ROTATIONAL-DYNAMICS AND VELOCITY SEGREGATION IN PLASTIC KCLO4 - A MOLECULAR-DYNAMICS SIMULATION

The individual and two-molecule rotational dynamics of ClO4 ions are s tudied via a molecular dynamics simulation of a potassium perchlorate crystal in the orientationally disordered phase. In order to overcome the difficulty of dealing with orientational variables, symmetry adapt ed rotator functions are used. The rotational matrix makes it possible to compute instantaneous rotational velocities and axes. Finally, an effective rotational angle is introduced also. The results obtained wi th these tools are consistent. The rotational motion is diffusive, but the relative orientations of two neighbouring molecules last longer t han that of a single molecule, showing coupled motion. Molecules which are far from the most probable orientations have greater rotational v elocity. A tentative interpretation is proposed.