Incoherent quasielastic neutron-scattering study of N(CH3)(4)MnCl3(TMMC)

Crystalline N(CH3)(4)MnCl3 (TMMC) exhibits a phase transition at T-c = 126 K from a high-temperature hexagonal phase, where the [N(CH3)(4)](+) (TMA) groups are orientationally disordered, to a low-temperature-ordered monoclinic phase. Incoherent quasielastic neutron scattering (IQNS) experiments were performed in order to investigate the reorientational dynamics of the TMA in both phases. The IQNS profiles, obtained on a medium resolution time-of-flight instrument (approximate to 110 mu eV) for the hexagonal phase and on a higher resolution spectrometer (approximate to 33 mu eV) for the monoclinic phase, were analyzed in the 305-145 and 100-60 K temperature ranges, respectively. A model has been proposed in which the TMA cations are in instantaneous general orientation so that each methyl group can probe 24 equivalent sites in the hexagonal phase. This model involves three basic reorientational motions of the cation. The first one is a precession of TMA around the 3-fold crystallographic axis, which slows down very quickly with decreasing temperature and can be no longer resolved below 200 K. The second one corresponds to a rotation of the cation about the 3-fold crystallographic axis. In the hexagonal phase, the third motion (flip), which is connected to the crystallographic mirror plane, is by far the faster one (characteristic time around 1 ps at 300 K). In the monoclinic phase, the thermal evolution of the IQNS profiles is essentially interpreted by a progressive freezing of the flip motion between two inequivalent orientations of the TMA (loss of the crystallographic mirror plane) as the temperature decreases. These results are then discussed in the context of the mechanism of phase transitions occurring in TMMC.