Věra Hamplová

Lead bromide and ternary alkali lead bromide single crystals — growth and emission properties

Abstract Single crystals of PbBr 2 , RbPb 2 Br 5 and CsPbBr 3 (20 mm in diameter and 50 mm long) were grown by the Bridgman method from starting materials purified by bromination with bromine and hydrogen bromide and by zone refining and were tested by measurements of the low-temperature exciton luminescence and decay kinetics. The emission of RbPb 2 Br 5 resembles that of PbBr 2 . Short (10–20 ps) decay times were found in CsPbBr 3 . The emission of all compounds tested is strongly influenced by defects and the exciton character from Frenkel to Wannier type going from PbBr 2 to CsPbBr 3 .

The effect of a lateral substituent on the mesomorphic properties in a series of ferroelectric liquid crystals with a 2-alkoxypropionate unit

A new series of ferroelectric liquid crystal materials containing a lateral methyl group on the aromatic ring of the alkoxybenzoate unit has been synthesized and investigated. These materials exhibit a wide temperature range of ferroelectric SmC* phase on cooling (including supercooling) with a very high spontaneous polarization. All phase transition temperatures are lower when compared with those of similar non-substituted materials. In comparison with the compounds containing the lateral methoxy group, only the SmC*-Cr transition changed significantly. The values of spontaneous polarization are mostly increased compared with both non-substituted and methoxy substituted compounds.

New series of ferroelectric liquid crystals with four ester groups

Three series of ferroelectric liquid crystals have been synthesized, having three ester groups in the mesogenic core and one ester group in the chiral chain. The ester groups introduced into the core decrease the temperature of crystallization, which results in a broad temperature range ferroelectric smectic C phase in homologues with a long non-chiral chain. These phases exhibit a high spontaneous polarization and relatively short pitch of the helical structure.

New ferroelectric and antiferroelectric liquid crystalline materials containing differing numbers of lactate units

A new series of chiral liquid crystalline materials having the same rigid core containing a keto group, and various numbers of (S)-lactate groups in the chiral segment, has been synthesized and studied. All the compounds contain a branched terminal alkyl chain derived from (S)-2-methylbutanol. The influence of the number of lactate units on mesogenic behaviour has been studied. All the compounds show the paraelectric SmA phase and the ferroelectric SmC* phase over rather wide temperature ranges. The antiferroelectric SmCA* phase was observed for the compound possessing two lactate groups.

Dielectric behaviour of the composite system: multiwall carbon nanotubes dispersed in ferroelectric liquid crystal

Ferroelectric liquid crystal–multiwall carbon nanotube (FLC-MWCNTs) composite systems have been prepared by dispersing low (0.03 and 0.10 wt.%) concentration of MWCNTs in the liquid crystal matrix. Relaxation behaviours of pure and composite systems have been investigated. Thermo-optic studies infer a small shift (∼5 K) in the clearing temperature of the dispersed systems. Dielectric measurements reveal that the MWCNTs influenced the permittivity. The relaxation time, relaxation frequency and dielectric strength have been evaluated and discussed. The influence of MWCNTs concentration on the conductivity of composite systems has been investigated.

Synthesis and mesomorphic properties of a new ferroelectric liquid crystal with 2-alkoxypropionate chiral groups

Abstract Two series of new chiral substances with 2-alkoxypropionate chiral groups have been synthetized. All compounds exhibit very wide ferroelectric SmC* phases with a high spontaneous polarization.

Helix twist inversion in ferroelectric liquid crystals with one chiral centre

Abstract The helix twist inversion in the chiral smectic C phase was studied for two new homologous series of lactic acid derivatives. The inversion phenomenon was attributed to a competition between spatial conformers leading to opposite helix handedness. The temperature of inversion was found to increase with increasing length of the chiral molecular tail. With all substances studied, high values of the spontaneous polarization P s were found which did not reach saturation on cooling, in spite of a saturated tilt angle θs.

Polar liquid crystalline monomers with two or three lactate groups for the preparation of side chain polysiloxanes

Two new chiral liquid crystalline monomers with bilactate or trilactate chiral units were synthesized and studied. The monomers show the paraelectric SmA and ferroelectric SmC* phases. The antiferroelectric phase was detected only for the monomer possessing the bilactate unit. The temperature dependences of the spontaneous polarization and spontaneous tilt angle were measured. Real and imaginary parts of the complex permittivity were studied as a function of frequency and temperature. The synthesized monomers contain a double bond at the end of the achiral terminal chain, and were used to prepare liquid crystalline polysiloxanes.

New series of chiral smectic chlorinated liquid crystals

Abstract Two series of ferroelectric liquid crystalline materials, containing a lateral chlorine group on the aromatic ring of the molecular core, have been synthesised and investigated. Sequences of mesophases and phase transition temperatures have been determined for all substances. The temperature dependences of the complex permittivity and the spontaneous polarisation were determined in the whole range of the ferroelectric SmC* phase. The substitution of chlorine increases the values of spontaneous polarisation more than 5 times in comparison with non-chlorinated compounds.

New chlorine‐substituted liquid crystals possessing frustrated TGBA and SmQ phases

Several series of chlorine‐substituted rod‐like liquid crystalline compounds have been synthesised containing a different number of lactate units in the chiral chain. Sequences of phases and phase transition temperatures were determined by differential scanning calorimetry, polarising optical microscopy and X‐ray diffraction studies. Depending on the molecule structure, the materials possess a rich variety of mesophases, i.e. the paraelectric chiral smectic A (SmA*), ferroelectric chiral smectic C (SmC*) and antiferroelectric smectic C (SmCA*) phases. Additionally, some compounds with one and two lactate groups exhibit the frustrated antiferroelectric twist grain boundary (TGBA) and smectic Q (SmQ) phases, respectively. The existence of the SmQ phase ordered in two directions was confirmed by X‐ray diffraction on a non‐oriented sample. The effects of the non‐chiral chain length and number of the chiral lactate groups on the mesomorphic properties, spontaneous polarisation, spontaneous tilt angle and complex permittivity have been studied and discussed.