Local order and lattice dynamics in the ordered phases of T.B.B.A. (Terephthal-Bis-Butyl-Aniline)

2014 This paper reviews the main results on the structure and the dynamics of the ordered smectic phases of T.B.B.A. The study of lattice dynamics by means of neutron coherent inelastic scattering bears out the X-ray measurements : the smectic Bc and Ec phases show a conventional three-dimensional order, however the molecules may perform motions with large amplitude, this makes the observation of collective modes rather difficult. Moreover we have restarted with neutron techniques the work done with X-rays on the local herring-bone order in the smectic Bc phase of T.B.B.A. Because of the energy analysis, the neutron measurements give better measure of the sizes of different types of domains. It seems that the molecule tilt angle induces only a little anisotropy in a given domain, and thus it is the coupling between smectic layers which favours the growth of a preferential domain near the transition SmBc ~ SmEc. Moreover a lifetime of the domains has been measured, and appears to be slightly greater than the relaxation time for individual rotational motions of the molecules. J. Physique 42 (1981) 1651-1663 DÉCEMBRE 1981, Classification Physics Abstracts 61.10 61.30 63.20 64.70E The ordered smectic phases can be classified in four families ; all these phases exhibit a mean three-dimensional order, but fluctuations around the equilibrium positions located on the crystalline lattice sites are large compared with those existing in an usual crystalline phase. The four phases BA, Bc, EA, Ec (1) are made of periodically stacked layers ; they differ from one another by the kind of order inside a layer and by the possible tilt angle of the molecular axis with respect to the normal to the layers. T.B.B.A. undergoes at 113.5 OC a transition from a stable crystalline phase to a smectic Bc phase. The (+) Associé au C.N.R.S. (L.A. no 2). (’) These phases are often called B instead of BA, H or G instead ofBc and E instead of EA. molecules, the tilt angle of which is about 30o, are arranged on a quasi hexagonal lattice. The Bc phase remains in a super-cooled state and at 84 OC it turns into a smectic Ec phase where the order inside a layer is different. Studies performed on the structure and on the dynamics of both smectic phases, have shown the following results : the X-ray diffraction measurements [1], yield information about the mean structure and the short range order, but cannot separate elastic from inelastic