Hexatic smectic mesophase

Note There are three types of hexatic smectic mesophases. smectic B (SmB), smectic F (SmF) and smectic I (SmI). Here, the term hexatic may be omitted because it is implicit for this group of smectic mesophases. 

Hexatic smectic mesophase in which the director is perpendicular to the layers with the long-range hexagonal bond-orientational order. 

Hexatic smectic mesophase the structure of which may be regarded as a C-centered monoclinic cell with a hexagonal packing of the molecules with the director tilted, with respect to the layer normals, towards the sides of the hexagons. 

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. 

The molecular packing in the sets of crystal smectic mesophases B, G, J and E, H, K, with director orientations similar to the hexatic SmB, SmF and SmI phases, respectively, is shown in Fig. 24. For two dimen-... 

SmA-C transitions, ferroelectric 481 SmA-hexatic smectic B transition 291 f SmA-SmC transition 65, 289 f, 326,478 SmA-TGBA-cholesteric, multicritical behavior 368 SmA-SmA critical point 302 SmAj/SmA fluctuations 384 SmC -SmA-TGB, multicritical behavior 368 SmC/C phases, flow/viscosity 470 f small angle neutron scattering (SANS) 684 smart pixels 810,814 smectic mesophase, difftision 590 smectic order parameters, XRD 646 smectic phases 17, 43, 60... 

Between crossed polars these defects appear as dark lines or brushes with curved or irregular shapes that correspond to extinction positions of the director and molecular long axes. Thus, the director can be either parallel or perpendicular to the polarizer and analyzer. The brushes tend to cover the specimen in rather a continuous way, indicating the liquid-like nature of the mesophase. The points where the brushes meet are called singularities in the texture (see Figure 3A). For nematic phases two forms of schlieren defect are found, one where two brushes meet at a point and one where four brushes meet. All tilted smectic phases (C, I, F, and ferrielectric C), except for the antiferroelectric phase, exhibit four brush singularities. Therefore, this provides a simple way of distinguishing between smectic and nematic phases. It should be noted that phases such as smectics A and B(hexatic) and crystal phases B(crystal), E, G, H, J, and K do not exhibit schlieren textures and so this narrows down the possibilities for phase identification. 

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