SMECTIC LAYERING IN POLYPHILIC LIQUID-CRYSTALS - X-RAY-DIFFRACTION AND INFRARED DICHROISM STUDY

We present X-ray diffraction measurements of lamellar ordering for a n ew class of mesogens - polyphilic compounds - which were reported to f orm achiral ferroelectrics. Two phases called smectic X and X' manifes t polar properties. Analysis of the scattering profiles parallel and p erpendicular to the smectic layers provides detailed data on the struc ture of smectic A, smectic X and X' phases of these compounds. Smectic X' phase corresponds to a strongly defective layered structure of the smectic C type with longitudinal correlation length xi(parallel-to) l ess-than-or-equal-to 200 angstrom. The molecules are in a zigzag confo rmation and are tilted with respect to the layer normals. Infra-red di chroism data show that the biphenyl moiety and the polyfluorinated cha in of molecules form angles of 56-degrees and 26-degrees with respect to the layer normals. In the smectic X phase, three types of layering coexist : (i) polar layers with tilted molecules, (ii) a modulated str ucture of the same type consisting of layers periodically shifted with respect to each other and (iii) the orthogonal smectic A layers with a spacing of 41 angstrom incommensurate with period 35 angstrom of the polar smectic layers. The coexistence of such incommensurate structur es is only possible in a strongly disordered medium. Infra-red dichroi sm data are consistent with this picture. The in-plane structure facto r in smectic X, X' phases reveals two features of polyphilic compounds : a dramatic increase in the short-range positional correlations (xi( perpendicular-to) almost-equal-to 35 divided-by 40 angstrom or approxi mately 7 or 8 molecules in the local in plane order) and splitting of the intensity profiles into three peaks. The latter corresponds to the appearance of nearest-neighbour molecular stacking at different dista nces. The formation of polar, either uniform or modulated layered stru ctures in the system of polyphilic molecules is discussed.