X-ray Studies on the Liquid Crystalline Phases of the 4-[4-n-Heptyloxy-Benzylidene-Amino]4'-cyanobiphenyl

By means of X-ray diffraction studies of oriented and non-oriented samples the lattice parameters of the orthorhombic cell of the S_E phase as well as of the hexagonal cell of the S_B phase were determined. By packing considerations a bimolecular building unit with oppositely arranged cyano-groups was assumed in the case of the S_E phase. The S_B phase is characterized by an ABAB sequence of the smectic layers in which the molecules are hexagonal arranged. The increasing mobility of the molecules at increasing temperature offers the possibility for a part of the molecules to slide in a more stable dimerized form. In the N_R and S_A phase the existence of monomers as well as of dimers is assumed. In this way a conception of the phase transitions S_E → S_B → N_R → S_A is given on the base of a decrease by steps of the dipole-dipole repulsion.