Antiferroelectric Transition Research Articles

Comparative study of the dielectric properties of an antiferroelectric liquid crystal in planar aligned cells and in microporous membrane

The dielectric properties of the antiferroelectric liquid crystal (AFLC) have been investigated under different confined geometries. The measurements of dielectric permittivities have been carried out in the frequency region from 10 Hz to 13 MHz in two planar aligned cells of thickness 2 μm and 10 μm, as well as in an Anopore membrane having pore diameter 0.2 μm and thickness 60 μm. In SmC ⁎ phase the Goldstone mode (GM) has been observed in both the 10 μm and 2 μm planar aligned cells. Dielectric strength of GM is significantly diminished whereas the relaxation frequency is increased in the 2 μm thick cell compared to that in the 10 μm thick cell. In SmC ⁎ A phase two antiferroelectric modes have been observed in two distinct frequency regions. In the lower frequency region below the Goldstone mode process another relaxation process is observed in both cells that arise due to the non-homogenous distribution of thermally generated ionic charges. The Goldstone mode present in planar aligned cells is not observed when the sample is confined in an anopore membrane. But two other modes are observed in the Anopore membrane. One is due to molecular reorientation around the short axis is usually observed in the quasi-homeotropic orientation of tilted smectics. The other mode is related to the internal curved surface of the anopore membrane. The results have been discussed.

NaNO2 + NaNO3 Phase Diagram: New Data from DSC and Raman Spectroscopy

The binary phase diagram of sodium nitrite and sodium nitrate is currently not well-understood because four earlier determinations of this binary system resulted in considerably different conclusions. Two works classify the system as eutectic and two as a continuous solid solution type, together with different sub-solidus curves. New Raman measurements and differential scanning calorimetry data on solidified mixtures of different compositions have provided support for a simple eutectic diagram with a solidus at 230 °C ranging from 0.25 to 0.80 in mole fraction of sodium nitrate. A sub-solidus transition was detected close to 183 °C and a second one close to 160 °C. The latter was considered to be related to the nitrite component and probably arose from the ferroelectric to antiferroelectric or paraelectric transition of sodium nitrite. The change of slope of the maxima of some Raman lines with temperature also gave support to these transitions.

Evidence for a New Non-Ferroelectric Phase Transition in (Pb1−xCax)TiO3 Ceramics for 0.60 ≤ x ≤ 0.90

Results of low temperature dielectric measurements on the mixed (Pb 1−x Ca x )TiO 3 system are presented to show that the strong dielectric anomaly observed for compositions with x < 0.60 is replaced by a very weak dielectric anomaly for x ≥ 0.60. The temperature dependence of the unit cell parameters, as obtained by Rietveld analysis of the neutron powder diffraction data, shows change of slope at the dielectric anomaly temperature. The observed dielectric peak for x ≥ 0.60 is shown to be due to a thermodynamic phase transition and not due to a relaxor or dipole glass type transition. Absence of hysteresis loop below the transition temperature reveals that this new phase transition is of non-ferroelectric type. Curie-Weiss fit to the dielectric constant gives negative Curie temperature which points towards an antiferroelectric transition.

Antiferroelectric Modes Behaviour in a Wide Temperature Range SmC*A Phase

ABSTRACT Two liquid crystalline partially fluorinated compounds: (S)–(+)–4-(1-metyloheptyloxycarbonylphenyl) 4′-(6-perfluorobutanoyloxyhex-1-oxy) biphenyl-4-carboxylate, in short MHPP(9F)HBC and (S)–(+)-4-(1-metyloheptyloxy-carbonylbiphenyl) 4′-(6-perfluorobutanoyloxyhex-1-oxy) benzoate, in short P(9F)HPMHBC, exhibiting paraelectric, ferroelectric and antiferroelectric phases have been studied by dielectric spectroscopy and spontaneous polarization measurements. These two compounds have the same molecular weight but their terminal chains at para positions are interchanged. Influence of molecular structure on the phase diagram and dielectric behaviour is presented. Two relaxation process revealed in the antiferroelectric phase are interpreted as collective modes: anti-phase and in-phase fluctuation of phase and amplitude. The reorientation around the short molecular axis does not contribute to the dielectric spectra.

Dielectric spectroscopy of an antiferroelectric liquid crystal showing an antiferroelectric–ferrielectric transition

We report the dielectric relaxation behaviour in the antiferroelectric SmCA* and ferrielectric SmCγ* phases of the antiferroelectric liquid crystal 4-[5-(4-octloxyphenyl)-2-pyrimidinyl]phenyl 4,4,4-trifluoro-3-(methoxyphenyl)butanoate which shows an antiferroelectric transition at around 88±0.1°C. In the SmCA* phase, two dielectric relaxation modes have been found, namely the usual antiferroelectric Goldstone mode and another arising from molecular rotation around its short axis. In the SmCγ* phase, one dielectric relaxation mode has been observed due to the ferrielectric Goldstone mode. Dielectric increments and relaxation frequencies of the antiferroelectric and ferrielectric phases are estimated from the fits of the Cole–Cole function of the dielectric spectrum. The dependence of the bias field in the ferrielectric phase is also discussed.

Annealing Effect of the Phase Transition in PbIn½Nb½O3 Crystal

The annealing effect of the phase transition in PbIn½Nb½O3 (PIN) crystal was investigated by means of the dielectric constant measurement and the polarizing microscopic observation. Using one and the same sample plate of the PIN, it was observed that the antiferroelectric transition gradually changes into the diffuse phase transition as the annealing temperature increases. On the basis of the observation by an optical microscope, the annealing effect of the diffuse phase transition in the relaxor was discussed.

