Reorientation ferroelectrics

Since P must remain normal to z and n, the polarization vector forms a helix, where P is everywhere normal to the helix axis. While locally a macroscopic dipole is present, globally this polarization averages to zero due to the presence of the SmC helix. Such a structure is sometimes termed a helical antiferroelectric. But, even with a helix of infinite pitch (i.e., no helix), which can happen in the SmC phase, bulk samples of SmC material still are not ferroelectric. A ferroelectric material must possess at least two degenerate states, or orientations of the polarization, which exist in distinct free-energy wells, and which can be interconverted by application of an electric field. In the case of a bulk SmC material with infinite pitch, all orientations of the director on the tilt cone are degenerate. In this case the polarization would simply line up parallel to an applied field oriented along any axis in the smectic layer plane, with no wells or barriers (and no hysteresis) associated with the reorientation of the polarization. While interesting, such behavior is not that of a true ferroelectric. 

Using this method, the M6R8/PM6R8 blend showed precisely the behavior expected for the achiral SmAPA structure. Specifically, the optical properties of the films were consistent with a biaxial smectic structure (i.e., two different refractive indices in the layer plane). The thickness of the films was quantized in units of one bilayer. Upon application of an electric field, it was seen that films with an even number of bilayers behaved in a nonpolar way, while films with an odd number of bilayers responded strongly to the field, showing that they must possess net spontaneous polarization. Note that the electric fields in this experiment are not strong enough to switch an antiferroelectric to a ferroelectric state. Reorientation of the polarization field (and director structure) of the polar film in the presence of a field can easily be seen, however. 

If it is possible to reorient the spontaneous polarization of a material between crystal-lographically equivalent configurations by an external electric field, then in analogy to ferromagnetics one speaks about ferroelectrics. Thus, it is not the existence of spontaneous polarization alone, but the switchability by an external field which defines a ferroelectric material. Figure 1.2 displays a characteristic hysteresis loop occurring during the reversal of the polarization in a ferroelectric. 

It is important to realize that thin films may differ in some substantial ways from bulk ceramics or single crystals of the same composition. One source of these differences is the substantial in-plane stresses that thin films are typically under, ranging from MPa to GPa [9], Because many ferroelectric materials are also ferroelastic, imposed stresses can markedly affect the stability of the ferroelectric phase, as well as the ease with which polarization can be reoriented in some directions. The phase diagram becomes considerably complicated by the presence of a dissimilar substrate [10]. It is obvious that the material coefficients are drastically changed. 

Cady in World War II realized that such a mechanical resonance of a vibrating crystal could be used in frequency control. This discovery had an important influence on radio communications.Alternating electric fields, such as those generated by the radio tubes of the time, were applied to plates of piezoelectric crystals and the expansions and contractions of the plates were caused to react on electrical circuits. If the natural frequency of the mechanical vibration of the quartz plate coincided with the frequency of oscillation of the electric circuit, resonance between the two took place and energy was acquired by the mechanical oscillators. Later. Rochelle salt and barium titanate, which are each both ferroelectric and piezoelectric, were used. ° In ferroelectric crystals, the polarization or dipole moment is reversed or reoriented upon application of an electric field. Ferroelasticity is another property displayed by some crystals in which stress can cause the interconversion between two stable orientational states. These physical properties of crystals are of great use in modern technology. 

As it was pointed out in the Introduction, the problem of the coexistence of displacive and order-disorder phenomena at the ferroelectric phase transitions of BaTiOs has met growing interest in recent time. Strong support of the order-disorder model comes 30 years ago from EPR measurements performed on Mn" " "-, Cr -, and Fe -doped BaTiOs [218-222] because in the low-temperature rhombohedral phase it was observed that Mn" " ", which substitutes isovalent Ti" " " sites, is displaced off-centre by 0.14 A along <111> directions with a reorientational hopping with correlation times 10 -10 s. 

A ferroelectric crystal shows a spontaneous electric polarization that can be reoriented between multiple equilibrium directions by application of an electric field The materials known as ferroelectric ceramics have attained a high level of importance". ... 

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