X-Ray study of the reentrant polymorphism N-SA-N-SA in a pure liquid crystal compound

X-ray diffraction patterns have been made on mesophases of 4-n-octyloxybenzoyloxy-4'-cyano-stilbene. In agreement with microscopic and magnetic studies we have reported the evidence of a thermotropic reentrant polymorphism nematic-smectic A-nematic-smectic A in this pure system. The evolution of the smectic layer thickness d as a function of temperature requires some kind of bilayer structure in the higher temperature S A whereas d corresponds to the molecular length in the lower temperature SA. In terms of local order the diffuse scattering intensity which appears in the higher temperature SA is the usually observed one, but in addition we have also seen an original two-dimensional superlattice consistent with smectic A symmetry in the lower temperature SA. The reentrant nematic phase seems to be a consequence of the competition between both short range smectic A orders.