title: Orientational fluctuations near the smectic A to smectic C phase transition in two "de Vries"-type liquid crystals.
abstract: On the basis of thorough analysis of 2D X-ray diffraction patterns from smectic monodomains, we examine the influence of orientational fluctuations on the weakly first-order smectic A (SmA) to smectic C (SmC) transitions in two nonchiral organosiloxane "de Vries"-type liquid crystals. We find that these materials exhibit very large molecular tilt fluctuations with magnitudes of up to 35°--thus larger than the average tilt itself. This is essential to understand the underlying molecular mechanism behind the practical absence of smectic layer contraction in these materials: in the SmA phase, the nematic order parameter is very low (molecular fluctuations correspondingly high), and the expected layer shrinkage at the SmA to SmC transition is almost fully compensated by the increase in orientational order, as the fluctuations diminish with decreasing temperature. In addition to the general tilt fluctuations, we observe intrinsic soft-mode fluctuations. They have a λ-shaped temperature dependence that peaks at the SmA-SmC transition with a maximum amplitude of about 2°.
authors.affiliation: Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart (Germany).
authors.forename: Dorothee
authors.initials: D
authors.lastname: Nonnenmacher
authors.forename: Stefan
authors.initials: S
authors.lastname: Jagiella
authors.forename: Qingxiang
authors.initials: Q
authors.lastname: Song
authors.forename: Robert P
authors.initials: RP
authors.lastname: Lemieux
authors.forename: Frank
authors.initials: F
authors.lastname: Giesselmann
keywords: X-ray diffraction; liquid crystals; orientational fluctuations; phase transitions; statistical thermodynamics