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

Reorientational motion of pyridinium cation in pyridinium iodide across the phase transition (T-c = 247 K) temperature is studied using a high-resolut ion quasi-elastic neutron scattering (QENS) technique. A clear evidence of the order-disorder transition with respect to the pyridinium ion reorientat ion is observed. In the high-temperature phase the pyridinium ion is found to undergo a sixfold jump rotation around the axis perpendicular to its pla ne. Weak quasielastic broadening is observed below the phase transition tem perature, at T = 240 K, indicating presence of residual disorder. Reorienta tional time and activation energy of rotation are also obtained in the high -temperature phase. (C) 2001 Elsevier Science B.V. All rights reserved.