Anti-ferroelectric phase

On the other hand, the proton potential of the 5-bromo compound is exactly symmetrical with reference to the reaction coordinate of the tautomerization. Consequently, the proton transfer can proceed through the tunnelling mechanism. This is the reason why the paraelectric behaviour is maintained even at 4 K. The suppression of the antiferroelectric phase transition may be derived from a quantum tunnelling effect. Such paraelectric behaviour can be regarded as quantum paraelectricity , which is a notion to designate the phenomenon that (anti)ferroelectric phase transitions are suppressed even at cryogenic temperatures due to some quantum-mechanical stabilization, proton tunnelling in this case. 

A coexistence region in SQA was also evidenced by 13C 2D temperature jump correlation NMR.172 Proton 1H spin lattice and spin-spin relaxation measurement on SQA was performed within 320 460 K.138 In addition to a typical critical behavior manifested at the known temperature of the anti-ferroelectric phase transition near Tc 373 K, both relaxation data also show a second critical behavior around 420 K, which opens the question of a second phase transition for SQA. 

These studies also showed two other interesting phenomena, firstly that certain members in the homologous series exhibit ferrielectric and antiferro-electric phases, and secondly for the materials that exhibit TGBA phases, a novel transition was found to occur in the isotropic liquid. The ferri- and anti-ferroelectric phases appear first for the n-undecyl homologue on ascending the series, and disappear once the chain length reaches sixteen carbon atoms in length. 

Pereverzev, Y.V., O.V. Prezhdo, and L.R. Dalton. 2002. Sample shape influence on the anti-ferroelectric phase transitions in dipolar systems subject to an external field. Phys Rev B 65 053104-1/4. 

ORDER-DISORDER THEORY AND APPLICATIONS. Phase transitions in binary liquid solutions, gas condensations, order-disorder transitions in alloys, ferromagnetism, antiferromagnetism, ferroelectncity, anti-ferroelectricity, localized absorptions, helix-coil transitions in biological polymers and the one-dimensional growth of linear colloidal aggregates are all examples of transitions between an ordered and a disordered state. 

There are numerous properties which make fluorinated LC attractive for applications. Short Rp-segments lead to de Vries phases and to 90° tilted anti-ferroelectric SmCA phases, useful for orthoconic switching in new display applications. Fluorination could also lead to enhanced polarization in ferroelectric and antiferroelectric LC phases. 

The low temperature anti-ferroelectrically ordered polymorph of ice VI with a ten molecule unit cell, and a sixteen-molecule super-cell for ice VIII, were also used. Both ice VII(A) and ice VII(B) consist of identical eight molecule sub-lattices anti-parallel to one another. Ice VII(A) consists of six C and two D type-hydrogen bonds respectively, while ice VII(B) contains four C and D type hydrogen bonds. The structure of these phases is considered in detail. 

Once the helical structure of the Sc phase is unwound, ferroelectricity is displayed (see Chapter 6 for the details). In recent years, many experimental studies have revealed that some liquid crystal compounds show new types of smectic phases with complex tilt and dipole order, such as the anti-ferroelectric smectic C phase, Sca phase, and the ferrielectric smectic C phase, Sc7 phase. For instance, in the Sca phase, the spontaneous polarization Ps is opposite for successive layers. It was found experimentally that the chiral So phase is in fact similar to the anti-ferroelectric Sca phase. 

Boemlburg et al. (1991) first discovered the anti-ferroelectric liquid crystal phase in the chiral side chain liquid crystalline polymer, Sca phase. Several other research groups followed with more such side chain liquid crystalline polymers. Boemlburg et al. (1992) reported an anti-ferroelectric liquid crystal in the molecule... 

This phase is stable above 23 GPa, which is the typical pressure at the upper/lower mantle boundary. At low pressures it is metastable, the stabler phases being pyroxenes. The orthorhombic phase is obtained from the cubic perovskite phase (Pm3m) by superposing rotations ofnearly-rigid oxygen octahedra to an anti-ferroelectric displacement of Mg ions perpendicular to the c-axis. This brings to twenty and ten the number of atoms per cell and structural parameters respectively. 

A.D.L. Chandani, E. Gorecka, Y. Ouchi, H. Takezoe and A. Fukuda, Anti-ferroelectric chiral smectic phases responsible for the tristable switching in MHPOBC, Jpn. J. Appl. Phys. 28(7), L1265-L1268, (1989). 

Nishiyama, 1., and Goodby, J. W., Effect of polymerization on the stability of anti-ferroelectric LC phases LC-properties of some chiral acrylates and their corresponding polyaci7lates, J. Mater. Chem., 3, 169 (1993). 

Ferrielectric and antiferroelectric chiral smectic C phases The constituent molecules of the antiferroelectric chiral smectic C (S g anti) phase have the tilted, lamellar structure of the ferroelectric phase but the tilt direction alternates from layer to layer to give a zig-zag stractme. Accordingly, the Pg of the antiferroelectric phase is zero. The ferrielectric chiral smectic C (S g ferri) phase also has an alternating... 

The use of CD as a probe of liquid crystalline properties has been rather limited. A helicoidal structure will induce circular dichroism at an absorption band of a nonchi-ral chromophore, and the magnitude of the induced CD absorption depends on the pitch of the helix, the sign of the CD changing if pitch inversion occurs. This technique has been used to investigate phase transitions between ferrielectric, ferroelectric and anti-ferroelectric smectic C states of MHPOBC... 

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.

Asymmetric synthesis of diastereomeric a- or P-CF3 liquid crystelline (LC) molecules is established through the catalytic asymmetric carbonyl-ene reaction with fluoral by a chiral binaphtiiol (BINOL)-derived titanium catalyst (1). Diastereomeric a- or P-CF3 LCs work as conformational probes for anti-ferroelectricity. The first example of spontaneous chiral resolution of racemates in fluid LC phases is further found with the diastereomeric LCs. 

A number of moleoiles that exhibit a ferroelectric (F) phase or an anti-ferroelectric (AF) phase (Figure 2) such as methyl-substituted MHPOBC (18) and the trifluoromethyl analogue, TFMHPOBC (19) with greater spontaneous polarization (Ps) (Figure 3), have been prepared and their physical properties have been investigated because of potential application in electro-optic devices for liquid-CTystalline displays (LCDs) (20). The AFLC molecules show quite useful characteristics such as a tri-stable switching, sharp DC threshold, and double-loop-hysteresis. 

If a mesophase possesses ferroelectric properties in the bulk its spontaneous polarization Pg can interact with a solid surface. The Pg-vector can be oriented, for example, in the direction toward substrates on both interfaces of a liquid crystal layer, that is, anti-symmetrically. In this case, the director of the smectic C phase, which is rigidly coupled with Pg, will change its orientation in the bulk of the layer resulting in the appearance of speciflc defects [33]. 

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