Structure and Correlations in Smectic B, F and I Phases

Abstract

The question of which of the known smetic phases exhibitis long range 3 dimensional order is first discussed briefly, The major theme of the paper is the nature of the structure and correlations in S_B, S_F, and S₁ phases. Results are presented about the temperture dependence of the interlayer stacking arrangement and the existence of strong layer undulations in crystalline S_B phases and about the existence of a S_B phase with very weak positional correlations between layers and short range order within the layers.

The raltion between the S₁ and S_F structures is established for the first time: both consist of uncorrelated layers in which the molecules are tilted relative to the layer normal and have limited positional correlations within layers. Both have 3-d long range bond orientational order of the C-centered monoclinic lattices. They differ in the direction of tilt of the molecules relative to the pseudo-hexagonal packing in the plane normal to the long axes: in S_F the tilet is directed towards an edge of the hexagon, while in S₁ it is directed towards an apex. These results enable previously described phases to be classified (or reclassified) as S₁ phases and emphasise the existence of more ordered phases with similar tilt to the S₁ phase but otherwise analagous to S_G and S_H—we call these S_G and S_H. Finally a preliminary repor is given of X-ray structural work which shows that the recently postulated S₁ and S_K phases do not exist.