Reorientational dynamics of aza-cyclohexadienyl radicals in pyridinium tetrafluoroborate

Pyridinium tetrafluoroborate is a ferroelectric with a paraelectric-ferroel ectric phase transition of second order, which is exceptional for multidire ctional ferroelectrics, Avoided-level-crossing muon-spin resonance has been used to show that the Mu substituted aza-cyclohexadienyl radical derived f rom the pyridinium ions undergoes fast uniaxial rotation around the axis pe rpendicular to the molecular plane, confirming earlier results by Czarnecki et al. The changes in reorientational dynamics between 239 and 244 K also verify the existence of a phase transition in this range, as found by Czarn ecki. Furthermore, the results give strong evidence that superimposed on th e rotation there is a wobbling motion of the rotational axis which freezes in at the first transition temperature, T-1 = 238.7 K. (C) 2000 Elsevier Sc ience B.V. All rights reserved.