Anti-ferroelectric material

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. 

In the Landolt-Bdmstein data collection, ferroelectric and antiferroelectric substances are classified into 72 families according to their chemical composition and their crystallographic structure. Some substances which are in fact neither ferroelectric nor antiferroelectric but which are important in relation to ferroelectricity or anti-ferroelectricity, for instance as an end material of a solid solution, are also included in these families as related substances. This subsection surveys these 72 families of ferroelectrics presented in Landolt-Bornstein Vol. III/36 (LB III/36). Nineteen of these families concern oxides [5.1,2], 30 of them concern inorganic crystals other than oxides [5.3], and 23 of them concern organic crystals, liquid crystals, and polymers [5.4]. Table 4.5-1 lists these families and gives some information about each family. Substances classified in LB 111/36 as miscellaneous crystals (outside the families) are not included. 

Within single crystals and ceramic crystallites, respectively, the dipole moments of neighbouring domains are either perpendicular or anti-parallel to each other. For polycrystalline materials the orientation of the crystallites and thus of the domains is randomly distributed. In the original state these materials do not exhibit a macroscopic polarization and thus no piezoelectric effect. However, the latter can be induced by applying a static electric field below the Curie temperature where the domains of uniform dipole moments arrange towards the polarization field (paraelectric polarization). The field strength applied should be between the saturation and the breakdown range. Due to this polarization the ferroelectric material becomes piezoelectric. 

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.

Anti ferroelectric Liquid Crystal Materials with Unusual Chemical Structures... 

Also surface optical properties of a material sometimes need to be changed, for example in making anti-reflection coating for lenses or reflective surfaces for CDs, the magnetic properties may need to be influenced as in the case of giving a ferroelectric surface to a plastic for magnetic recording, and, perhaps most extensively of all, the surface electrical properties need to be controlled in microelectronic devices used in computers and all modern electronic equipment. 

These materials have the ilMnOs R = Sc or small, rare earth cation) stoichiometry,and have been erroneously referred to as hexagonal perovskites. The compounds do not exhibit the perovskite structure. The Mn cations are not octahedrally coordinated, rather the cation is surrounded by five oxide anions in a trigonal prismatic coordination environment. Also the R cations are not 12-coordinate, as would be the case in a perovskite, but are in seven-fold coordination. The materials are multi-ferroic, with anti-ferromagnetic and ferroelectric properties.The nature of the polarity and therefore the ferroelectric behaviour was only recently described. Careful structural studies indicated that although the dipole moments are attributable to the R-O bonds and not the Mn-O bonds, the R-cations are not directly responsible for the ferroelectric behaviour. The noncentrosymmetry is attributable to the tilting of the MnOs polyhedra, which in conjunction with the dipole moments in the R-O bonds results in ferroelectric behaviour. Thus the ferroelectric behaviour in these materials is termed improper " and occurs by a much different mechanism than BaTiOs or even BiFeOs. 

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. 

---