I. Levstik

Dielectric properties of CsH2PO4 and CsD2PO4

Abstract Orthorhombic CsH 2 PO 4 undergoes a ferroelectric transition at T c = −119.5° C , whereas the ferroelectric transition temperature in isomorphous CsD 2 PO 4 is T c = −5.55° C . The transitions are of first order in both cases. The rather large isotope effect demonstrates the importance of the OHO bonds in the transition mechanism.

Angular dependence of the proton spin-spin and spin-lattice relaxation in nematic MBBA-EBBA mixtures

Abstract The dependence of the proton T1, T1ϱ and the dipolar splitting of the proton resonance line δ on the angle between the nematic director and the magnetic field was measured in a nematic 67 mol.% MBBA and 33 mol.% EBBA mixture by exploiting the fact that the magnetic field orientation time is much longer than the proton T1. The results show that — as in pure MBBA — translational diffusion rather than order director fluctuations (ODF) represent the dominant relaxation mechanism for T−11 in this system whereas there is a 30% ODF contribution to T−11ϱ. It is further shown that the T1∝“ω 1 2 L” ODF dispersion law does not hold at arbitrarily low Larmor frequencies and that (T1)ODF approaches a constant value as ωL→0.

Dielectric properties of the 250 K phase transition in BaMnF4 and of CsH2PO4 and CsD2PO4

The dielectric constant of BaMnF4 has been measured close to the 250 K phase transition along a, b and c crystal-lographic axes. The dielectric constant along the a axis shows a small peak at -23.3°C (∼250 K), whereas along the other two axes the dielectric constant changes monotonically with the temperature.Orthorhombic CsH2PO4 undergoes a ferroelectric transition at Tc = -119.5°C, whereas the ferroelectric transition temperature in isomorphous CsU2PO4 is Tc = -5.55°C. The transitions are of fust order in both cases. The rather large isotope effect demonstrates the importance of the O-H-O bonds in the transition mechanism.

A dielectric method for determining the rotational viscosity in ferroelectric liquid crystals

Abstract A dielectric method for determining the rotational viscosity in thick samples of ferroelectric liquid crystals is described and the results for the DOBA-1-MPC and for the DOBAMBC are presented. In both cases the temperature dependence of the rotational viscosity is of Arrhenius type.

Spontaneous polarization of helicoidal ferroelectric smectic C* liquid crystals

Measurements of the spontaneous polarization of DOBAMBC in helicoidal smectic C* phase is presented. It is shown that the polarization shows an anomalous temperature dependence characterized by one square root behaviour close to Tc and by another one far from Tc. This behaviour is explained theoretically to be a consequence of a nonchiral biquadratic coupling between the polarization and the tilt of molecules in the Landau expansion of the free-energy density. Also the tilt is predicted to exhibit a (less pronounced) crossover region of the same type as the polarization within the proposed model. It is also noted that this crossover region coincides with the temperature interval where the pitch of the helix displays anomalous properties.

A room temperature ferroelectric liquid crystal

Abstract Temperature dependences of the dielectric constant and of the helical pitch have been measured for the mixture 71.76 wt.% of 4-n-butiloxybenzylidene-4′-n-octylaniline, 12.92 wt.% of 4–4′-bis-n-heptyloxy-azoxybenzene and 15.32 wt.% of 4″-(2-methylbutylphenyl)-4′-(2-methylbutyl)-4-biphenylcarboxylate which is in the ferroelectric helicoidal smectic C* phase at room temperature. The anomalies close to the smectic C*-smectic A transition are of the same type as in DOBAMBC and cannot be understood in terms of the standard Landau free energy expansion.

Rotational viscosity in ferroelectric liquid crystal dobambc determined by dielectric method

Abstract Spontaneous polarization, tilt, pitch of the helix, and low frequency dielectric dispersion were measured as functions of temperature in the smectic C* phase of the DOBAMBC. It is shown that measured thermodynamic quantities satisfy simple relations, predicted by a phenomenological model for ferroelectric liquid crystals. These relations allow the evaluation of the rotational viscosity, which temperature dependence is of Arrhenius type with an activation energy of 0.51 eV.

Dielectric properties of biphenyl

The dielectric constant along the crystallographic axes was measured. The dielectric anomalies at TI along the a and b axes show critical exponent beta =0.50+or-0.01 in the incommensurate phase II. At the second phase transition TII no anomaly in dielectric constant was observed.

The dielectric properties of DOBA-1-MPC with large spontaneous polarization

Abstract The frequency and temperature dependence of the complex dielectric constant in DOBA-1-MPC with large spontaneous polarization was investigated close to SmA-SmC phase transition. The frequencies and dielectric strengths of a Goldstone and soft mode were determined. The experimental results were compared with theoretical model; the agreement is quite good.