High frequency dynamics of an orientationally disordered molecular crystal

The dynamic structure factor of the polycrystalline plastic crystal 1-cyanoadamantane has been measured in the THz frequency region by inelastic X-ray scattering as a function of the exchanged momentum Q in the range 1–10 nm−1 and as a function of the temperature in the two solid disordered phases: plastic crystal and glassy crystal. We find that the dispersion of the acoustic excitations is crystal-like in the two phases and that the attenuation Γ(Q) shows a negligible T dependence similarly to structural glasses and a Qα behavior with 1.15 < α < 1.50 at variance with structural glasses characterized by a Q2 dependence.