C. Filipič

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.

Thermodynamic model of ferroelectric chiral smectic C* liquid crystals

Abstract A thermodynamic model of the ferroelectric SmC* phase, based on an extended Landau expansion of the free-energy density, is presented. We discuss how the signs of the parameters entering the model are depending on whether the pitch of the helix is right-handed or left-handed and whether the compound is of the (+) or (-) type in regard to the sign of the polarization. With the introduced Landau expansion as a basis, we derive the equations governing the behaviour of the tilt, polarization, pitch, dielectric susceptibility and heat capacity of the system. We show that by rewriting the equations into dimensionless form, we can transform the original set of eleven parameters in the Landau expansion into a new set, consisting of six dimensionless parameters and five scaling factors. The six dimensionless parameters are the only ones which enter the calculations of the quantities mentioned above, i.e. we need six dimensionless parameters to describe the temperature dependence of the five basic quantiti...

Thermodynamic model of ferroelectric liquid crystals and its microscopic basis

Abstract The properties of the extended thermodynamic model (Phys. Rev. A 36, 1484 (1987)) of ferroelectric liquid crystals, which can describe the anomalous thermodynamic behaviour of these systems in the chiral smectic C* phase, are reviewed. In the model the bilinear as well as biquadratic coupling between the tilt of molecules and the spontaneous polarization is included. The bilinear coupling is of a chirak nature and therefore small. This leads in the smectic C* phase close to the transition from the smectic A phase to cross-over effects between the regime, where because of small tilt only bilinear coupling is important, to the regime, where the biquadratic coupling also becomes relevant. A microscopic model is introduced, which describes the coupling between the tilt of molecules and the orientation of the molecular dipole, which is perpendicular to the long molecular axis. It is shown that the microscopic model is consistent with the extended thermodynamic model and the parameters of the thermodyn...

Dielectric properties of (CnH2n+1NH3)2CdCl4 and (NH3(CH2)nNH3)CdCl4 perovskite layer compounds

Abstract The dielectric properties of (CH3NH3)2CdCl4, NH3(CH2)3NH3.CdCl4 and NH3(CH2)5NH3.CdCl4 single crystals as well as of (CnH2n+1NH3)2CdCl4 (n = 1, 2, 3) and (NH3(CH2)nNH3)CdCl4 (n = 2,3,4,5) pressed powder pellets were investigated as a function of temperature between 77 and 373°K and significant dielectric anomalies were found to occur at all structural phase transitions. The dialectric constant in the alkyl-ammonium as well as alkylene-diammonium crystals is found to be much higher parallel to the octahedral planes than in a direction perpendicular to these layers.

Ferroelectric Liquid Crystals

Abstract The main advances in the field of bulk ferroelectric liquid crystals since 1984 are reviewed. Recent experiments have shown that the molecular tilt and the in-plane spontaneous polarization are not strictly proportional to each other as assumed before. Microscopic experiments such as NMR and NQR have further allowed for a determination of the basic order parameters as a function of the nuclear position along the molecule without unwinding the chiral helix.

Experimental and theoretical study of the influence of a bias electric field on the dielectric properties of the chiral smectic A* phase

Abstract The influence of a bias electric field on the temperature and frequency dependence of the soft mode part of the complex dielectric constant of the chiral smectic A* phase has been studied experimentally. It was found that only close to T S*CS*A does the bias field have any influence on the experimentally determined quantities. Here the relaxation frequency increased with increasing bias field, while the corresponding dielectric strength decreases. We also present a theoretical calculation of the influence of a bias field on the complex dielectric constant of the chiral smectic A* phase which, apart from minor details, is in accordance with the observed behaviour. Finally we discuss how the nature of the electroclinic effect can be correlated with the influence of the bias field on the smectic A* dielectric constant.

Weak ferromagnetism and ferroelectricity in K3Fe5F15

Here, we report on the observation of a weak ferromagnetic transition at TN=122K in a K3Fe5F15 system which is ferroelectric and ferroelastic below Tc=490K. The magnetization and the susceptibility continuously increase with decreasing T down to 2K. The Mossbauer spectra show a spontaneous magnetic ordering and at least three sites corresponding to Fe2+ and Fe3+. The ratio between Fe2+ and Fe3+ is 60:40. At 6K, there are two magnetically ordered sextets with internal fields of 585 and 263kOe. The “slim” hysteresis loops observed are as well characteristic of weak ferromagnetism. At 122K and at low frequencies, the system shows dielectric anomaly characteristic of magnetoelectric behavior.

Critical exponents of ferroelectric TSCC determined from dielectric measurements

Abstract The temperature dependence of the static dielectric constant, the spontaneous polarization and the electric field dependence of the polarization in TSCC have been measured. The critical exponents γ, β and δ were determined. The values obtained are consistent with the predictions of the mean field theory, and β in particular seems to be different from the one determined with EPR techniques.

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.

The magnetoelectric coefficients of Pb(Fe1/2Nb1/2)O3 and 0.8Pb(Fe1/2Nb1/2)O3−0.2Pb(Mg1/2W1/2)O3

The value of the magnetoelectric coefficient γ was estimated for Pb(Fe1/2Nb1/2)O3 and 0.8Pb(Fe1/2Nb1/2)O3−0.2Pb(Mg1/2W1/2)O3 from the measured values of their magnetic susceptibility and remanent polarization.