C₆₀ (D_r=2.4 10¹⁰ s⁻¹ at 400 K) do not differ within the experimental errors. Upon cooling we witness strong changes in the inelastic spectra indicative of intra-molecular bond formation. The build-up of intensities in the gap region separating internal and external vibrations in pure C₆₀ is the most striking of these changes. The details of the spectra in the low-temperature phases are compatible with the formation of linear polymers (upon slow cooling from the fcc phase) and dimers (upon fast cooling), respectively. Using lattice dynamical model calculations reliable information about the restoring forces is obtained. We find that the inter-cage bonds are weaker in the dimer than in the polymer. The same holds for the cage deformations. The conversion of the metastable dimer phase into the polymer phase is followed in real time and turns out rather complex. The transition between the polymer and fcc phases is accompanied by inelastic precursor effects which show a strong hysteresis. These precursors can be identified with fast reorientational movements of monomers. There is strong evidence that the polymer phase is heterogeneous at elevated temperature, containing monomer regions, the amount of which is time and temperature dependent and can be determined quantitatively from the inelastic spectra.