Pretransitional Effects Research Articles

Pre-Transitional Effect and an Intermediate Phase in the Antiferroelectric PbZr0.80Sn0.20O3 Single Crystal

The temperature dependences of electrostrictive strain and birefringence for a PbZr0.80Sn0.20O3 single crystal have been measured with a high accuracy close to the high temperature phase transitions into the paraelectric phase. Characteristic features are the existence of local maxima on M 11(T) dependence and irregular Δn(T) dependences that indicate existence of an intermediate phase between the antiferroelectric and paraelectric phase. Above TC a local Δn(T) maximum appears on heating caused probably by interactions of polar regions embedded in the non-polar cubic matrix. The existence of these regions could be accounted for by a disorder of Sn ions in the lead zirconate lattice.

Dielectric and orientational properties of nematogenic 4-n-octyl(4′-cyanophenyl)benzoate

AbstractExperimental studies of the dielectric properties in the nematic and isotropic phases of 4-cyanophenyl-4′-n-octylbenzoate (8CPB) have been performed in the frequency range from 50 kHz to 100 MHz. The relaxation process related to the rotation around the short molecular axis has been analysed. The results obtained have revealed that in the vicinity of the I-N phase transition, the molecular subdiffusional rotation process occurs. It is connected with the pretransitional effects which are observed both in the static and dynamic dielectric measurements. On the basis of the temperature dependence of the relaxation time in the nematic and isotropic phases, the orientational order parameter 〈P2〉 has been determined. The values of 〈P2〉 obtained in this way have been compared with those evaluated from the measurements of the polarized electronic absorption of the dichroic dye, dissolved as guest probe in the mesogenic host.

Phase Transition Studies in Liquid Crystals across I-N and N-Sc Phases in Alkoxy Benzoic Acids – Density Measurements

Density is one of the simple and accurate techniques to obtain the information regarding the nature of the transition and pre transitional effects across any phase transformation in liquid crystals. Density is measured with temperature in well known alkoxy benzoic acids with alkoxy chain number from 3 to 12 and 16. Even though the density measurements are carried on some of the homologues in this series, we have carried out systematic studies on all the homologues available and the results are compared with the literature values. Whereever there is deviation the reasons for such deviations are explained.

Thermodynamic Functions of a Liquid Crystal through Density Measurements

Thermodynamic functions of a material are determined using ultrasonic measurements coupled with volumetric measurements at different temperatures. It is also possible to determine some of these thermodynamic functions using only volumetric measurements. Such studies are important in the case of liquid crystal as they give insight in the structural changes arising out of transition between two phases. Apart from this, these studies are also found to give clear information regarding the nature of phase transition, pre-transitional effects, and molecular ordering. Some of the thermodynamic functions calculated are adiabatic compressibility, order parameter, Sharma's number, etc. In the present study these different parameters have been determined using only volumetric measurements in case of 4,4 – azoxy anisole at different temperatures. Variations in various parameters with respect to temperature have been discussed in the light of the results obtained. In the literature the data for these parameters are available for the above said liquid crystal calculated in conjunction with ultrasound measurements. The results obtained in the present work are compared with these results. It has been shown that the variation of the properties observed with respect to temperature can be explained on the basis of Frenkal's heterophase fluctuation theory.

PMRD investigations of molecular dynamics and phase transitions in the liquid crystal 6CHBT

Proton Magnetic Relaxation Dispersion (PMRD) measurements were carried out over a wide Larmor frequency range near the isotropic-nematic transition temperature (T NI) and in the mid nematic phase of a low viscous liquid crystal 4-(trans-4′-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT), with a view to examining the pre-transitional effects just above isotropic-nematic transition (T > T NI) and the role of director fluctuations (DF) on the nematic phase stability of this system. The results near the transition indicate critical slowing down of the short-range nematic order modes just above T NI with a mean field critical exponent characterizing the coherence length (ξ). The observed temperature independence of the correlation time associated with the reorientations (τR) and the extension of DF mode spectrum, in the mid-nematic phase, to relatively higher resonance frequencies seem to suggest fairly low hindering barriers for the tumbling of the molecules about their short axes.

The double-peak phenomenon of the phase transition of Rb substituting K in LiKSO4, competition between Rb sub-lattice and smearing the phase transition

The influence of Rb in the triple sulfate LiK(1−x)RbxSO4 (0 ≤ x ≤ 0.2) compound on the structural and thermal properties of the bulk material is discussed. Samples were prepared by fusing the sulfates in stoichiometric ratios in a platinum crucible at 1273 K for 30 min. The x-ray powder diffraction measurements were performed at ambient temperature. The crystal structure parameters were refined by the Rietveld method using the FullProf program. The analysis indicated a single-phase having a hexagonal symmetry with space group P63 regardless of the Rb content. Thermal analysis by differential scanning calorimetry shows a considerable shift of the phase transition point towards lower temperatures with increasing Rb content. A pre-transitional effect is observed by differential thermal analysis of the sample without Rb contribution while for the sample with 20% Rb the transition is represented by double endothermic peaks. The energy of transition has been estimated using the Kissinger method for samples with different concentrations of Rb.

