Smectic crystals

In the crystalline version of smectic B, there is positional as well as bond-orientational order, and this ordering is called hexagonal, as opposed to hexatic. In the tilted version of the hexatic smectic B, there are two different directions the tilt has been found to take with respect to the hexagonal bond orientation, namely toward nearest and toward next nearest neighbors, producing the two different smectics I and F (see Fig. 10-27). Similarly, there are two different tilted crystalline hexagonal phases, smectics J and G. Even in smectic crystals, the molecules retain freedom to rotate cooperatively about theii axes, hence smectic ciystals are not as solid as most simple crystals. Nature is wonderfully creative in contriving... 

FIGURE 5.2 Liquid crystals, (a) Smectic crystals the ends of the molecules are on a plane, (b) Nematic crystals the ends of the molecules do not match, (c) Cholesteric crystals the molecules in each layer are arranged in a manner similar to nematic crystals, but the angle changes from plane to plane of the molecules, forming a helix of pitch length p. 

Since the first discovery of the liquid crystalline phase over one hundred years ago, the classification of the distinct liquid crystalline phases in small-molecule liquid crystals has been well established (7,2). As shown in Figure 1, the least ordered liquid crystalline phase is the nematic phase that only possesses molecular orientational order due to the anisotropy of the molecular geometric shape. The next ordering level introduced is the layer structure in addition to the molecular orientation to lorm a smectic A (S/J or a smectic C (Sc) phase. Following the phase the hexatic B (Ho), smectic crystm B (So) and smectic crystal E (S ) phases are observed. In this series the long axis of the molecules is oriented perpendicular to the layer surface while order is increasingly developed from positional order normal to the layer in bond... 

Soft smectic crystal state Long-range translational order Rotational disorder... 

Typical temperature dependences of the principal refractive indices for three nematic liquid crystals are given in Fig. 2.11 (information on the refractive indices of smectic crystals is given in [25]). 

In smectic crystals the ratio is noticeably greater than one (e.g.,... 

Heating a thermotropic liquid crystal results in decreasing the molecular order. The general pattern is as follows below, but not all possible phases may appear, and there are many types of smectic crystals. In addition, the LC phase may appear upon cooling rather than upon heating. 

Cr=crystalline Sm=smectic CrSmB = smectic crystal B N=nematic Ch = cholesteric I = isotropic. 

These two phases are equivalent to the smectic crystal E phase except for the molecules being tilted with respect to the layer... 

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