Journal de Physique II

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DOI: 10.1051/jp2:1993187
J. Phys. II France 3 (1993) 1121-1139

Smectic layering in polyphilic liquid crystals : X-ray diffraction and infra-red dichroism study

L. M. Blinov¹, T. A. Lobko¹, B. I. Ostrovskii¹, S. N. Sulianov¹ and F. G. Tournilhac²

¹ Institute of Crystallography, Academy of Sciences of Russia, Leninsky pr. 59, Moscow, 117333, Russia
² ESPCI, CNRS-URA429, 10 rue Vauquelin, 75231 Paris Cedex 05, France

(Received 8 February 1992, accepted in final form 23 March 1993)

Abstract
We present X-ray diffraction measurements of lamellar ordering for a new class of mesogens - polyphilic compounds - which were reported to form achiral ferroelectrics. Two phases called smectic X and X ' manifest polar properties. Analysis of the scattering profiles parallel and perpendicular to the smectic layer provides detailed data on the structure 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 $\xi_{\parallel} \leqslant 200$ Å. The molecules are in a zigzag conformation and are tilted with respect to the layer normals. Infra-red dichroism data show that the biphenyl moiety and the polyfluorinated chain of molecules form angles of 56° and 26° 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 structure 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 Å incommensurate with period 35 Å of the polar smectic layers. The coexistence of such incommensurate structures is only possible in a strongly disordered medium. Infra-red dichroism data are consistent with this picture. The in-plane structure factor in smectic X, X ' phases reveals two features of polyphilic compounds : a dramatic increase in the short-range positional correlations $(\xi_{\perp}\approx 35~\div ~40$ Å or approximately 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 distances. The formation of polar, either uniform or modulated layered structures in the system of polyphilic molecules is discussed.