Hexatic smectic B phase

2 Physical Properties of Non-Chiral Smectic Liquid Crystals 

The molecular bond direction is defined by an imaginary line between one pair of the nearest neighbour molecules [23]. This means that the measurement of bond-orientation correlation requires a four-point correlation function. To date no appropriate experimental tool allows us to conduct this kind of measurement. Thus the degree of bond-orientation correlation can only be inferred indirectly. Moreover, no available physical field exists which can couple directly to the hexatic order in the hexatic B phase. Consequently, the heat-capacity measurement is one of the most powerful experimental probes used to investigate the nature of the SmA-SmBhex transition and to complement structural identification by X-ray or electron diffraction. 

To test the theoretically predicted thermal properties [37,38] related to the two-dimensional liquid-hexatic transition critically, we have carefully characterized the hexatic B phase in compound 54COOBC [30] and found that it does not display herringbone order. To make a clear distinction between these two cases, we propose to use hexatic B to denote the phase found in 54COOBC and use SmB for the phase found in the nmOBC compounds which has a clear indication of some degree of herringbone order. 

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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 hexatic smectic B phase, the molecules tend to be perpendicular to the layers. Each layer is again a two-dimensional liquid, but locally the molecules are distributed on a triangular lattice. The number of defects is such that the positional order does not propagate over distances larger than a few hundreds of angstroms but the bond order extends over macroscopic distances (Fig. 2.9). A monodomain sample is optically uniaxial. 

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