Asymmetry ferroelectrics

Distribution of strain about the dislocation is thus highly asymmetric. One contribution to this asymmetry, as between compressed and dilated sides, has been considered. There is little evidence on which to interpret the considerable differences between the strain on the right and left sides. Whether some ferroelectric distortion -bringing small changes in cell dimensions and a polar axis no longer collinear with the crystallographic c axis - could contribute to the asymmetry is speculative. 

Raman spectra as a function of temperature are shown in Fig. 21.6b for the C2B4S2 SL. Other superlattices exhibit similar temperature evolution of Raman spectra. These data were used to determine Tc using the same approach as described in the previous section, based on the fact that cubic centrosymmetric perovskite-type crystals have no first-order Raman active modes in the paraelectric phase. The temperature evolution of Raman spectra has indicated that all SLs remain in the tetragonal ferroelectric phase with out-of-plane polarization in the entire temperature range below T. The Tc determination is illustrated in Fig. 21.7 for three of the SLs studied SIBICI, S2B4C2, and S1B3C1. Again, the normalized intensities of the TO2 and TO4 phonon peaks (marked by arrows in Fig. 21.6b) were used. In the three-component SLs studied, a structural asymmetry is introduced by the presence of the three different layers, BaTiOs, SrTiOs, and CaTiOs, in each period. Therefore, the phonon peaks should not disappear from the spectra completely upon transition to the paraelectric phase at T. Raman intensity should rather drop to some small but non-zero value. However, this inversion symmetry breakdown appears to have a small effect in terms of atomic displacement patterns associated with phonons, and this residual above-Tc Raman intensity appears too small to be detected. Therefore, the observed temperature evolution of Raman intensities shows a behavior similar to that of symmetric two-component superlattices. 

An important group of piezoelectric ceramics are solid solutions of PbZrOj and PbTiOs represented as Pb(Zr, Ti)03 (and commonly referred to as PZT). At high temperature these compounds have the cubic perovskite structure (Fig. 1). In the ferroelectric phase, which is stable at room temperature, the lattice is distorted, and the asymmetry of the positive and negative ions results in a net dipole moment. Spontaneous polarization is the dipole moment per unit volume. 

As mentioned in the introduction, chiral compounds can exhibit chiral mesophases and these are important due to the important physical properties that they may exhibit, including thermochroism, ferroelectric and electroclinic effects [15], In 1975, Meyer predicted the existence of a spontaneous polarization (Pg) in chiral, tilted smectic phases [86], and the existence of such polar order within a liquid crystal phase has important implications both scientifically and industrially [19]. The asymmetry associated with the chirality may also produce a beneficial lowering of transition temperatures. 

In summary, whatever the films studied (metastable or not), asymmetry in the switching properties was commonly observed. Such asymmetry was attributed in ferroelectric films to an internal built-in electric field existing at the bottom interface or/and to the tip/film/electrode structure, which brings its own asymmetry [113-118]. 

In order to complete the constitutive theory, the form of must be specified. As previously noted, ferroelectric ceramics exhibit an asymmetry in the attainable... 

As an example, the joint analysis of IR and Raman spectra provided evidence of the partial ordering of cations in a Fe-Cr corundum-type mixed sesquioxides, which are used industrially as high temperature water-gas shift catalysts, but are also active in olefin oxidative dehydrogenation. X-ray diffraction (XRD) patterns of these solids indicate the conmdum-type structure without any superstructure. This implies that iron and chromium ions are randomly distributed. IR and Raman spectra instead definitely show that cations are at least partially ordered in layers such as in the ilmenite-type superstructure. Similarly, XRD analysis shows a cubic (non-ferroelectric) structure of nanometric BaTi03, while vibrational spectroscopies reveal microscopic asymmetry of this structure. Similarly, IR spectroscopy allowed the determination of the state of vanadium in solid solution in Ti02 anatase catalysts, and the presence of Ti" + in the silicalite framework of TSl catalysts, " used for the selective oxidation of phenol and the ammoximation of cyclohexanone with hydrogen peroxide. 

Chirality has become arguably the most important topic of research in liquid crystals today. The reduced symmetry in these organized phases leads to a variety of novel phase structures, properties, and applications. Molecular asymmetry imparts form chirality to liquid crystal phases, which is manifested in the formation of helical ordering of the constituent molecules of the phase. Similarly, molecular asymmetry imposes a reduction in the space symmetry, which leads to some phases having unusual nonlinear properties, such as ferroelectric-ity and pyroelectricity. 

Furthermore, it is also noticed, in Figure 5, that the symmetry of C-V curves is destroyed after the films electric-annealed. For the films annealed at +200 V, the negative part of the C-V curve exhibits slightly stronger tuning property than the positive part, whereas the films annealed at -200 V shows the opposite phenomenon. Such asymmetry of the C-V characteristics might also result from the appearance of the nano-polar-regions in the paraelectric state, since the similar asymmetries observed in ferroelectric films due to the hysteresis effects. 

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