Order-disorder transition in pyridinium iodide: QENS study

Reorientational motion of the pyridinium cation in pyridinium iodide as studied by the high-resolution, quasi-elastic neutron scattering (QENS) technique both above and below the order–disorder transition (Tc=247 K) temperature are reported. The pyridinium ion is found to undergo reorientational motion around the axis perpendicular to its plane and is best described by a 6-fold jump rotation model in the high-temperature phase. Below the phase transition temperature, in the ordered phase, some residual disorder is observed. Reorientational time and activation energy of rotation are also obtained in the high-temperature phase.