Two classification systems for smectic phases, based on symmetry and order

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Abstract
On the basis of evidence in the literature, it is concluded that the basic molecular packing underlying all smectic structures is a herringbone arrangement of the molecular planes with an orthogonal lattice of the molecular centers. Starting from this basic packing, two sets of five orthorhombic and three monoclinic smectic layer structures are derived which can all be classified with crystallographic space-group symbols. One set is for centrosymmetric ’’molecules,’’ the other for noncentrosymmetric ’’molecules.’’ A second classification of the same set of structures is developed on the basis of the symmetry of the molecular positions. This classification is equally specific but more easily visualized, and it leads to simpler classification symbols. With an additional symbol to indicate order or disorder in the stacking of the smectic layers, a set of 14 possible smectic structures is obtained (each structure allowing centrosymmetric as well as noncentrosymmetric ’’molecules’’), and all known layered smectic phase types are classified within this system, each phase type assigned to a different structure. The structure of the smectic B phase appears to be different than usually assumed, and the so far not understood smectic F phase has been assigned a specific structure.