A. 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.

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...

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.

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.

Lifshitz-Like Behaviour in Ferroelectric Liquid Crystals

Abstract In order to explain the anomalous behaviour of the critical magnetic field and the helical pitch in the vicinity of the Λ-line and the Lifshitz point in the ferroelectric liquid crystal DOBAMBC a model involving not only chiral but also non-chiral biquadratic coupling between the polarization and the tilt has been proposed. The model predicts an “S - like” dependence of the modulation wave vector close to the smectic C* - smectic A transition. High temperature resolution measurements of the helical pitch and the intensity of the diffraction sattelites indeed support the proposed model and seem to show that the pitch is finite at Tc. In such a case the Lifshitz field HL is finite too.

Glassy to inhomogeneous-ferroelectric crossover in (Pb, La)(Zr, Ti)O3 ceramics

The temperature dependence of the dielectric nonlinearity a3=e3/e14 has been determined in 9/65/35 (Pb, La)(Zr, Ti)O3 ceramics. In zero electric field a deep in the temperature behavior of the static a3 has been observed indicating a crossover from paraelectric-like to glass-like behavior on cooling towards the freezing transition. By varying a bias electric field another glass-to-ferroelectric crossover has been observed in the temperature dependence of the static a3. The observed crossovers as well as the temperature dependence of the linear static dielectric constant agree with the predictions of the spherical random-bond–random-field model of relaxor ferroelectrics.

Dielectric and pyroelectric properties of Pb5Ge3O11 ceramics

The pyroelectric coefficient of the (PbO)x(GeO2)(8−x) ceramics as a function of the PbO or GeO2 content close to x=5 was studied. The value of the pyroelectric coefficient increases with increasing content of the PbO up to x=5 and decreases above with an irregularity around x=5. From the frequency dependence of the complex dielectric constant the electric conductivity was determined. The temperature dependence of the electric conductivity is a function of poling. Unpoled ceramics have an activation energy of 0.70 eV. The observed dielectric relaxations in ceramics are equivalent to those in monocrystals.

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.