Anti-ferroelectric

In order to understand these extreme changes in physical and chemical properties of hydrogen-bonded systems, first attempts to model their dynamics were related to rather simple structures, as exhibited by the KDP family or squaric acid and its analogues. The isotope effects on their ferro- or anti-ferroelectric transition temperatures are listed in Table 1 together with the corresponding isotope exponent. 

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. 

There are certain crystals in which dipoles are spontaneously aligned in a particular direction, even in the absence of electric field. Such substances are called ferroelectric substances and the phenomenon is called ferroelectricty. The direction of polarisation in these substances can be changed by applying electric field. Baruion titanate (BaTi03), sodium potassium tartarate (Rochelle salt), and potassium hydrozen phosphate (KH2I04) are ferroelectric solids. If the alternate dipoles are in opposite directions, then the net dipole moment will be zero and the crystal is called anti-ferroelectric. Lead zirconate (PbZr03) is an anti-ferroelectric solid. 

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. 

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. 

We have been able to compare the difference that full anti-ferroelectric ordering present in ice VIII has with the incomplete anti-ferroelectric ordering in two ice VII structures in terms of bond stiffness Xi As and charge movement The ability of the O—O... 

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. 

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. 

In the last few years the anti-ferroelectric liquid crystal Sca was discovered by Chandani et al. (1988, 1989) in MHPOBC... 

Figure 6.39. The molecular packing of unwound samples (a) ferroelectric and (b) anti-ferroelectric liquid crystals. (From Goodby et al, 1993.)...

Compared with conventional ferroelectric liquid crystals, anti-ferroelectric liquid crystals have the following advantages ... 

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... 

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. 

Figure 11.6 Schematic relationship between dielectric solids (E is an applied electric field, and a is an applied stress), (a) Dielectric (i) E — 0, (ii) E is finite a dielectric, normally unpolarised, becomes polarised is an electric field), (b) Piezoelectric (i) cr = 0, (ii) a is finite (a piezoelectric, normally unpolarised, developes a polarisation when subjected to stress, even is no electric field), (c) Pyroelectric and ferroelectric E — 0, a — 0. (d) Anti-ferroelectric E = 0,

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. 

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