NEUTRON-SCATTERING STUDY OF PRETRANSITIONAL DYNAMICS IN THE DEUTERATED DIACETYLENE MONOMER 2,4-HEXADIYNYLENE BIS(P-TOLUENESULFONATE)

Coherent neutron scattering is used to make a dynamical study of the d isubstituted and fully deuterated diacetylene 2,4-hexadiynylene bis(p- toluenesulfonate) (pTS-D) in the monocrystalline monomer state. The ai m of this study is to make a comparison with the dynamics of the same monocrystal in the polymer state [Phys. Rev. B 49, 11602 (1994)]. The mechanism leading to the incommensurate phase of pTS-D monomer is also discussed. The experimental study shows an overdamped soft mode above T-i. Its evolutions as a function of temperature and its dispersion a re described. A central peak related to the presence of defects is als o observed. The soft mode presents roughly the same mean-field behavio r for pTS-D monomer and pTS-D polymer. pTS-D mixed monomer-polymer cry stals are then probably very good prototypes for the study of the infl uence of pure random field on a structural phase transition.