2H NMR study of dynamics, ordering, and phase transitions in ferroelectric pyridinium tetrafluoroborate

Dynamic H-2 NMR spectroscopy employing line shape studies and spin-lattice relaxation experiments is used to investigate the molecular dynamics of the perdeuterated pyridinium cations in pyridinium tetrafluoroborate in the temperature range between 120 and 290 K. Special interest lies in the two solid-solid phase transitions occurring at 204 K and at 238.7 K. The experimental spectra show that the pyridinium cations perform fast rotation around the pseudo C6 axis at high temperatures. Decreasing the temperature leads to successive slowing down of the rotational motion resulting in nearly completely immobile and preferentially oriented pyridinium cations at 120 K. Simulating the spectra with a 3-site or a 6-site jump model shows that both are appropriate to describe the experimental NMR data. The models yield population probabilities which permit the calculation of the orientational contribution to the polarization and the enthalpy change. For both models distinct deviations from the macroscopic properties are found. We therefore suggest that, in addition to the cation orientation, the classical ferroelectric mechanism operates which has a partly compensating effect. A displacement of anion vs cation sublattice by 0.23-0.25 Angstrom is sufficient to account for the missing polarization contribution. Both these ferroelectric mechanisms are potentially continuous in their onset, which is compatible with second-order transitions, but it appears that they may both be borderline cases between second and first order.