PHONONS IN ORIENTATIONALLY DISORDERED NEOPENTANE C(CD3)4

The phonons of deuteriated neopentane C(CD3)4 single crystals in the orientationally disordered phase were measured at T = 173 K by coherent inelastic neutron scattering, yielding very broad bands that spread, at a given energy, over a large portion of the Brillouin zone while sitting on an intense background. No librational mode was detected. Selection rule violations, presumably linked to disorder, were observed. The elastic constants are discussed in terms of rotational-translational coupling, and inelastic scattering results are compared with the molecular center of mass translational disorder as obtained from diffraction experiments providing a confirmation of a previous interpretation of these experiments.