Phonon damping by translation-rotation coupling in orientationally disordered molecular crystals

A simple model for the translation-rotation coupling in orientationally disordered molecular crystals (ODIC) is used to investigate the effect of the coupling to the damping of acoustic phonons in an fcc-ODIC composed of tetrahedral molecules. The influence of the coupling is calculated as a function of molecular and lattice parameters in the framework of linear response theory. The results are applied to the plastic phase I of solid CD₄, revealing a strong damping of short-wavelength phonons. This is consistent with experimental and molecular dynamics observations.