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Electronic Properties of Perovskite Oxides
Published in Gibin George, Sivasankara Rao Ede, Zhiping Luo, Fundamentals of Perovskite Oxides, 2020
Gibin George, Sivasankara Rao Ede, Zhiping Luo
Antiferroelectric (AFE) materials are the materials exhibiting antiparallel dielectric polarizations from two adjacent domains; as a result, the net polarization is zero. The material behaves as a paraelectric under a weak electric field in the direction of an electric field and ferroelectric under a strong field. Therefore, antiferroelectric materials exhibit a double-hysteresis loop against changes in the electric field, as shown in Figure 6.26. Since the adjacent dipoles in antiferroelectrics are antiparallel, the spontaneous polarization does not exist, but antiferroelectrics exhibit low remnant polarization and dielectric loss at a low electric field.
Optical properties and applications of ferroelectric and antiferroelectric liquid crystals
Published in L. Vicari, Optical Applications of Liquid Crystals, 2016
Emmanouil Ε Kviezis, Lesley A Parry-Jones, Steve J Elston
Following the same chain of argument, SmCA* materials are anti-ferroelectric. This is because each smectic layer, considered individually, has the same symmetry as a SmC* layer. Therefore each layer has a net spontaneous polarization along its C2 axis. However, because of the alternating tilt directions in adjacent layers, the polarizations also alternate in direction from layer to layer, and hence the material is described as being antiferroelectric.
Unusual electric-field-induced transformations in the dark conglomerate phase of a bent-core liquid crystal
Published in Liquid Crystals, 2014
M. Nagaraj, K. Usami, Z. Zhang, V. Görtz, J.W. Goodby, H.F. Gleeson
To explain the unusual field-induced transformations of the DC phase, it is important to know the in-layer molecular organisation in the DC phase. Therefore, the temperature dependence of the dielectric permittivity, ε′, of the material was recorded. The plot of the temperature dependence of the ε′ (Figure 6), as expected, shows a clear first order transition between the nematic and the DC phase. It also shows that ε′ is maximum at the N–DC transition and its value decreases by ~20% as we go further down in temperature in the DC phase. This indicates an antiferroelectric arrangement of the in-layer polarisation direction between successive layers. There is a sharp increase in the ε′ values at TN-DC – T ≈ 35 K. This corresponds to the same temperature where an increase in the layer spacing is observed by small angle X-ray diffraction. There is no DSC peak found corresponding to this temperature and no change in the texture is observed under polarising microscope at these temperatures. The ε′ was measured several times in different cells, but, the behaviour at TN-DC – T ≈ 35 K is repeatable. An electric field applied (~20 V/μm) to this lower temperature DC phase did not induce any change in its texture, possibly because of the high viscosity of the phase at these temperatures.
Influence of carbon chain length on physical properties of 3FmHPhF homologues
Published in Liquid Crystals, 2019
A. Deptuch, M. Marzec, T. Jaworska-Gołąb, M. Dziurka, J. Hooper, M. Srebro-Hooper, P. Fryń, J. Fitas, M. Urbańska, M. Tykarska
There are two classes of relaxation processes in chiral smectic phases: molecular and collective ones. Molecular processes include rotations and precessions of molecules, while collective processes involve correlated movements of molecules that lead to fluctuations of the order parameter. Collective processes are further divided into amplitudons and phasons, which are the collective fluctuations of the tilt angle and azimuthal angle , respectively [1,2]. In the paraelectric phase the only possible collective process is the soft mode, which relates to collective fluctuations of (an amplitudon). The collective processes in the ferroelectric phase include both the soft mode and collective fluctuations of molecules around the tilt cone. The latter is referred to as a Goldstone mode and is much stronger than the soft mode [2,4,5]. In the antiferroelectric phase the polarisation vectors of neighbouring smectic layers do not cancel each other completely and, therefore, four collective processes, called non-cancellation modes, are possible. The fluctuations of and in neighbouring smectic layers can be either in-phase or anti-phase, so the non-cancellation modes include two amplitudons, in-phase AL and anti-phase AH, and two phasons, in-phase PL and anti-phase PH [2,3].
Helix parameters in bi- and multicomponent mixtures composed of orthoconic antiferroelectric liquid crystals with three ring molecular core
Published in Liquid Crystals, 2014
Instead of grooving number of new synthesised compounds with antiferroelectric phase,[9–12] the easiest method to obtain materials with broad temperature range for chiral anticlinic phase is by the creation of mixtures. It allows to control other parameters important for applications, that is helical parameters. The limited number of antiferroelectric mixtures with long helical pitch [8,11–14] as well as continuous development of new fields of application of liquid crystal materials [15–18] make the formulation of new antiferroelectric mixtures with desired helical parameters a very important action.