Ferroelectric phase-anisotropy

An important aspect of the enhanced intrinsic response of ferroelectrics is anisotropy, the direction dependence of properties. By symmetry, ferroelectric crystals are anisotropic with respect to dielectric, mechanical, and electromechanical properties, and this issue is essential when designing devices that exploit the highest material responses by correctly orienting single crystals [8, 33-35]. The crystal axes, along which the highest material response occurs, may not coincide with the polar directions of spontaneous polarization for a given ferroelectric phase. This is the case... 

In the single-domain state, many ferroelectric crystals also exhibit high optical nonlinearity and this, coupled with the large standing optical anisotropies (birefringences) that are often available, makes the ferroelectrics interesting candidates for phase-matched optical second harmonic generation (SHG). 

Lead titanate exhibits only one stmctural phase transition from paraelectric cubic mhm to ferroelectric tetragonal 4mm phase. Curie temperature is relatively high 490° C. Lead titanate has the biggest anisotropy in the tetragonal lattice parameter ratio c/a = 1.06 among all perovskite crystals. Due to the anisotropy it exhibits relatively high hydrostatic piezoelectric coefficient dh (higher than PZT ceramics). 

The nematic phase of nonlinear mesogens may be biaxial - a translationally disordered fluid phase with two directors n and o specifying the orientational order (Fig. 5.3). Biaxial order in a nematic is predicted to occur [29] if the shape anisotropy of the idealized molecule (discoid) representing the nonlinear meso-gen is appropriately intermediate between the prolate shape of calamities and the oblate shape of discotics. Discoid-shaped mesogens lend themselves to a variety of stratified phases. Ferroelectric (Sapp) and antiferroelectric (Sapa) layer motifs in the normal smectic phases of discoid-shaped mesogens are readily envisioned (Fig. 5.4), but less obvious is the possibility of generating chiral supramolecular structures from such achiral discoid-shaped mesogens (Fig. 5.5) [30]. 

In other words, molecular design and the synthesis were carried to express various characteristics for the required purposes that depend on the specific application of the ferroelectric liquid crystal. For example, ferroelectric liquid crystals with large spontaneous polarization, a large tilt angle with small temperature dependence, a specific viscosity, a small rotational viscosity of the molecules along their long axis, a chiral smectic C phase over a wide temperature range, a characteristic anisotropy of the dielectric constant, a suitable phase sequence, a suitable helical pitch, photochemical stability, and so on have been newly synthesized [3-5]. 

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