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In this work, we report on the results of the examination of the nonlinear dielectric response of layered ferrielectic CuInP2S6 and Cu0.9Ag0.1InP2S6 crystals. The temperature behaviour of the nonlinear dielectric properties of ferrielectic CuInP2S6 and Cu0.9Ag0.1InP2S6 is found to be consistent with Landau theory. The additional peak of the third-order nonlinear susceptibility in these crystals, below the phase transition temperature, is strongly influenced by the electric field amplitude, and it is caused by the ferroelectric domain dynamics. The nonzero value of the second-order susceptibility and the mismatch between the temperatures of linear and third-order susceptibilities indicate that some polarisation exists above the phase transition temperature. [ABSTRACT FROM AUTHOR]

The ferrielectric intermediate liquid crystal phases are typically narrow in temperature range, but significant stabilisation of these phases can be achieved by doping. We report an achiral dopant that may induce a significant conformational change in the host molecule, characterised by tilt ratio inversion across the ferrielectric phase range. Preferred molecular structures are predicted for dopants that could lead to stabilisation of the ferrielectric phases for potential device applications. [ABSTRACT FROM AUTHOR]

The synthesis and mesomorphic properties of 25 novel esters derived from 4-substituted thiophene-2-carboxylic acids and either (S)-1-methylheptyl 4-hydroxybenzoate or (S)-1-methylheptyl 4'-hydroxybiphenyl-4-carboxylate are described. A structure-property relationship has been deduced, and mesogenic properties are found to be dependent on the total number of rings in the molecular core and the nature of the substituent in the 4-position of the thiophene ring. Four-ring systems have greater thermal stability and are more likely to form mesophases than their three-ring counterparts. A 1,4-phenylene substituent in the 4-position of the thiophene ring (Series I and II) gives better thermal properties than does a 2,5-thienyl substituent (Series III and IV). A rationale is suggested in terms of conjugative effects that may be important in mesophase thermal stability. A preliminary molecular modelling study (B3LYP/6-31G(d)) reveals that 2,4-disubstituted systems offer a slight preference for a folded geometry structure compared with their 2,5-counterparts. Homologues (n = 7-10,12) in Series II show an interesting chiral phase behaviour between the SmA and SmC* ferrielectric phase, tentatively assigned SmX*, which then gives a SmC* antiferroelectric phase. Textures have been characterised by thermal polarising microscopy using a conventional glass slide cover-slip and free-standing film. Although the SmX* phase has still to be fully characterised, this is the first reported occurrence of such a phase in 2,4-thiophene systems. [ABSTRACT FROM AUTHOR]

• First direct measurements of the electrocaloric effect in a ferrielectric (NH 4) 2 SO 4. • Small effect of the electric field on the temperature and entropy of the transition. • High extensive and low intensive electrocaloric efficiency. • No conversion between conventional and inverse electrocaloric effects. • Contribution of Joule heat to the temperature change under the electric field. [Display omitted] The influence of an electric field on the phase transition parameters and the electrocaloric response in ferrielectric (NH 4) 2 SO 4 was studied using a universal multifunctional adiabatic calorimeter. The very low sensitivity of the phase transition temperature to the electric field, dT 0 /dE ≈ 5·10-3 K/(kV/cm), was shown to be the reason for the low maximum value of the intensive electrocaloric effect, ΔT AD , at E = 11 kV/cm: direct and indirect measurements gave the values of 0.01 K and 0.05 K, respectively. However, the extensive electrocaloric response revealed by an analysis of the entropy–temperature–electric field phase diagram was relatively large, with a value of ΔS ECE ≈ − 19 J/kg K, due to the significant contribution of a jump in the excess entropy to the total value, δS 0 /ΔS 0 ~ 0.37. An additional irreversible increase in temperature under the electric field was associated with the Joule effect and indicated a significant electrical conductivity of ammonium sulphate. [ABSTRACT FROM AUTHOR]

A numerical modeling of the three-layer SmC*FII phase behavior under an external electric field is performed. A discrete free energy of the model is analyzed by means of genetic algorithms. We focus our attention on the structure evolution induced by the field and its relation to the field dependence of the in-plane optical anisotropy. We show the multisolition character of the azimuthal molecular reorientation under the field. It permits us lo explain the unusual field-induced behavior of the conoscopic figures in the SmC*FII phase. [ABSTRACT FROM AUTHOR]

Dielectric properties of a chiral smectic liquid crystal exhibiting ferroelectric and antiferroelectric forms of dipole order have been studied in the frequency range 10 Hz ÷ 1 MHz. Three relaxation bands were found in the hexatic ferroelectric phase and in the antiferroelectric phase, and six bands in the apparently ferrielectric state. The number of relaxation processes observed in the studied frequency range reduces to one in the smectic C α * and A phases. The results of measurement suggest that phases exhibiting ferrielectric properties might be in fact a mixture of ferroelectric and antiferroelectric clusters. This mixed state is stabilized by surface interactions. [ABSTRACT FROM AUTHOR]

Chiral twin compounds, possessing two chiral moieties in such a way that the chiral centres are located at the both peripheral sides of a molecular twin structure, have been prepared and the liquid-crystalline properties investigated. The chiral twins showed stable ferrielectric and antiferroelectric phases. A twist grain boundary phase was found to appear by mixing the twin and the analogous monomer. [ABSTRACT FROM AUTHOR]

The synthesis and physical properties of a liquid-crystalline material exhibiting the ferrielectric C γ *-phase in a broad temperature range are described. The contradictory literature data on this substance are critically reviewed. Dielectric, optical, electrooptic, calorimetric and miscibility studies allowed us to verify the existing data and to establish the complicated phase sequence of the studied substance. The substance possesses many phases specific for materials called antiferroelectric without having the classical antiferroelectric C a *-phase. The range of existence of the ferrielectric C γ *-phase is very broad (about 20°C). This feature can be useful in structural investigations of the C γ * phase. [ABSTRACT FROM AUTHOR]

We propose a thermodynamic model to the study the antiferroelectric (AFE) phase transitions in antiferroelectric-ferroelectric (AFE-FE) superlattices in which the coupling at the interface between two layers is mediated by local polarizations. Phase diagram of the AFE layer in term of the degree of interfacial effect λ and temperature T involving ferrielectric (FI) and ferroelectric (FE) phases is investigated. These two phases are stabilized by the interfacial effect and internal electric field. AFE thickness L AFE versus T phase diagram is also constructed. Intermediate regions of two-phase coexistence (IM) emerge in the λ - T and L AFE - T phase diagrams, if certain interface properties λ and layer thickness L AFE criteria are met. These IM regions are metastable states, which exist as a transition state between two phases. A tricritical point locates at the boundaries across the FI, IM and FE phases is found in the L AFE - T phase diagram. Competition among the internal electric field due to the electrostatic coupling, the FE ordering arises from the interfacial effect and the antiferroelectric ordering within the AFE layer giving rises to the rich AFE phase diagram., (© 2020 IOP Publishing Ltd.)