Antiferroelectrics dielectric properties

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. 

Parab SS, Malik MK, Dabrowski R, Deshmukh RR (2013) Thermodynamic and bias field characterization of quickly operating antiferroelectric liquid crystal. J Mol Liq 183 20-25 Parab SS, Malik MK, Deshmukh RR (2014a) Investigation of dielectric properties of poly(methyl methacrylate)-E7 composite films. Int J ChemTech Res 6 1836-1839 Parab SS, Malik MK, Bhatia PG, Deshmukh RR (2014b) Investigation of liquid crystal dispersion and dielectric relaxation behavior in polymer dispersed liquid crystal composite films. J Mol Liq 199 287-293... 

The presence of clusters in BC nematics is now well established from various measmements. Recent studies " have in fact indicated a ferroelectric or an antiferroelectric response to an applied electric field, and an unusual low-frequency (presumably collective) mode has been detected in the dielectric spectra of bent-core nematics, which might also be related to clusters. In spite of the intense studies, however, the exact structure and the physical properties of the clusters are still unknown. Therefore, not surprisingly, a precise physical model for the role of polar clusters in the flexoelectric response of BC nematics and a quantitative estimation of the resulting increment of the flexocoefiicients has not yet been worked out. 

Here, the first term describes the nematic-like elastic energy in raie crmstant approximation (K in 9). This term allows a discussion of distortions below the AF-F threshold (a kind of the Frederiks transition as in nematics in a sample of a finite size). In fact, the most important specific properties of the antiferroelectric are taken into account by the interaction potential W between molecules in neighbour layers the second term in the equation corresponds to interaction of only the nearest layers (/) and (/ + 1). Let count layers from the top of our sketch (a) then for odd layers i, i + 2, etc. the director azimuth is 0, and for even layers / + 1, / + 3, etc. the director azimuth is n. The third term describes interactimi of the external field with the layer polarization Pq of the layer / as in the case of ferroelectric cells. Although for substances with high Pq the dielectric anisotropy can be neglected, the quadratic-in-field effects are implicitly accounted for by the highest order terms proportiOTial to P. ... 

Part k covering functional materials is organized in a two-step approach. The first step corresponds to searching for the substance of interest, that is, the relevant group of substances. The second step corresponds to the physical property of interest. Materials covered are semiconductors, superconductors, magnetic materials, dielectrics and electrooptics, and ferro- and antiferroelectrics. 

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.

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