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 disubstituted and fully deuterated diacetylene 2,4-hexadiynylene bis(p-toluenesulfonate) (pTS-D) in the monocrystalline monomer state. The aim of this study is to make a comparison with the dynamics of the same monocrystal in the polymer state [Phys. Rev. B 49, 11 602 (1994)]. The mechanism leading to the incommensurate phase of pTS-D monomer is also discussed. The experimental study shows an overdamped soft mode above ${\mathit{T}}_{\mathit{i}}$. Its evolutions as a function of temperature and its dispersion are described. A central peak related to the presence of defects is also observed. The soft mode presents roughly the same mean-field behavior for pTS-D monomer and pTS-D polymer. pTS-D mixed monomer-polymer crystals are then probably very good prototypes for the study of the influence of pure random field on a structural phase transition.