Dielectric pretransitional effects in the isotropic phase of chiral liquid crystals

We have performed dielectric measurements on chiral liquid crystalline systems. From the values of the static dielectric permittivity, it is observed that the dielectric pretransitional effect in the isotropic phase on approaching the isotropic to cholesteric or isotropic to blue phase transition is virtually not influenced by the chirality of the system. The relaxation times do not show anomalous behaviour near the transition, although a small overall slowing down is observed in mixtures with increasing chirality.

Singular Pretransitional Behavior of the Electric Field-Dependent Part of the Thermodynamic Quantities of Strongly Polar Mesogenic Liquids in the Isotropic Phase

A new interpretation of the pretransitional temperature behavior of the static dielectric permittivity and its derivative in terms of the basic thermodynamic quantities: the internal energy, the entropy, and the Helmholtz free energy is presented. It was shown that in the case of strongly polar mesogenic compounds (as the homologous series of alkylcyanobiphenyls) the electric field-induced increments of both the internal energy and the entropy exhibit a critical-like temperature dependence in the vicinity of transition from the isotropic liquid to the nematic phase. As a most important pretransitional effect, it is shown that at a temperature of about 10 degrees before the phase transition, an electric field applied to the isotropic liquids induces an increase in the entropy (∆S > 0) instead of its decrease, as observed far from the phase transition or for the non-mesogenic dipolar liquids.

Field theory for size- and charge-asymmetric primitive model of ionic systems: Mean-field stability analysis and pretransitional effects

The primitive model of ionic systems is investigated within a field-theoretic description for the whole range of diameter-, lambda , and charge, Z ratios of the two ionic species. Two order parameters (OP) are identified. The relation of the OP's to physically relevant quantities is nontrivial. Each OP is a linear combination of the charge density and the number-density waves. Instabilities of the disordered phase associated with the two OP's are determined in the mean-field approximation (MF). In MF a gas-liquid separation occurs for any Z and lambda is not equal to 1 . In addition, an instability with respect to various types of periodic ordering of the two kinds of ions is found. Depending on lambda and Z , one or the other transition is metastable in different thermodynamic states. For sufficiently large size disparity we find a sequence of fluid-crystal-fluid transitions for the increasing volume fraction of ions, in agreement with experimental observations. The instabilities found in MF represent weak ordering of the most probable instantaneous states, and are identified with structural loci associated with pretransitional effects.

The solid state phase transformation of potassium sulfate

Potassium sulfate single crystals that are grown from aqueous solutions lose, upon the first heating, up to 1% of mass that is assumed to be water. This mass loss occurs in the vicinity of the PT from orthorhombic to hexagonal K 2SO 4. Only in the first heating run of K 2SO 4 that has not yet released water, pretransitional thermal effects can be observed in the DTA curve. If K 2SO 4 crystals are grown from solutions containing 4 wt% Cd, Cu, or Fe, only Cu or Fe can be incorporated significantly with concentrations of several 0.1%. The phase transformation temperature measured for such solid solutions depends on the heating rate. For pure K 2SO 4, the phase transformation temperature is independent of heating rate 581.3 ∘C and the enthalpy of transformation is (5.8±0.2) kJ/mol.

Characterization of Some Liquid Crystals through Sound and Thermodynamic Parameters

The structural changes arising from transition between the mesophases of liquid crystalline materials can be better understood by ultrasound in conjunction with specific volume properties. In particular, ultrasonic studies are found to give clear information regarding the nature of the phase transition, pretransitional effects, and molecular ordering. For example, temperature corresponding to the maximum value of acoustic impedance, adiabatic compressibility, and temperature corresponding to the minimum value of ultrasonic velocity, Rao's number, order parameter, and molar compressibility, in thermotropic liquid crystals are very useful in understanding their structural behavior. In view of these facts, it was thought worthwhile to study ultrasonic velocity with data available for density measurements on pure samples of nematics, namely, 4,4′-azoxy anisole and 4,4′-dibutoxy azoxy benzene. The results of this work, are presented in this article and discussed.

