Ferroelectricity of Chiral Compounds in Highly Ordered Smectic Phases
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FERROELECTRICITY OF CHIRAL COMPOUNDS IN HIGHLY ORDERED SMECTIC PHASES STANISAW WR6BEL*.
MATTHIAS PFEIFFER. ASH)K M. BIRADAR+
AND WOLFGANG HAASE
Institut fdr Physikalische Chemie, Technische Hochschule Darmstadt, Petersenstr.20, D-6100 Darmstadt, Federal Republic of Germany
ABSTRACT Measurements of the complex electric permitttivity for the Sm C* and for three highly ordered smectic phases: Sm I*, Sm J* and Sm K* of 4- (2-methylbutyl)phenyl -4'- (octyloxy) - (1,1') - biphenyl -4- carboxylate ( 80S I ) have been done in the frequency range from 5 Hz to 13 MHz. For the first three chiral phases the Goldstone mode was found with temperature independent critical frequencies, Vc= 1.5kHz and the relaxation time -G = 106 s - for the Sm Ca phase, and Vc = 1 kHz and -_ = 160 ps - for the Sm IC one. In two highly ordered smectics, i.e. Sm J and Sm K with inter-layer correlations, the Goldstone mode seems to be suppressed. By applying a D.C. bias field of 1.4 kV/cm the Goldstone mode has been suppressed and shifted towards higher frequencies in both the Sm C* and Sm I phase. For the Sm J* phase the Goldstone mode is very weak and practically insensitive to the bias field used. In the Sm K* phase there is no Goldstone mode like relaxation, and the only contribution to electric permittivity comes from the fast molecular motion connected with reorientation of molecules about their long axis.
INTRODUCTION Ferroelectric liquid crystals( FLCs) were theoretically predicted by R.B. Meyer and the first substance exhibiting ferroelectric properties (DOBAMBC) was synthesized in 1975 [1]. Ferroelectric properties of thermotropic liquid crystals are due to both the molecular structure ( chirality of polar molecules ) and the quasicrystalline structure of tilted smectics ( Sm C , Sm I*, Sm F , Sm J*, Sm G',
Sm K * and Sm H *). The spontaneous polarization within a smectic layer is proportional to the vector product of the unit vector along the layer normal ( Z 0) and the director ( n), Ps ~ (ioxn) (1) Because of chirality of molecules the tilted smectics form helicoidal structure and z is the axis of the helix. Fluctuations of the 9p-angle, the angle in the layer plane which gives the pitch, create a signal in the dielectric spectrum, the Goldstone mode. However, it is known that in highly ordered tilted smectics with strong inter-layer correlations (Sj, S G, SK and S. ) the helicoidal structure is Mat. Res. Soc. Symp. Proc. Vol. 173. @1990Materials Research Society
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suppressed [2, 3]. FLCs exhibit the spontaneous polarization between 1 and - 300 nC/cm2 , which is very small in comparison with classical ferroelectrics. However, the exsistence of spontaneous polarization within the layers allows one to induce electrooptic effects with switching times in ms and its region depending on temperature, phase structure and so on. The electric permittivity tensor of the Sm CC phase, having a helicoidally arranged polarization vector, has the following form in the layer normal frame: S xxcos
xyz
(xyy
-
2 6
(
Ey sin2 4p, (Syy - E
xx
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