Anomalies of periodicity in some liquid crystalline cyano derivatives

Abstract

Unusual modes for mass density fluctuations are observed by X-ray experiments in nematic and smectic phases of some liquid crystalline materials with cyano end group. In particular, for the first time, we give evidence for a S A-SA transition in a pure compound (DB7) : in the high temperature SA1 phase a long-range density wave with a wavelength equal to 1.6 molecular length coexists with a short-range wavelength of about one molecular length. At low temperature a transition towards an SA2 phase occurs and we see two sharp Bragg spots which correspond to the two first-orders of diffraction by a modulation of two molecular length periodicity. Further another cyano derivative which exhibits both reentrant nematic and reentrant smectic A phases reveals the simultaneous existence and the competition of two modulations in these phases. But the occurrence of a reentrant phase does not seem to be connected to a commensurate or incommensurate lock-in of the two characteristic wavelengths.