Field-induced phase transitions in an antiferroelectric liquid crystal using the pyroelectric effect
N. M. Shtykov,
1,2
J. K. Vij,
1,
* R. A. Lewis,
3
M. Hird,
3
and J. W. Goodby
3
1
Department of Electronic and Electrical Engineering, Trinity College, University of Dublin, Dublin 2, Ireland
2
Institute of Crystallography, Russian Academy of Sciences, Leninskii Prospekt 59, 117333 Moscow, Russia
3
Department of Chemistry, University of Hull, Cottingham Road, Hull HU6 7RX, United Kingdom
Received 23 July 1999
The antiferroelectric liquid crystal AFLC under investigation possesses different helical polar phases.
Measurements of pyroelectric response of these phases as a function of temperature and bias field have
elucidated the ability of this method for investigating the nature of antiferroelectric phases and phase transi-
tions under the bias field. The pyroelectric signal as a function of the bias field at fixed temperatures and as a
function of temperature for fixed bias fields was measured for different phases of the investigated AFLC
material. A theoretical model describing the pyroelectric response in different phases of AFLC is given, and
the experimental results are interpreted. The threshold fields for field induced phase transitions are determined.
The type of field induced phase transition from the AF phase in particular is found to be dependent on the
temperature within its range. The properties of an unusual ferrielectric phase existing between ferrielectric
chiral smectic-C (SmC *) and antiferroelectric AF phases are studied in a great detail. The results confirm that
this phase is one of the incommensurate phases, predicted by the axial next-nearest neighbor Ising model and
Landau model for this temperature region.
PACS numbers: 42.70.Df, 61.30.-v, 64.70.Md
I. INTRODUCTION
Antiferroelectric liquid crystals AFLC’s exhibit several
chiral phases between paraelectric smectic A (SmA * ) and
antiferroelectric smectic C
A
(SmC
A
* ). These phases were
tentatively designated as SmC
* , SmC
* , and SmC
* in or-
der of decreasing temperature 1. Among these phases
SmC
* seems to be more complicated than the other phases.
Finally, this series of phases was added by the discovery of a
number of additional ferrielectric and antiferroelectric
phases. The existence of some of the ferrielectric phases is
very sensitive to the optical purity of the AFLC’s. It was
reported that phases SmC
* and SmC
* , which have ferri-
electric properties, disappear with decreasing optical purity
2. The SmC
* phase is usually considered to be the same as
the ferroelectric chiral smectic C (SmC * ). But some re-
searchers believe that the SmC
* phase in optically pure
samples should be considered as ferrielectric and not ferro-
electric. It was also confirmed that a decrease in the optical
purity causes the phase transition SmC
* to SmC * phase 3.
X-ray resonant technique employed on a thiobenzoate liquid-
crystal compound has recently shown 4 the existence of
four phases with different superlattice periodicities. These
phases are SmC
A
* , SmC
FI 1
* , and SmC
FI 2
* , with two-, three-,
and four-layer superlattices, respectively, and SmC
* , with a
periodicity incommensurate with the layer spacing. In the
SmC
* phase an incommensurate periodicity was shown to
lie roughly between from eight and five layers with decreas-
ing temperature.
The appearance of antiferroelectric and ferrielectric
phases in the tilted chiral smectic liquid crystals can be un-
derstood to be a result of the competition between the anti-
ferroelectric and ferroelectric interactions in adjacent smectic
layers. This competition produces different periodic ( A
+F ) sequences of antiferroelectric A and ferroelectric F
orderings among the smectic layers. Several different theo-
retical approaches have been advanced for explaining a va-
riety of the ferrielectric phases, and these postulates are
based mostly at the expanded Landau model 5–7 or on the
one-dimensional Ising model 8 and the axial next-nearest-
neighbor Ising ANNNI model 9–12. Recently, the short
pitch modes model 13 was presented, which describes an-
tiferroelectric and ferrielectric phases as structures with cer-
tain ‘families’ of modulation modes.
We take the ANNNI model, advanced for the antiferro-
electric and ferrielectric phases in an electric field 12, as
the basis for explaining a variety of different phase transi-
tions observed in our pyroelectric experiments. The Hamil-
tonian of the system with the third-nearest-neighbor interac-
tion in the electric field E is given by
H =-J
i , j
s
i
s
j
-J
1
i
A
s
i
s
i +1
-J
2
i
A
s
i
s
i +2
-J
3
i
A
s
i
s
i +3
-E
i
s
i
, 1
where the molecular state is expressed in terms of the Ising
spin ( s
i
=1), designating the direction of the molecular
tilt, J, J
1
, J
2
, and J
3
the interaction parameters, the first
summation is taken over the nearest-neighbor pairs of mol-
ecules in the same smectic layer and the second, third, and
fourth ones are over those between the first-, second-, and
third-nearest-neighbor layers, respectively. The superscript A
denotes the axial direction. An essential and characteristic
point of this model is the negative value for the energy
parameter J
2
. *Corresponding author. Email address: jvij@tcd.ie
PHYSICAL REVIEW E AUGUST 2000 VOLUME 62, NUMBER 2
PRE 62 1063-651X/2000/622/22799/$15.00 2279 ©2000 The American Physical Society