Order–disorder transition in pyridinium iodide: quasi-elastic neutron scattering study

Reorientational motion of pyridinium cation in pyridinium iodide across the phase transition (Tc=247 K) temperature is studied using a high-resolution quasi-elastic neutron scattering (QENS) technique. A clear evidence of the order–disorder transition with respect to the pyridinium ion reorientation is observed. In the high-temperature phase the pyridinium ion is found to undergo a sixfold jump rotation around the axis perpendicular to its plane. Weak quasi-elastic broadening is observed below the phase transition temperature, at T=240 K, indicating presence of residual disorder. Reorientational time and activation energy of rotation are also obtained in the high-temperature phase.