X-RAY STUDY OF ORDERED SMECTIC PHASES IN SOME BENZYLIDENEANILINES

From X-ray experiments, performed on both single domain and powder samples, the two ordered smectic phases (S4 and S3) exhibited by a few n-(alkoxybenzylidene)-p-n-alkylaniline compounds appear respectively as SB//c (tilted) and SB//A (normal) phases: the phase transition between these two phases is a first order transition. The description of the nature of these phases must also take into account an extra feature observed on some X-ray powder patterns: the splitting of the ring corresponding to the Bragg reflection on smectic layers into two rings. To interpet this phenomenon, the authors assume that there occurs a segregation into domains of different layer thickness, implying the existence of two kinds of molecular conformations differing in length. In the last part of this report, they review the classification problems related to the smectic ordered phases using new information provided by the study of the S4 and S3 phases.