Dielectric and viscous properties of mesogenicn-nonylcyanobiphenyl (9CB)

Static and dynamic linear dielectric properties and shear viscosity offreely flowing 9CB in the isotropic (I), nematic (N) and smectic A(SA) phases were investigated. Pretransitional effects observed in the vicinity of the I to N phasetransition manifest themselves as anomalies in the temperature dependence of the staticpermittivity, the rotational diffusion exponent and the activation energy of moleculesrotating around their short axis. As a presmectic effect, a jump in the temperaturedependence of the shear viscosity of freely flowing nematic 9CB is observed at about0.5 °C before thetransition to the SA phase.

Weakly first-order character of the nematic-isotropic phase transition in liquid crystals

The classification of phase transitions in first-order and second-order (or continuous) ones is widely used. The nematic-to-isotropic (NI) transition in liquid crystals is a weakly first-order transition, with only small discontinuities in enthalpy and specific volume at the transition which are not always easy to measure. On the other hand, fluctuation effects near the transition, typical for a continuous transition, are present because of the only weakly first-order character. In a recent paper [Phys. Rev. E 69, 022701 (2004)], it was concluded from the static dielectric permittivity in the isotropic phase near the NI transition that less polar mesogens (with little or no pretransitional effects) are characteristic for a first-order NI phase transition, whereas in the case of strongly polar ones (with large pretransitional effects) the NI transition is close to second order. In this paper, we address the question whether it is, indeed, possible to use these fluctuation effects in the isotropic phase to quantify the "strength" of a weakly first-order transition, i.e., how far it is from second order. Therefore, we measured the temperature dependence of the enthalpy near the NI transition of seven liquid crystals with adiabatic scanning calorimetry and compared the measured values of the latent heat with pretransitional effects in the dielectric constant and the specific heat capacity. The compounds used in the comparison are MBBA, 5CB, 8CB, 5NCS, 5CN, 8CHBT, and D7AB. From our analysis we find, contrary to the assertion in the above reference, no correlation between the strength of the NI transition of a given compound and the pretransitional effects observed, neither dielectrically, nor thermally.

Phase Transitions and Pre-transitional Effects in N-(p-n-Pentylbenzylidene)-p-n-pentylaniline (5.5) and its Oxygen Derivatives – A Dilatometric Study

The density of N-(p-n-pentylbenzylidene)-p-n-pentylaniline (5.5) and its two oxygen derivatives, viz. N-(p-n-pentylbenzylidene)-p-n-pentyloxyaniline (5.O5) and N-(p-n-pentyloxybenzylidene)-p-npentyloxyaniline (5O.O5), is reported for the isotropic (I), nematic (N), smectic-G (SG) and crystalline (K) phases. Dilatometric results reveal the first order nature of the I-N, N-SG and N-K transitions. A rare occurrence of the N-K transition is found for the compound 5O.O5 at a considerably higher temperature. From the observed density data pre-transitional effects are estimated for the isotropic to the first mesomorphic phase. The results are discussed in the light of data reported on other Schiff base benzylidene anilines

Infrared and Raman studies of the phase transition in the organic conductor (TTM-TTP)I 3

We report the polarized IR reflectance as well as Raman scattering investigations of the organic charge transfer salt (TTM-TTP)I 3 as a function of temperature, below and above the metal–insulator phase transition at T = 160 K. The IR reflectance was measured in the frequency region from 600 to 10,000 cm −1, for the electrical vector of the polarized light parallel and perpendicular to the TTM-TTP stacking axis. For the polarization parallel to stacks the IR spectra are typical for semiconducting charge transfer salts. The electronic part of IR spectra was analysed in terms of a Lorentz model and temperature dependence of the optical transport parameters was determined. For the polarization perpendicular to the stacks we observed two electronic bands at about 5000 and 8000 cm −1. The phase transition at 160 K has nearly no influence on the IR spectrum. The Raman scattering for different excitations ( λ = 514.5, 632.6 and 785 nm) was mainly studied within the region of C C stretching vibrations. In this frequency range, three Raman lines at 1426, 1453 and 1486 cm −1 attributed to TTM-TTP molecules are observed. Below 160 K a splitting of the band 1486 cm −1 into two peaks at about 1488 and 1498 cm −1 is found. The intensity and temperature behaviour of the split band at 1498 cm −1 is strongly dependent on sample. The observed spectral modifications are related to an asymmetric deformation of TTM-TTP. Taking into account temperature dependence of bands attributed to the C H stretching and SCH 3 bending vibrations, we suggest that the TTM-TTP deformation can exist also above the phase transition temperature. Above 160 K molecules fluctuate between distorted and symmetrical state forming non-stable domains (pre-transitional effects), but below 160 K the molecular distortion and domains are stable. The existence of electronic band at 5000 cm −1 for the polarization perpendicular to TTM-TTP supports this picture.

Structure and fluctuations of phosphatidylcholines in the vicinity of the main phase transition.

