Dependence Of Spontaneous Polarization Research Articles

Physical properties of new binary antiferroelectric liquid crystal mixtures

ABSTRACTThree newly prepared binary mixtures exhibiting chiral tilted smectic phases have been studied using differential scanning calorimetry, dielectric spectroscopy and electro-optic method, as well as X-ray diffraction. Broad temperature range of ferroelectric and antiferroelectric phases was detected in these mixtures and temperature dependence of spontaneous polarization, tilt angle and switching time were measured for all of them. It's occurred that all of the studied mixtures are orthoconic antiferroelectric liquid crystals. Based on the X-ray diffraction results, the temperature dependence of layer thickness in the paraelectric, ferroelectric and antiferroelectric phases was found. By using dielectric spectroscopy, Goldstone mode was identified in the ferroelectric phase, while antiphase fluctuations of azimuthal angle have been found in the antiferroelectric phase. Based on the results of the complementary methods, the transition temperatures were found as well as the order of the para-ferroelectric phase transition was determined as non-continuous one with critical parameter β equal to ca. 0.25.

PLZT microfibers volume gradients and anisotropy

ABSTRACTElectrocaloric materials for a new generation of cooling elements utilize the temperature dependence of spontaneous polarization in crystalline materials to harvest waste heat. From published results in particular (Pb0.93La0.07)(Zr0.65Ti0.35)O3 composition, have the largest pyroelectric coefficient. The most effective form for such applications is micro-fiber form, due to small heat capacitance allowing quick response time, even for nano second laser excitation. The presented work provides a consistent set of examinations of structural, ferroelectric and piezoelectric properties of damaged ceramics in fiber form and its comparative analysis to bulk to get practical knowledge of the processes occurring during fiber manufacture, to effectively prevent its destruction.

New antiferroelectric liquid crystal for use in LCD

In this work, the physical properties of newly synthesized liquid crystalline compound exhibiting two liquid crystalline phases (ferroelectric and antiferroelectric) were studied. Based on the results of differential scanning calorimetry, polarizing microscopy, and photoelastic modulator methods, the temperature dependences of spontaneous polarization, tilt angle, switching time, and birefringence in the ferroelectric, as well as antiferroelectric phases were determined. Furthermore, the influence of the external electric field on the liquid crystalline textures was studied and the phase sequences at heating and cooling were revealed. The temperature dependence of spontaneous polarization was analysed by means of Landau mean-field theory, and the critical parameter β obtained for ferroelectric liquid crystalline and isotropic liquid transition was 0.21 which is close to 0.25, the value characteristic for tri-critical point.

Dependence of spontaneous polarization on stacking sequence in SiC revealed by local Schottky barrier height variations over a partially formed 8H-SiC layer on a 4H-SiC substrate

Ballistic electron emission microscopy was used to measure the increase of local Schottky barrier (compared to the surrounding 4H-SiC area) over a partial 8H-SiC layer that is the surface-exposed tail of an 8H stacking fault inclusion extending from 4H substrate. This local increase is believed to be due to polarization charge induced at the interface of partial 8H layer and underlying 4H host, resulting from the spontaneous polarization (SP) difference between SiC regions with different bilayer stacking. This is a direct experimental probe of the dependence of SP in SiC on local stacking sequence by measuring carrier transport.

Lattice Dynamics and Spontaneous Polarization Calculation of Disorder Solid Solution Thin Ferroelectric Films

In the framework of nonempirical model of polarizable ions, lattice dynamics and polarization for solid solution thin ferroelectric films PbB′1/2B″ 1/2O3 (B′ = Sc, Ga, In, Lu; B″ = Nb, Ta) were calculated. The thickness of films was varied from 3 to 41 layers and PbO <B>O2 terminations of film were considered. It was found, those unstable polar modes are existing in spectra of all compounds for every thickness and eigenvectors of this mode has different character for different terminations. The frequency of “soft” polar mode tends to its corresponding value in bulk crystal with increasing of film thickness. In phase, distorted by eigenvector of this mode, the film thickness dependence of spontaneous polarization was calculated. Also it was found that layer-by-layer polarization profile for films with different terminations are differing.

An analysis of the temperature dependence of the spontaneous polarization of LiNbO3 crystals

The temperature dependence of spontaneous polarization and the primary pyroelectric coefficient of stoichiometric lithium niobate crystals are calculated and the specific features of structural ordering of lithium niobate in the ferroelectric phase are analyzed using a modified electrostatic model. The reasons that experimentally observed anomalies of the primary pyroelectric coefficient occur are discussed.

