The problem of orientational order in tilted smectic phases : a high resolution neutron quasi-elastic scattering study

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Abstract
High resolution neutron quasi-elastic scattering data in the supercooled smectic H, smectic VI and smectic VII phases of terephtal-bis-butyl-aniline — TBBA - are presented. The elastic incoherent structure factor — EISF — and the line shapes are analyzed in terms of models permitting orientational ordering around the long axis. The conclusions are : (i) In the H phase, the molecules rotate uniformly around their long axis, on a time scale of 10 -11 s, implying no orientational order of the dipoles. (ii) In the VI phase, the molecules probably reorient, on the same time scale, by jumps of π around the long axis, again implying no orientational order of the dipoles. (iii) In the VII phase, large amplitude rotational motions on a time scale of 10-11 s do not exist any more. From (i) and (ii), it is concluded that the tilted or non-tilted smectic phases should be characterized by a criterion other than the existence or non-existence of a strong intermolecular dipole-dipole interaction