| Issue |
J. Phys. II France
Volume 2, Number 5, May 1992
|
|
|---|---|---|
| Page(s) | 1195 - 1213 | |
| DOI | https://doi.org/10.1051/jp2:1992195 | |
J. Phys. II France 2 (1992) 1195-1213
X-ray studies of the smectic
phase and transitions between reentrant nematic, smectic A
1, A
and
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)
Abstract
We report X-ray diffraction measurements in nematic,
smectic A
1, A
and
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
), monolayer smectic (A
1), overlapped smectic (A
) and commensurate biaxial
phase was found in binary mixtures of 4OSACB and 9OSACB homologous.
It includes A
1-
, N
-
and A
-
phase transition lines, as well as triple N
A
and N
A
points. The parameters of the monoclinic two-dimensional
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
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
types, thus confirming the theoretical description of
phase as a two-dimensional escape from the incommensurability.
61.30 - 64.70M - 61.10
© Les Editions de Physique 1992