Ferroelectric properties of achiral banana-shaped and calamitic-chiral thioesters

Two thioester compounds: (S)-(+)-4-(1-methylheptyloxy)biphenyl 4′-heptylthiobenzoate (in short: MHOBS7) having rode-like, calamitic-chiral molecules, and 1,3-phenylene bis{4-[(4-octyloxybenzoyl)-sulfanyl]benzoate} (in short: 8OSOR) built of banana like achiral molecules, have been studied by complementary methods to show differences in their linear dielectric as well as ferroelectric properties. To this end the following experimental methods were used: differential scanning calorimetry, polarizing microscopy, dielectric spectroscopy and reversal current method. The first compound shows a ferroelectric SmC* phase with temperature dependent Goldstone mode displaying negative activation energy. At the N*--SmC* transition spontaneous polarization changes discontinuously showing first order character of this transition. For 8OSOR compound with bent core molecules only B1 phase shows up, which does not switch. However, it was possible to record reversal current spectra but temperature dependence of spontaneous polarization was unusual suggesting antiferroelectric order of this phase.

Critical Exponents of Ferroelectric SrTi18O3

Measurements of the temperature dependence of spontaneous polarization and dielectric constant in the vicinity of the phase transition at ∼26 K in SrTi 18 O 3 were performed and the critical exponents were determined. The critical exponents β and γ have classical Landau values. These values confirm that we are dealing with the classical paraelectric to ferroelectric transition of the soft mode type in agreement with recent optical studies. We can also conclude that the Lyddane-Sachs-Teller relationship is obeyed in this system.

Ferroelectric Compounds with Strong Polar Cyano Group in Terminal Position

The synthesis, phase transition parameters, the temperature dependence of spontaneous polarization, tilt and pitch of (S)-(+)-4-(1-methylheptyloxycarbonyl)phenyl 4′-(cyanoacetoxy hexoxy)biphenyl-4-carboxylates and (S)-(+)-4-(1-methylheptyloxycarbonyl)biphenylyl 4′-(cyanoacetoxy)benzoate are described. These compounds exhibit the synclinic smectic C phase and are useful as the components of antiferroelectric mixtures with high tilt angle. The (S)-(+)-4-(1-methylheptyloxycarbonyl)phenyl 4′-(cyanoacetoxyhexoxy)biphenyl-4-carboxylate shows the ability to induce an antiferroelectric phase in mixtures based on the other ferroelectric components.

Electrical properties of Pb2Bi3DyTi5O18 ceramic

Single-phase orthorhombic tungsten bronze (OTB) ceramics, Pb2Bi3DyTi5O18, were prepared by a high-temperature solid-state reaction method. Electrical impedance, phase angle, dielectric constant and dielectric loss were measured in the frequency range 0.1 kHz to 3 MHz at room temperature. The temperature variation of the dielectric constant shows a diffuse phase transition having a diffusivity parameter value of 1.71 at 1 kHz. Pyroelectric current was measured using a direct measurement technique. The temperature dependence of spontaneous polarization was evaluated from the pyroelectric coefficient. The values of the pyroelectric coefficient and spontaneous polarization at room temperature were found, respectively, to be 1.308 × 10–10 C cm–2 °C–1 and 1.993 × 10–7 C cm–2. The activation energy (Ea = 0.516 eV) was estimated from the temperature variation of the dc resistivity. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Detection of Alpha Sub-Phase Between Para- and Ferroelectric Phase of a Fluorinated Compound

Measurements of spontaneous polarization and tilt angle vs. temperature were performed for a fluorinated ferroelectric compound displaying large spontaneous polarization and tilt angle as well. Transition between the paraelectric and alpha sub-phase was identified as a second order type whereas the alpha sub-phase—ferroelectric phase transition seems to show a tri-critical point behavior. Spontaneous polarization measurements as well as texture observation on a mono-domain thin layer of the sample being under strong electric filed confirm the existence of the sub-phase alpha between para- and ferroelectric phase. It is demonstrated for the first time that in the narrow temperature range of the alpha sub-phase there is a pronounced distortion on the curve presenting temperature dependence of spontaneous polarization.

