Theory of phonon linewidth in orientationally disordered crystals

The dynamics of coupled translational (T) and rotational (R) modes is studied with special reference to the experimental situation in the alkali cyanides. The dynamic equations are extended and, in addition to the bilinear T-R coupling, anharmonic interactions of the form T-T-R are considered. These interactions lead to additional friction coefficients for translational and rotational motion. Transport coefficients are evaluated with mode-mode coupling theory. The validity of orientational relaxation models coupled to lattice vibration is confirmed as a particular case of the present theory at low frequencies. An extension to the high frequency regime is proposed.Dedicated to Prof. Dr. H.E. Müser ob the occasion of his 60th birthday