Optical and x-ray evidence of the "de Vries" Sm-A*-Sm-C* transition in a non-layer-shrinkage ferroelectric liquid crystal with very weak interlayer tilt correlation.
TL;DR: A non-layer-shrinkage fluorinated ferroelectric liquid crystal compound, 8422[2F3], has been characterized by means of optical, x-ray, and calorimetric methods and there is strong evidence that the second-order Sm-A*-Sm-C* transition is well described by the diffuse cone model of de Vries.
Abstract: A non-layer-shrinkage fluorinated ferroelectric liquid crystal compound, 8422[2F3], has been characterized by means of optical, x-ray, and calorimetric methods. The orientational distribution within macroscopic volumes, determined through wide-angle x-ray scattering and birefringence measurements, was found to be identical in the $\mathrm{Sm}\ensuremath{-}{A}^{*}$ and helical $\mathrm{Sm}\ensuremath{-}{C}^{*}$ phases. Together with the absence of layer shrinkage, this constitutes strong evidence that the second-order $\mathrm{Sm}\ensuremath{-}{A}^{*}--\mathrm{Sm}\ensuremath{-}{C}^{*}$ transition in this material is well described by the diffuse cone model of de Vries. The absolute values of the layer spacing show that the molecules aggregate to antiparallel pairs. The molecular interaction across the layer boundaries will then occur only between fluorine atoms, leading to unusually weak interlayer tilt direction correlation. This explains the experimental observations of a very easily disturbed $\mathrm{Sm}\ensuremath{-}{C}^{*}$ helix and a peculiar surface-stabilized texture. Tilt angle and birefringence values as a function of field and temperature have been evaluated in the $\mathrm{Sm}\ensuremath{-}{A}^{*}$ and $\mathrm{Sm}\ensuremath{-}{C}^{*}$ phases and the results corroborate the conclusions from the x-ray investigations.
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TL;DR: In this paper, Kapernaum et al. showed that azobenzene is indeed an SmC-promoting group in ILCs and showed that the aromaticity of the imidazolium head group plays an important role in the formation of smectic C phases.
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TL;DR: In this article, the characteristics of various phases of antiferroelectric liquid crystal and their electrical and electro-optical parameters are discussed, and the advantages of these phases are discussed.
Abstract: Anti-ferroelectric liquid crystal structure was proposed in 1989 on the basis of tristable switching behavior of a new chiral smectic C phase (now abbreviated as SmC * a phase). Materials showing SmC * a phase along with several other phases have now been termed as Antiferroelectric Liquid Crystals (AFLCs). The prospects are high for AFLC s application in displays with outstanding performance as compared to conventional nematic and other displays. Some of the attractive features of AFLCs are microsecond response, intrinsic dc compensation, video speed capability, gray scale and very wide viewing angle, which make them promising for future displays. Some issues hindering the commercialization of AFLCs are being solved at the level of the synthesis and/or formulation of mixtures. Beside technological applications, AFLCs are interesting for the basic studies in soft condensed matter fi eld as these materials are showing numerous new sub-phases, viz. SmC * α, SmC * β, SmC * γ and many others with distinct macroscopic properties. In the present chapter, we will discuss the characteristic structure of various phases of AFLCs and their electrical and electro-optical parameters.
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TL;DR: In this article, it has been shown that the de-Vries electroclinic liquid crystals, with a wide smectic A (Sm-A) phase and a high electro-clinic effect, do not obey the Curie-Weiss law.
Abstract: The Curie–Weiss law is obeyed where there is a change in the order parameter and the symmetry of molecules at the transition temperature (Tc) of ferro-to-para phase. The ferroelectric transition is not strictly governed by the Curie–Weiss law. In fact, a number of solid ferroelectrics show deviation from the Curie–Weiss law. It has been shown in the present letter that the de-Vries electroclinic liquid crystals, with a wide smectic A (Sm-A) phase and a high electroclinic effect, do not obey the Curie–Weiss law. The nonlayer shrinkage leads to symmetry conservation that makes the observance of soft mode impossible and hence the deviation.
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TL;DR: In this paper , the double-fluorinated glassformer 3F7FPhF has been investigated by several experimental methods (differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction, electro-optic measurements, dielectric and infra-red spectroscopy) complemented with quantum mechanical density functional theory (DFT) and semi-empirical calculations.
11 citations
TL;DR: Four of the authors' five mesogens turned out to show de’Vries-like behavior with a layer shrinkage that is far less than that expected for conventional materials, and can be considered as the first de Vries-type materials among ionic liquid crystals.
Abstract: In ionic liquid crystals the orthogonal smectic A phase is the most common phase while the tilted smectic C phase is rather rare. We now present a new study with five novel ionic liquid crystals exhibiting both a smectic A as well as the rare smectic C phase. Two of them have a phenylpyrimidine core while the other three are imidazolium azobenzenes. Their phase sequences and tilt angles were studied by polarizing microscopy and their temperature dependent layer spacing as well as their translational and orientational order parameters were studied by X-ray diffraction. The X-ray tilt angles derived from the X-ray layer contraction and the optical tilt angles of the five ionic liquid crystals were compared to obtain their de Vries character. Four of our five mesogens turned out to show de Vries-like behavior with a layer shrinkage that is far less than what has to be expected for conventional materials. These materials can thus be considered as the first de Vries-type materials among ionic liquid crystals.
11 citations