NMR Study on Ferroelectric (CH3)2NH2H2AsO4

The dimethylammonium dihydrogen arsenate (CH3)2NH2H2AsO4 (DMADA), which belongs to KDP-type crystals, undergoes a ferroelectric phase transition of second order at 274.5K (1⁄4 Tc). The phase transition is characterized by the symmetry change from the monoclinic-space group P21=n to Pn. 1) It was also reported that this crystal shows the drastic isotope effect on the phase transition temperature by deuteration. Moreover, the critical slowing-down phenomenon of polarization has been observed near Tc. 2) These facts suggest that the phase transition at 274.5K is of the order– disorder type and that the hydrogen plays an important role in the phase transition of DMADA. In addition to these properties, this crystal shows the unique and interesting features that spontaneous polarization increases slowly with decreasing temperature below Tc and is not saturated even at 77K. On the other hand, the spontaneous polarization is saturated at a temperature T 1⁄4 Tc 10K in the KH2AsO4 crystal. The replacement of the K ion of KDP family with the dimethylammonium ion will lead to such a tempeature dependence of spontaneous polarization in DMADA. Therefore, investigations on not only hydrogen dynamics at the order–disorder phase transition of 274.5K but also the motion of dimethylammonium ion will be helpful for generalizing the ferroelectricity and ferroelectric phase transition of KDP-type crystals. Recently, we have carried out H-NMR measurements for DMADA. In the present paper, we report the dynamics of hydrogen from the results of the spin–lattice relaxation and the NMR line measurements in DMADA. The DMADA single crystals were grown by slow evaporation method at 303K from an aqueous solution containing (CH3)2NH2 and H3AsO4 in equimolar ratio. The measurement of the H spin–lattice relaxation time T1 was performed using pulse spectrometer (Thamway Prot1002MR) with the frequency of 10.5MHz. The spin–lattice relaxation time was obtained from the recovery of the intensity of the spin echo signal following the saturation comb pulses. The H-NMR lines were observed with the NMR spectrometer by the Q meter detection method. In the NMR measurement, the powder specimens were used. The temperature dependence of H spin–lattice relaxation ratio at 10.5MHz is shown in Fig. 1. The spin–lattice relaxation rate T 1 1 takes a maximum at about 107K, and a sharp peak at 275K (see the inset of Fig. 1). The anomaly of T 1 1 near 275K is characterized by a very sharp peak. This crystal undergoes the phase transition of the order–disorder type at 274.5 K. Therefore, this anomaly of T 1 1 accompanies the phase transition observed at 274.5K. Moreover, we note that T 1 1 shows the change from 83.3 s to 0.285 s below room temperature. In the case that the rotation of methyl group exists, T 1 1 is given by

Dielectric and pyroelectric studies of an antiferroelectric liquid crystal

Polymorphism and polar properties of an antiferroelectric (R)-2-methylheptyloxycarbonylphenyl-4-[(4-decyloxy-3-fluoro)benzoyloxy]benzoate liquid crystal are studied. The phases are identified, and the phase transition points are determined. Dielectric constant, dielectric losses, and pyroelectric properties are studied for the orthogonal smectic SmA and tilted smectic, SmCα*, SmC*, SmCγ*, and SmCA* phases. The temperature dependence of spontaneous polarization is measured by the repolarization current technique and integration of the pyroelectric constant.

Electrical, Optical, Electro-Optical and Electro-Mechanical Properties of Liquid Crystals in Freely Suspended Films and in Periodic Three-Dimensional Array of Nano-Scale Voids in Synthetic Opals

Pyroelectric effect has been observed in freely suspended film (FSF) of ferroelectric liquid crystal (FLC). The temperature dependence of spontaneous polarization evaluated using this effect has been found to be nearly the same as that in the sandwich cell. In the FLC FSF, no threshold has essentially been observed in the electro-optic effect. On the other hand, in transferred film (TF) the threshold exists even at extremely slow scan rate. The threshold field is found to increase with decreasing thickness of TF. In the case of TF the substrate influences strongly the electro-optic effect. The influence of the substrate was found to extend to about 200 smectic layers. Upon acoustic sound irradiation and also electric field application on FLC FSF, the mechanical vibration of the film was effectively excited. For both excitations, resonance vibrations were observed but their oscillating modes were different. The application of FLC FSF to a space electric field sensor has also been discussed. Unique electrical and optical properties of liquid crystals infiltrated in nano-scale voids of synthetic opals with three-dimensional regular array have also been observed.

