J. Phys. II France
Volume 2, Numéro 5, May 1992
Page(s) 1195 - 1213
DOI: 10.1051/jp2:1992195
J. Phys. II France 2 (1992) 1195-1213

X-ray studies of the smectic $\tilde{\rm C}$ phase and transitions between reentrant nematic, smectic A 1, A $_{\rm D}$ and $\tilde{\rm C}$ phases in binary mixtures of polar mesogens

T. A. Lobko1, B. I. Ostrovskii1 and W. Haase2

1  Institute of Crystallography Academy of Sciences of the U.S.S.R., Leninsky prosp. 59, Moscow, 117333, Russia
2  Institut für Physikalische Chemie, Technische Hochschule Darmstadt, 6100 Darmstadt, Germany

(Received 5 December 1991, accepted 8 January 1992)

We report X-ray diffraction measurements in nematic, smectic A 1, A $_{\rm d}$ and $\tilde{\rm C}$ phases for terminally polar three-ring compounds belonging to 4-[ N-(4- n-alkoxysalicylidene)] - 4'-cyanobiphenyl series ( nOSACB). A new topology of phase diagram with the reentrant nematic (N $_{\rm re}$), monolayer smectic (A 1), overlapped smectic (A $_{\rm d}$) and commensurate biaxial $\tilde{\rm C}$ phase was found in binary mixtures of 4OSACB and 9OSACB homologous. It includes A 1- $\tilde{\rm C}$, N $_{\rm re}$- $\tilde{\rm C}$ and A $_{\rm d}$- $\tilde{\rm C}$ phase transition lines, as well as triple N $_{\rm re}$A $_1\tilde{\rm C}$ and N $_{\rm re}$A $_{\rm d}\tilde{\rm C}$ points. The parameters of the monoclinic two-dimensional $\tilde{\rm C}$ unit cell and tilt angles of molecule have been determined. The intensity asymmetry and the positions of the off-axis reflections which are characteristic of the $\tilde{\rm C}$ phase are discussed on the basis of models with competing order parameters. Our results directly indicate on the strong coupling between incommensurate density modulations of A 1 and A $_{\rm d}$ types, thus confirming the theoretical description of $\tilde{\rm C}$ phase as a two-dimensional escape from the incommensurability.

61.30 - 64.70M - 61.10

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