Types of Smectics

Within the family of untilted smectics, there is a hierarchy of phases with order ranging from that of smectic A, in which there is no positional order within each layer, to the crystalline smectics B and E, which have long-range positional order within each layer, namely hexagonal and orthorhombic for the B and E phases, respectively. This inplane order, represented by the dots within circles or ellipses of Fig. 10-27, propagates from layer to layer, producing fully three-dimensional crystalhne order. The family of tilted smectics includes similar ordered phases J, G, K, and H. 

Within a smectic layer, a state of order called hexatic can exist that is intermediate between the liquid order of the smectic A, and the crystalline order of crystalhne smectic B. This state of order characterizes the hexatic smectic-B phase. Flexatic order consists of long-range bond orientational order, but no long-range positional order. This is illustrated 

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 C2.2.4. Types of smectic phase. Here tire layer stacking (left) and in-plane ordering (right) are shown for each phase. Bond orientational order is indicated for tire hexB, SmI and SmF phases, i.e. long-range order of lattice vectors. However, tliere is no long-range translational order in tliese phases.

A lattice model of uniaxial smectics, formed by molecules with flexible tails, was recently suggested by Dowell [29]. It was shown that differences in the steric (hard-repulsive) packing of rigid cores and flexible tails - as a function of tail chain flexibility - can stabilize different types of smectic A phases. These results explain the fact that virtually all molecules that form smectic phases (with only a few exceptions [la, 4]) have one or more flexible tail chains. Furthermore, as the chain tails are shortened, the smectic phase disappears, replaced by the nematic phase (Fig. 1). 

Fig. 5. Nearest neighbour triplet dipole configurations on triangular lattice, which play an important role in stabilization of different types of smectic A packings...

In the smectic mesophases the molecules are oriented, as in a nematic mesophase, with their principal axis roughly parallel to the director, but they are also defining layers. These layers can be perpendicular to the director, as in the smectic A mesophase (SmA), or tilted, as in the smectic C (SmC). The SmA and SmC mesophases are the less ordered and more common smectic mesophases. Other less common types of smectic mesophases are known, which differ in the degree or kind of molecular ordering within and between the layers [2]. 

Note 1 There are several types of smectic mesophases, characterised by a variety of molecular arrangements within the layers. 

The question of predicting the type of smectic polymorph that a given mesogen... 

Table 3. LC smectic polymers with indefinite type of smectic mesophase...

Sigaud G, Hardouin F, Achard MF, Levelut AM (1981) A new type of smectic A phase with long range modulation in the layers. J Phys (Paris) 42 107-111... 

When used in conjunction with microscope observations, the X-ray diffraction spectra, examples of which are shown in Fig. 10, can usually be used to accurately identify the mesophase. Oriented and quenched samples can also provide information about these phases. Because there are several types of smectic mesophases, one must... 

Once the helical structure of the Sc phase is unwound, ferroelectricity is displayed (see Chapter 6 for the details). In recent years, many experimental studies have revealed that some liquid crystal compounds show new types of smectic phases with complex tilt and dipole order, such as the anti-ferroelectric smectic C phase, Sca phase, and the ferrielectric smectic C phase, Sc7 phase. For instance, in the Sca phase, the spontaneous polarization Ps is opposite for successive layers. It was found experimentally that the chiral So phase is in fact similar to the anti-ferroelectric Sca phase. 

Liquid-like layers with the molecules upright on the average (fig. 1.1.5(a)) negligible in-plane and interlayer positional correlations. Thus the structure may be described as an orientationally ordered fluid on which is superimposed a onedimensional density wave. A number of polymorphic types of smectic A have been discovered (see 5.6). 

The first evidence that more than one form of smectic A exists came from an observation of Sigaud et of a phase transition that was detected by calorimetry, but could not be observed optically. X-ray studies revealed that this was a transition between two forms of the A phase the higher temperature phase was characterized by a pair of refiexions corresponding to a layer spacing and the lower temperature one by two pairs of reflexions corresponding to fi / and 21 respectively. The former type of smectic A is called the monolayer (A ) phase and the latter the bilayer (Aj) phase. A third type which has a layer spacing intermediate between / and 21, has been identified and is called the partially bilayer (A ) phase. The structures of these phases are represented schematically in fig. 5.6.1. 

A number of different smectic states exist among the low molecular weight mesomorphic compounds. The molecules, for example, might be normal to the planes or tilted relative to the normal. The different states have been given various letter designations (Smectic A, B, C etc.) and deVries (2) has classified the x-ray patterns obtained from the various types of smectic (and also nematic) states. 

Both cholesteric and smectic mesophases are layered. In the former case, the periodicity arises from a natural twist to the director field, and in the latter, from a center-of-mass correlation in one dimension. There are many types of smectic phases distinguished by their symmetry and order. The set of field-induced phenomena is quite different for these two materials, owing primarily to the very different layer compressibility. That is, the cholesteric pitch can be unwound by an external field, whereas the smectic layering is typically too strong to be altered significantly. However, because of the common layered structure, there are also strong similarities. 

The smectic mesophase is more ordered than the nematic phase and furthermore, whereas only one nematic phase exists, the smectic phase exhibits polymorphism (see Figure 3. 2), i.e., there are many different types of smectic phases. As for the nematic phase, the smectic phases can be identified by optical polarising microscopy (see Chapter 9). In 1917, Grandjean was studying (by microscopy) a sample of smectic liquid crystal (later classified as smectic A) which showed stepped edges, indicating that the smectic phase was lamellar in natnre. The lamellar nature of smectic phase allows various combinations of molecular correlations both within the layers and between the layers , each of which constitutes a different type of smectic mesophase. 

Many questions now need answering, such as what type of stmctural units and combinations of stmctural units allow the generation of the nematic phase How do these differ from those required to generate smectic phases How do stmctural features vary from one type of smectic phase to another What stmctural features are responsible for the generation of a smectic phase where the constituent molecules are tilted (e.g., S ) ... 

In general, similar stractural features can be used to generate all types of smectic mesophase, although the phase is by far the most common smectic phase exhibited... 

In addition to the special nature of the chiral nematic phase, there are maity different types of smectic liquid ciystal phases (S, Sj and S ) and ciystal smectic mesophases... 

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. 

The smectic phase has at least two unique directions, the director and the layer normal. Eleven types of smectic mesophases have been identified, ranging from smectic A to smectic K. They differ in the arrangement of the mesogens within each individual layer. For example, in the smectic-A phase, these two directions are collinear (orientational order), whereas in the smectic-C phase the director could make an angle with the layer normal (positional order). In addition to orientational order and positional order which we have just mentioned, there is another order called bond orientation order. There are several smectic phases which contain this order and they are called hexatic smectics. 

---