Hypothetical phase transition, structural disorder and spontaneous polarization in ammonium sulfate crystals

EPR spectra of SeO3 − radicals in ammonium sulfate crystal were studied in the high temperature phase. Two magnetically inequivalent SeO3 − radicals exist in the crystal as a consequence of different coupling to two inequivalent ammonium groups: (NH4)1 and (NH4)11. It is estimated that the distinction between the two SeO3 − radicals would disappear at 585 K, which is interpreted as the hypothetical ferroelastic phase transition. The intensity ratio of the two SeO3 − radicals can be changed by appropriate heat treatment of the sample. It is inferred that the intensity ratio is related to the disorder of protons in the asymmetric potential minima in hydrogen bonds. The unusual temperature dependence of spontaneous polarization was studied in crystals grown from solutions with pH in the range 2.0 to 5.0 as well as in crystals after heat treatment.

The unusual temperature dependence of spontaneous polarization in ammonium sulfate crystals

Abstract EPR spectra of SeO3 radicals in ammonium sulfate crystals were studied in the temperature range (430 – 100) K. Two magnetically inequivalent SeO3 radicals are differently coupled to two crystallographically inequivalent ammonium groups: (NH4)I and (NH4)II. At the ferroelectric phase transition temperature Tc = 223 K, both SeO3 radicals become distorted and displaced from the ab plane towards the polar c-axis. One of these SeO3 radicals and (NH4)I remains temperature independent in the ferroelectric phase, while the other one and (NH4)II change their orientation with temperature in the ferroelectric phase. The unusual temperature dependence of the spontaneous polarization can be attributed to different behavior of (NH4)1 and (NH4)II, which can be studied by EPR of SeO3 radicals.

Dielectric Study on the Phase Transitions in Ca2Pb(C2D5COO)6(DDLP) and Ca2Ba(C2D5COO)6(DDBP)

Dielectric properties of deuterated dicalcium double propionates, Ca 2 Pb-(C 2 D 5 COO) 6 (DDLP) and Ca 2 Pb(C 2 D 5 COO) 6 (DDBP), have been studied. The I–II (the Curie temperature) and II–III transition temperatures of annealed DDLP crystal are 313.1 K and 172 K, respectively. They are 14∼20 K depressed by deuteration. The isotope effect on DLP is much conspicuous in comparison with that of Ca 2 Sr(C 2 H 5 COO) 6 (DSP). The temperature dependence of spontaneous polarization in DDLP is measured and compared with that of Ca 2 Pb(C 2 H 5 COO) 6 (DLP). The I–II and II–III transition temperatures of DDBP are 275 K and 204.5 K, respectively. The deuteration causes an increase of about 8 K in the I–II transition temperature, but does not change virtually the II–III one.

Synthesis, polymorphy and ferroelectric properties of chiral 3,6-diphenyl-1,2,4-triazines

Abstract Seven new ferroelectric esters of 2-chlorocarboxylic acids with 3-(4-hydroxyphenyl)-6-(4n-alkyloxy-phenyl)-1,2,4-triazines were synthesized and their ferroelectric properties were investigated. These compounds exhibit wide range Sc phases (up to 83.5 K) and in two cases high values of spontaneous polarization (up to 260 nC/cm2) were observed. Moreover, in binary mixtures of a 3,6-diphenyl-1,2,4- triazine with 4,4‘-diheptyloxyazoxybenzene the Sc phase range was enhanced to about 100 K and for small mole fraction of the chiral dopant a linear concentration dependence of spontaneous polarization was found.

Specific features op dipole ordering in ferroelectric liquid crystal mixtures

Abstract It has been experimentally demonstrated that dependence of spontaneous polarization of a ferroelectric liquid crystal on the tilt angle can be essentially nonlinear. Concentration dependences of the degree of dipole ordering have been measured for binary mixtures of chiral and achiral cmectic C compounds. The type of these relationships has been shown to be determined by the component molecular structures.

Dielectric properties of ferroelectric Rb2CoBr4

Abstract Single crystals of Rb2 CoBr4 were grown by the Bridgman method. DTA and the dielectric constant measurements revealed that the normal-incommensurate transition point T i is 60°C and the lock-in transition (Curie temperature T c) is—81°C (on heating). The temperature dependence of spontaneous polarization P s, coercive field E c, and antiferroelectric polarization P AF were measured for a single-crystal sample down to liquid nitrogen temperature.