Structure in the Hexatic B Phase

The structure [3,4,23,39,40] of the hexatic B phase is relatively close to that of the smectic A phase, in that the molecules are arranged in layers so that their long axes are orthogonal to the layered planes. Loeally, the molecules are essentially hexagonally close packed and are undergoing rapid reor-ientational motion about their long axes on a similar time scale to the smectic A phase. 

Long range bond orientational order simply means that when we consider molecules which are arranged in a hexagonal close packed domain in one part of a bulk sample, and then move a long distance away from that particular area of the specimen, we will find a similar hexagonally close packed domain that has the same orientation, however, there will be no relationship between the positional ordering of the two domains (see Fig. 7). 

Between the layers there is no correlation of the molecules, thus, out of the plane the order is only short range. The hexatic B phase is, therefore, easily distinguished from the crystal B phase, where the molecules have long range periodic order in three dimensions. 

When the hexatic B phase is formed from the smectic A phase on cooling it does so via a first order phase transition. This phase transition however is relatively weak and sometimes approaches becoming second order. 

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