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Stabilization of the smectic-c-alpha* phase in mixtures with chiral dopants

Chang, H.S. and Jaradat, S. and Gleeson, H.F. and Dierking, I. and Osipov, M.A. (2009) Stabilization of the smectic-c-alpha* phase in mixtures with chiral dopants. Physical Review E, 79 (6). ISSN 1539-3755

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A series of mixtures comprising an antiferroelectric liquid-crystal host and a chiral dopant is described in which the layer spacing variation at the orthogonal smectic-A* (SmA*) to tilted smectic-C* or smectic-C-alpha* (SmC* or SmC alpha*) phase transition changes from the usual strong contraction in the pure system to one in which there is almost no layer spacing change observed across the transition for dopant concentrations of 7%. The nature of the orthogonal to tilted phase transition is examined using Raman spectroscopy, to determine the order parameters < P-2 > and < P-4 > in the SmA* phase, and via a generalized Landau expansion to reveal the details of the phase transition itself. The results show that the value of < P-2 > at the orthogonal to tilted transition increases from around 0.6 to 0.7 as the dopant concentration increases, while < P-4 > remains constant at approximately 0.4 irrespective of dopant concentration. Further, the generalized Landau potential measurements prove that the transition is purely second order, while electro-optic measurements confirm that the tilt angle at the transition becomes smaller with increasing dopant concentration. The combined data show that the high-temperature tilted phase regime corresponds to a SmC alpha* phase rather than the mechanism suggested by de Vries that is inferred by the layer spacing data alone. We demonstrate that the lower-temperature SmC alpha*-SmC* phase transition is of first order. Further, the temperature range of the SmC alpha* phase increases dramatically with concentration, from around 2 K in the pure system to around 21 K in the 8% doped mixture, showing that the chiral dopant plays a role in stabilizing this phase. Indeed, we particularly note that for the 8% doped mixture all other SmC*-like phases disappear and that the only tilted phase remaining is SmC alpha*. This implies that we are reporting a liquid-crystalline phase sequence, namely, cryst.-SmC alpha*-SmA*-iso., i.e., a direct transition between the SmC alpha* phase and the crystalline phase.