INCOHERENT QUASI-ELASTIC 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-o rdered monoclinic phase. Incoherent quasielastic neutron scattering (I QNS) experiments were performed in order to investigate the reorientat ional dynamics of the TMA in both phases. The IQNS profiles, obtained on a medium resolution time-of-flight instrument (approximate to 110 m u eV) for the hexagonal phase and on a higher resolution spectrometer (approximate to 33 mu eV) for the monoclinic phase, were analyzed in t he 305-145 and 100-60 K temperature ranges, respectively. A model has been proposed in which the TMA cations are in instantaneous general or ientation so that each methyl group can probe 24 equivalent sites in t he hexagonal phase. This model involves three basic reorientational mo tions of the cation. The first one is a precession of TMA around the 3 -fold crystallographic axis, which slows down very quickly with decrea sing temperature and can be no longer resolved below 200 K. The second one corresponds to a rotation of the cation about the 3-fold crystall ographic axis. In the hexagonal phase, the third motion (flip), which is connected to the crystallographic mirror plane, is by far the faste r one (characteristic time around 1 ps at 300 K). In the monoclinic ph ase, the thermal evolution of the IQNS profiles is essentially interpr eted by a progressive freezing of the flip motion between two inequiva lent orientations of the TMA (loss of the crystallographic mirror plan e) as the temperature decreases. These results are then discussed in t he context of the mechanism of phase transitions occurring in TMMC.