A chiral axial next nearest neighbour xy-model for antiferroelectric liquid crystals
A. Roy - N.V. Madhusudana
Raman Research Institute, C.V. Raman Avenue,
Bangalore-560 080, India
nvmadhu@rri.ernet.in
Received 23 August 1999
Abstract
We describe a chiral axial next nearest neighbour
xy-model to account for the various subphases exhibited
by antiferroelectric liquid crystals made of chiral
rod-like molecules. The assumed form of the interlayer
interactions is based on physical processes which are
discussed. Using a discrete model, the predicted sequence
of transitions is
SmA-SmC-SmC
-FIH-FII-FIL-SmC*A,
where FI stands for a ferriphase, as seen in many
compounds. The ferri and SmC
phases are
characterized by relatively large angles between the
c-vectors of successive layers and occur only when the
compounds have high optical purity. The calculated
field induced structures exhibit a plateau of the apparent
tilt angle at
,
where
is the tilt
angle of the molecules in the ferriphase. The conoscopic
figures in the presence of a field and ellipsometric
parameters in the absence of a field have also been
generated, which agree extremely well with the experimental
results. Recent anomalous X-ray scattering studies prove
the xy-character of the configurations, though the
commensurate structures that are found in the ferriphase
require an extension of the model to include lock-in terms.
PACS
61.30.Cz Theory and models of liquid crystal structure -
64.70.Md Transitions in liquid crystals -
77.84.Nh Liquids, emulsions and suspensions; liquid
crystals
Copyright EDP Sciences, Società Italiana di Fisica, Springer-Verlag