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Abstract
Phonon density of states g(ω) in librational and lattice modes of trimethylammonium molecule are studied across the rotational phase transition temperature (Tc = 308 K) of (CH3)3NHCl by incoherent inelastic neutron scattering using a time of flight instrument, AGNES at JAERI. The librational modes show remarkable softenings across the phase transition with increasing temperature. An extra boson like peak at around 2 meV appears in the ratio of the g(ω) above the Tc divided by that below the Tc in the region of the lattice phonon. This might be due to a coupling between the phonon and the molecular rotor. The drastic change of the g(ω) is also simulated by a molecular dynamics calculation.