Ferroelectrics definition

In Science, every concept, question, conclusion, experimental result, method, theory or relationship is always open to reexamination. Molecules do exist Nevertheless, there are serious questions about precise definition. Some of these questions lie at the foundations of modem physics, and some involve states of aggregation or extreme conditions such as intense radiation fields or the region of the continuum. There are some molecular properties that are definable only within limits, for example, the geometrical stmcture of non-rigid molecules, properties consistent with the uncertainty principle, or those limited by the negleet of quantum-field, relativistic or other effects. And there are properties which depend specifically on a state of aggregation, such as superconductivity, ferroelectric (and anti), ferromagnetic (and anti), superfluidity, excitons. polarons, etc. Thus, any molecular definition may need to be extended in a more complex situation. 

Here we focus on the effect of dipolar dispersion laws for high-frequency collective vibrations on the shift and width of their spectral line, with surface molecules inclined at an arbitrary angle 6 to the surface-normal direction. For definiteness, we consider the case of a triangular lattice and the ferroelectric ordering of dipole moments inherent in this lattice type.56,109 Lateral interactions of dynamic dipole moments p = pe (e = (sin os, sin6fcin , cos )) corresponding to collective vibrations on a simple two-dimensional lattice of adsorbed molecules cause these vibrations to collectivize in accordance with the dispersion law 121... 

By our definition, the tilt plane is normal to the polarization in the ferroelectric state in the illustration in Figure 8.13 this is a vertical plane normal to the plane of the page. Since there is no tilt of the director projected onto this plane, the phase should be considered a type of SmA. We name this structure SmAPp (an untilted polar smectic the subscript F referring to a ferroelectric structure, in this case a ferroelectric state of an antiferroelectric phase). The antiferroelectric phase is therefore also an SmA denoted SmAPA (the subscript A for antiferroelectric). While this idea is certainly intriguing, no such antiferroelectric has yet been discovered. 

It may be noted that simple MOPAC AMI calculations suggest that the dipole moment of NOBOW is oriented antiparallel to the molecular arrow. As indicated in Figure 8.25, this means that for an up field, the molecular arrows are pointing down. Given the definition of the sign of P in FLCs, this also means that domains of the ShiCaPa phase with positive chirality have negative ferroelectric polarization, and vice versa. 

The problem of the structural multidomaining below Ta makes it difficult to reach a definite conclusion, as shown in Figs. 5 and 13. Measurements for defect-free and stress-free STO samples are indispensable for a definite conclusion about the symmetry of the ferroelectric phase of STO 18. Finally, we can conclude that STO 18 may be a typical soft mode ferroelectric. 

The difference between the two definitions rests with the fact that a pyroelectric crystal must possess an overall (observable) permanent electric dipole. Thus a pyroelectric crystal is built from unit cells, each of which must contain an overall electric dipole, (Figure 4.11b). The pyroelectric effect will only be observed, however, if all of these dipoles are aligned throughout the crystal, (Figure 4.11c). [Note that a ferroelectric crystal is defined in a similar way. The difference between a pyroelectric crystal and a ferroelectric crystal lies in the fact that the direction of each overall electric dipole in a ferroelectric crystal can be altered by an external electric field.]... 

A ferroelectric crystal is one that has an electric dipole moment even in the absence of an external electric field. This arises because the centre of positive charge in the crystal does not coincide with the centre of negative charge. The phenomenon was discovered in 1920 by J. Valasek in Rochelle salt, which is the H-bonded hydrated d-taitrate NaKC4H406.4H20. In such compounds the dielectric constant can rise to enormous values of 10 or more due to presence of a stable permanent electric polarization. Before considering the effect further, it will be helpful to recall various definitions and SI units ... 

Displacements that occur between several equilibrium sites for which the probability of occupancy of each site depends on the strength of the external field. This mechanism is also known as dipolar or ion jump polarization and is depicted schematically in Fig. 14.10. Another definition of ion jump polarization is the preferential occupation of equivalent or near-equivalent lattice sites as a result of the applied field biasing one site with respect to the other. If the alignment occurs spontaneously and cooperatively, nonlinear polarization results and the material is termed ferroelectric. Because of the relatively large displacements, relative dielectric constants on the order of 5000 can be attained in these materials. Nonlinear dielectrics are dealt with separately in Chap. 15. But if the polarization is simply due to the motion of ions from one adjacent site to another, the polarization behavior is linear with voltage. These solids are discussed below. 

Given the definition of ferroelectricity as the spontaneous and reversible polarization of a solid, it is not surprising that ferroelectricity and... 

Figure 19. Definition of the different angular parameters involved in ferroelectric switching.

If a material is ferroelectric, local field effects must be taken into account in order to determine the intrinsic individual relaxation time. The collective relaxation times are characterized by a critical slowing down, as observed for the reorientations of HjO and HjO in HUP or of Hse04 and in NH4HSe04 . The Ising model and molecular field approximation lead to the definition of a new relaxation time ... 

Ferroelastics are the materials where the order parameter is mechanical deformation that spontaneously appears at transition temperature. As it was mentioned in Chap. 1, Aizu [74, 75] was the first who gave the formal definition of ferroelasticity as the property that can exist by itself in the materials which are neither ferroelectrics nor ferromagnets. 

The definition of ferroelectric liquid crystals needs some comments see remark / in Sect. 4.5.3.I. 

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