Inelastic neutron scattering by coupled rotational and translational modes in KCN

The TA [100] phonon branch of the molecular crystal KCN was studied by inelastic neutron scattering in the cubic phase. In addition the distribution of the quasielastic scattered neutrons was investigated. The results are analysed by assuming a coupling of the phonon modes to the rotational degrees of freedom of the CN-dumb-bells which are regarded as a system of interacting quadrupoles. A good description of the present results and also of the existing ultrasonic and Brillouin data is achieved by a simple model which uses a single collective rotational mode of finite excitation energy and line width.