We have determined the structural properties and bending fluctuations of fully hydrated phosphatidylcholine multibilayers in the fluid (Lalpha) phase, as well as the structure of the ripple (Pbeta') phase near the main phase transition temperature (TM) by x-ray diffraction. The number of carbons, nHC, per acyl chain of the studied disaturated lipids varied from 14 to 22. All lipids exhibit a nonlinear increase of the lamellar repeat distance d in the Lalpha phase upon approaching TM, known as "anomalous swelling." The nonlinear increase reduces with chain length, but levels off at a constant value of about 0.5 A for lipids with more than 18 hydrocarbons per chain. A detailed analysis shows that anomalous swelling has two components. One is due to an expansion of the water layer, which decreases with chain length and finally vanishes for nHC >18. The second component is due to a bilayer thickness increase, which remains unchanged in its temperature dependence, including a nonlinear component of about 0.5 A in the vicinity of TM. Thus, anomalous swelling above 18 hydrocarbons per chain is due to the pretransitional effects on the membrane only. These results are supported by a bending fluctuation analysis revealing increased undulations close to TM only for the short chain lipids. We have further calculated the electron density maps in the ripple phase and find no coupling of the magnitude of the ripple amplitude to the chain length effects observed in the Lalpha phase. Hence, in agreement with an earlier report by Mason et al. [Phys. Rev. E 63, 030902 (2001)] there is no connection between the formation of the ripple phase and anomalous swelling.

Switching Behaviour of Antiferroelectric Liquid Crystals

We present a model of antiferroelectric liquid crystals that explains a number of phenomena observed in their switching properties. These include the pretransitional effect, the thresholded transitions between the antiferroelectric and ferroelectric states, the frequency dependence of the hysteresis loop and the origin of the v-shaped switching observed in some antiferroelectric materials.

Study of dipole dynamics and pre-transitional effects at isotropic to nematic phase transition by low-frequency dielectric relaxation measurements

Thermal microscopy (TM) as well as low-frequency (LF) dielectric relaxation studies are carried out on a nematic liquid crystal (NLC), viz E7 (Merck Ltd, UK). The isotropic to nematic (IN) transition temperatureT IN determined by TM and LF-dielectric permittivity measurements agrees with the available data. Dielectric loss studies in the frequency region of 5–10 MHz indicate a relaxation (in the kHz region) akin to Debye type off-centered dispersion. The observed nematic relaxation is found to correspond to reorientation (about the short axis) of the nematic dipole to the external field. The temperature variation of the nematic relaxation frequencyf R is found to follow an Arrhenius shift, with an activation energy of 1.7 eV. Temperature variation of the dielectric strength (Δε = ε o − ε∞) and the distribution parameter α in the nematic phase are discussed. The dynamic response of the nematic dipoles and growth of pre-transitional fluctuations are found to be nonlinear in the vicinity of the IN transition. The value of the exponent αeff = 0.072 indicates weak growth of transitional fluctuations across the IN transition.

Ultrasonic and Piezoelectric Investigation of Sn2P2S6 Type Photosensitive Ferroelectric-Semiconductor Crystals

The investigation results of ultrasonic and piezoelectric properties of Sn2P2(S,Se)6, CuInP2S6 and CuCrP2S6 ferroelectric-semiconductors single crystals are presented. In Sn2P2S6 type crystals ultrasonic attenuation caused by acoustoelectric interaction after illumination showed fast and long term relaxations. Anomalies of ultrasonic velocity and attenuation at phase transitions have been established in CuInP2S6 and CuCrP2S6 crystals. The piezoelectric detection of longitudinal ultrasonic signal by thin plate of CuCrP2S6 crystal was observed. In the high temperature phase pretransitional effects in ultrasonic and piezoelectric properties were clearly seen. Such effects can be determined by polar clusters, existing above phase transition temperature.

Tilt plane orientation in antiferroelectric liquid crystal cells and the origin of the pretransitional effect.

The optic, electro-optic, and dielectric properties of antiferroelectric liquid crystals (AFLCs) are analyzed and discussed in terms of the local tilt plane orientation. We show that the so-called pretransitional effect is a combination of two different electro-optic modes: the field-induced antiphase distortion of the antiferroelectric structure and the field-induced reorientation of the tilt plane. In the presence of a helix, the latter corresponds to a field-induced distortion of the helix. Both electro-optic modes are active only when the electric field has a component along the tilt plane. Thus, by assuring a horizontal surface-stabilized condition, where the helix is unwound by surface action and the tilt plane is everywhere parallel to the cell plates, the pretransitional effect should be suppressed. We also discuss the dielectrically active modes in AFLCs and under which circumstances they contribute to the measured dielectric permittivity.