PHONONS IN ORIENTATIONALLY DISORDERED NEOPENTANE C(CD3)4

The phonons of deuteriated neopentane C(CD3)4 single crystals in the o rientationally 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 si tting on an intense background. No librational mode was detected. Sele ction rule violations, presumably linked to disorder, were observed. T he elastic constants are discussed in terms of rotational-translationa l coupling, and inelastic scattering results are compared with the mol ecular center of mass translational disorder as obtained from diffract ion experiments providing a confirmation of a previous interpretation of these experiments.