Anti-ferroelectric ordering

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... 

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. 

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. 

These findings allow us to conclude that with suitable synthesis, we may be able to engineer discotic materials which acquire large dipoles and long range polarization order only as a result of temperature molecules which change from anti- to ferroelectric or to a super-paraelectric order when heated. 

It was recognized as early as 1981 [54] that in order to achieve the ferroelectric state, a chiral liquid crystal is necessary. Chirality puts two molecules in the ideal unit cell, and this allows new modes of fluctuations and also new forms of organisation which also give possible minima in the free energy [ structure ] function. An electric field can now take the liquid crystal into one of these alternative forms of molecular packing, some of which can have a net ferroelectric moment per unit cell and therefore an overall ferroelectric moment. Columnar pyrenes made recently by Bock and Helfrich [55] have saturation polarization of -llOnCcm" and switch from the anti- to the ferro-electric state on a time scale of = 10 s with switching fields -10 V tm . Similar behavior has been... 

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