Lattice dynamics of the molecular crystals KCN and NaCN in their disordered cubic phase

A model for the lattice dynamics of the disordered cubic high temperature phase of KCN and NaCN is proposed using a shell model for the translationnal motion, while the librational degrees of freedom of the CN − ‐ddumbbells are taken into account in the approximation of rigid molecular ions. The calculted dispersion curves are in adequate agreement with the experimental data from inelastic neutron scattring especially for the TA‐branches, which show an anomalous dispersion.