Dielectric relaxation behavior of a liquid crystal showing an unusual type
of antiferroelectric-ferroelectric-antiferroelectric phase sequence
S. K. Kundu and B. K. Chaudhuri*
Solid State Physics Department, Indian Association for the Cultivation of Science, Calcutta-700032, India
A. Seed
Chemistry Department, Kent State University, Kent, Ohio 44242-0001
A. Ja
´
kli
Liquid Crystal Institute, Kent State University, Kent, Ohio 44242-0001
Received 28 April 2002; revised manuscript received 26 December 2002; published 14 April 2003
The temperature and frequency dependent dielectric relaxation behavior of a liquid crystalline S-+-1-
methylheptyl 4-2-4-alkoxyphenyl thiophene-5-carbonylthiooxy benzoate system is reported. Interesting
successive antiferroelectric-ferroelectric-antiferroelectric AF-FE-AF phase transitions are observed in this
system resembling the successive phase transitions observed in crystalline Rochelle salt. The smectic-C *
(SmC *) to AF1 phase transition around 103.0 °C is first order in nature, predicted from the use of Orihara
and Ishibashi theory. It is also found that a contribution of the ferroelectric SmC * phase ordering penetrates
even in the antiferroelectric AF1 (SmC
A
*) and AF2 (SmC
A
*) phases very close to the SmC *-AF1 and
SmC *-AF2 phase boundaries critical regions. It is suggested that this type of mixing of AF and FE phases
might cause surface induced ferroelectric- or ferroelectric-type ordering near the AF-FE phase transitions. A
soft mode with Debye-type dispersion was observed in the SmA phase. The thermal behaviors of dielectric
dispersion, absorption, and dielectric strength in different phases are also reported and discussed.
DOI: 10.1103/PhysRevE.67.041704 PACS numbers: 64.70.Md, 77.84.Fa, 77.84.Nh, 61.30.-v
I. INTRODUCTION
Ferroelectric FE or antiferroelectric AF phase transi-
tions are well known in many H-bonded molecular crystals
like KH
2
PO
4
1, Rochelle salt 2, etc. However, similar
ferro- and antiferroelectric ordering has also been discovered
in liquid crystals ferroelectricity in 1974 and antiferroelec-
tricity in 1989. Since the first observation of antiferroelec-
tricity in MHPOBC 4-1-methylheptyloxy-carbonyl phenyl
4 ' -octyloxybiphenyl 4-carboxylate3–5, much attention
has been paid to the study of antiferroelectric liquid crystals
AFLCs both experimentally and theoretically. Many
AFLCs have also been found to show successive phase
changes and other interesting behavior like frustration 6
and phase penetration 7. An optically pure TFMHPOBC
liquid crystal 8 showed a second order phase transition di-
rectly from smectic A SmA to smectic C
A
* (SmC
A
* ). An-
other antiferroelectric liquid crystal system, viz., MHPOBC,
showed 4 a SmC * phase between SmA and SmC
A
* phases.
Strictly speaking, optically pure MHPOBC has three SmC *
subphases, viz., SmC
* , SmC
* , and SmC
* as reported by
Fukai et al. 3 and Chandani et al. 5. On the other hand,
for mixtures of R and S enantiomers, the phase sequence
changes to SmA -SmC * -SmC
A
* , where the transition from
SmC * to SmC
A
* is first order in nature. In recent years, a
great deal of effort has also been directed to determining the
detailed structures of various chiral smectic-C phases exhib-
iting antiferroelectric (SmC
A
* ) or ferrielectric
(SmC
FI1
* ,SmC
FI2
* ,SmC
* ) electro-optic responses. Direct
structural observation of superlattice periodicities associated
with the AF and ferrielectric phases 9 has also been
marked.
Recently an unusual antiferroelectric-ferroelectric-
antiferroelectric phase sequence has been reported in an an-
tiferroelectric S-+-1-methylheptyl 4-2-4-alkoxyphenyl
thiophene-5-carbonylthiooxy benzoate system 10hereaf-
ter referred to as MHATCTB. It has the phase sequence
cryst ↔
67.8 °C
SmC
A
* ↔
103.0 °C
SmC * ↔
110.0 °C
AF2 ↔
116.0 °C
SmA ↔
129.0 °C
isotropic.
In this paper we report a thorough investigation of the
dielectric relaxation behavior of this interesting MHATCTB
system, confirming the transitions. The successive phase
transitions from isotropic to crystalline phases were studied
using both temperature and frequency dependent dielectric
relaxation measurements. The present dielectric study sup-
ports the above phase sequence observed from differential
scanning calorimetry DSC and polarization measurements
10. We observed interesting phase penetration behavior, *Author to whom correspondence should be addressed.
PHYSICAL REVIEW E 67, 041704 2003
1063-651X/2003/674/0417045/$20.00 ©2003 The American Physical Society 67 041704-1