Crystal smectic phases

Smectic phases are more highly ordered than nematic phases, and with an ordering of the molecules into layers. There are a number of different smectic phases which reflect differing degree of ordering. Crystal smectic phases are characterised by the appearance of inter-layer structural correlations and may in some cases be accompanied by a loss of molecular rotational freedom. 

The crystal smectic phases differ from those already discussed in that the layers are rather well correlated. The crystal smectic E phase is an orthogonal, orthorhombic phase. 

The final group of lamellar phases to be introduced here are the so-called crystal smectic phases. These are more ordered than the previous smectic examples and are characterized by the appearance of interlayer correlations and, in some instances, by the removal of freedom of molecular rotation. Consequently the (crystal) B, G and J phases are derived from the Sb, Sf and S] phases, respectively, but with the presence of interlayer correlations. Further the (crystal) E, H and K phases are B, G and J phases which have lost rotational freedom. These phases are still disordered and hence, still intermediate between the solid and liquid states. 

While all of the previously described phases are genuine liquid-crystalline phases, there are other phases in which there is long-range positional order. They differ from true crystal phases in one important aspect the molecules in them have the freedom of (at least) rotation around their long axis their thermal motion is not completely frozen out. These phases are labelled crystal smectic phases (denoted by the letters B, E, G, H, J, K). 

The true liquid-crystal mesophases of calamitic mesogens are divided into two classes—the nematic and smectic phases and these will be described in turn. There is, in addition, a series of crystal smectic phases that are not really liquid-crystal phases but which for many years were classified as such these will be mentioned briefly below. 

Rotationally (3-D) disordered crystals that may be derived from globular molecules leading to isotropic phases. This classification does not apply to crystal smectic phases composed of elongated molecules, although these could be described as anisotropic plastic crystals. 

The symmetry arguments for achiral or chiral smectic C phases can also be applied in similar ways to the other smectic and soft crystal smectic phases. For example, all of the tilted phases (smectics I and F, and crystal phases J, G, H, and K ) would have broken symmetries leading to polar noncentrosymmetric structures resulting in ferroelectric properties. Even orthogonal phases, such as the smectic A phase would have different symmetries for the chiral versus the achiral forms. For instance, the smectic A phase has symmetry, where-... 

Table 1. Symmetries of common liquid crystal phase types only those phases with well-established phase structures are included. Crystal smectic phases, including the cubic D phase have been omitted, as have recently discovered twist grain boundary phases, and structurally modulated variants of smectic phases.

However in chiral tilted liquid crystal smectic phases, the polarization is driven by the tilt angle, and the phase transition will not necessarily be of second order. 

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