Antiferroelectric transition behavior

Initially the octyl to dodecyl compounds were prepared and these were found to exhibit relatively normal behavior, i.e. smectic A phases were found for the lower homologues with smectic and ferroelectric smectic C phases occurring for the higher members. However, when the tetradecyl homologue was examined in the polarizing transmitted light microscope, an iridescent helical mesophase was observed which upon cooling underwent a further phase transition to a ferroelectric smectic phase. In addition, this compound was also found to exhibit antiferroelectric and ferrielectric phases. 

Studies of the dielectric properties will certainly provide greater insight into the molecular processes that determine the applications of supramolecular structures. The onset of ferroelectric and antiferroelectric behavior and the ordering-ordering transitions in supramolecular structures, as well as the dynamics of molecular segments in such structures, can be investigated by studying their dielectric spectra. 

Likewise, in lead lanthanum zirconate titanate (PLZT) relaxors, the substitution of La for Pb at A sites produces randomly distributed Pb vacancies. In bismuth sodium titanate (BNT [BiNa]Ti206), the replacement of Bi by Na creates charge imbalances and vacancies. Ceramics with compositions Bai, jNaxTii Nbx03 are either classical ferroelectrics (for 0relaxor ferroelectrics (for 0.075 < x< 0.055) with a diffuse transition temperature without any frequency dispersion. The relaxor behavior increases with increasing compositional deviation from both BaTi03 and NaNbOs (Khemahem et al., 2000). 

Figure 5. Response of polar dielectrics (containing local permanent dipoles) to an applied electric field from top to bottom paraelectric, ferroelectric, ferrielectric, antiferroelectric, and helielectric (helical anti-ferroelectric). A pyroelectric in the strict sense hardly responds to a field at all. A paraelectric, antiferro-electric, or helieletric phase shows normal, i.e., linear dielectric behavior and has only one stable, i.e., equilibrium, state for E=0. A ferroelectric as well as a ferrielectric (a subclass of ferroelectric) phase shows the peculiarity of two stable states. These states are polarized in opposite directions ( P) in the absence of an applied field ( =0). The property in a material of having two stable states is called bistability. A single substance may exhibit several of these phases, and temperature changes will provoke observable phase transitions between phases with different polar characteristics.

Investigadon of tbe Beld dependence of the apparent till angle in the antiferroelectric phase of polymer 79 revealed that the tilt angle gradually increases with the field before tbe transition from tbe antiferroelectric state to the electrically induced ferroelectric stale occurs (Fig. 97). The hysteresis behavior observed for low-molar-mass antiferoelectric LCs was not observed for tbe polymer. 

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