Defect-induced phase separation in relaxor Pb(In1/2Nb1/2)O3 crystals

Thermal properties of ordered and disordered Pb(In1/2Nb1/2)O3 (PIN)single crystals were investigated. For the ordered PIN, typical thermalanomalies were observed at the antiferroelectric transition temperature.These anomalies were degraded and disorder appeared during heatingprocesses. The defect-induced phase separation provided a good account of theinstability of the relaxor and antiferroelectric states of the PIN crystals.

Structural, electric and magnetic properties of Pb 2Mg 1− xCu xWO 6

X-ray diffraction, EPR and electric measurements performed on Pb 2Mg 1− x Cu x WO 6 (PMCW) are reported. By Cu substitution for Mg, the diffraction lines are similar to that of Pb 2MgWO 6 (PMW), but a small shift of peaks toward small angles is observed. For 0.3≤ x≤0.5, new lines appear due to an unknown phase. The EPR spectrum is due to the Cu 2+ ions. Dielectric anomaly at T c (38 °C) related to the antiferroelectric transition in Pb 2MgWO 6 is affected by Cu substitution for Mg. T c increases linearly with Cu content and transition becomes more diffuse.

Quantum Theoretical Evidence for Two Distinct Hydrogen-Bonding Networks and for an Ising Chain Model of the Antiferroelectric Transition in Squaric Acid

The electronic structure of squaric acid (H2C4O4, SQA) has been investigated by density functional techniques at the B3LYP/6-31G** level to understand the mechanism of the paraelectric-to-antiferroelectric phase transition in the compound at 373 K. A nonsymmetrical structure without a C4 axis has been used for the first time. Monomer, dimer, and pentamer models of the SQA system were used to assess the possible differences between the two intersecting O−H···O chains. Despite the use of a small cluster size, we felt that the noted differences in the energetics and related characteristics of the two chains in the crystal lattice are realistic. This result suggests different proton dynamics along the chains and thus argues against the generally accepted 2-D phase-transition behavior. The calculations also help explain the literature discrepancies involving earlier 13C NMR and diffraction data. They support a model in which the phase transition is triggered by thermally induced disorder of the H atoms in the more π-resonant O···H−O bonds, which in turn leads to the 3-D ordering of the two mutually perpendicular Ising-type chains.

Coupled spin-charge dynamics inNaV2O5and its effect on the optical absorption

We argue that in the quarter filled ladder compound ${\mathrm{NaV}}_{2}{\mathrm{O}}_{5}$ the quasi-one-dimensional spin system is strongly coupled to an antiferroelectric mode with a broad band. This mode is the single-electron excitation on a rung of the two-leg ladder, which can hop from rung to rung due to the Coulomb interactions between electrons. We show that the coupling of the antiferroelectric mode to spins gives rise to the infrared absorption continuum, observed in ${\mathrm{NaV}}_{2}{\mathrm{O}}_{5}.$

A Density Functional Study of the Complex Nature of the Hydrogen-Bond Network and Mechanism of the Antiferroelectric Transition in Squaric Acid

An extensive electronic structure analysis of squaric acid (H 2 C 4 O 4 , SQA) was performed by density functional techniques, including a serie of hydrogen-bonded systems to model the solid state compound. In addition, some magnetic properties were calculated by GIAO method. As result, we obtain a theoretical model able to characterize with accuracy the 13 C NMR data of this type of systems with an antiferroelectric phase transition. A closer connection between hydrogen bonding and aromaticity was found in SQA. Thus, there are two different hydrogen bonds in each molecule of the solid, associated to the two clearly distinct O=C-C=C-OH resonant chains. Consistently, the study of the proton transfer process in SQA shows two energetically different coordinates for the hydrogen movement. Moreover, the theoretical NMR results show that a hydrogen bond lengthening is not coherent with the experimental evidences for the phase transition in SQA. Current theoretical results support a new description of the phase transition mechanism in SQA.

Structures and phase transitions in some Pb2B′B″O6 complex perovskite-type compounds

In order to study the influence of substitutions and of the ionic radii of B’ ions on structure and phase transitions, we present some physical measurements and data on Pb2B′B″O6 (B”=W, Nb, Mo) complex perovskite compounds. By Mn and Fe substitution for Mg in Pb2MgWO6 compound a change in lattice parameters and shift of antiferroelectric transition temperature is observed. It appears that in all three series of compounds a linear dependence between ferroelectric or antiferroelectric transition temperature and B’ ionic radii is observed.

High temperature x-ray diffraction studies on antiferroelectric and ferroelectric phase transitions in (Pb1−xBax)ZrO3 (x=0.05,0.10)

We have carried out high temperature x-ray diffraction studies on (Pb1−xBax)ZrO3(PBZ) to correlate the large thermal hysteresis (∼100 °C for x=0.05) and irreversibility (for x=0.10) of the antiferroelectric (AFE)–ferroelectric (FE) phase transition observed in dielectric measurements with structural changes. It is shown that for both the compositions, the sequence of phase transitions during heating is orthorhombic antiferroelectric (AO) to rhombohedral ferroelectric (FR) and then to cubic paraelectric (PC). The wide phase coexistence region (∼80 °C for x=0.05 and ∼160 °C for x=0.10) and the arrest of the FR to AO transition for x=0.10 during cooling strongly indicate first order character of the AO–FR transition. It is shown that the transformation strains associated with the AO to FR transition increases with Ba2+ concentration from a value of 0.6% for x=0 to 0.9% for 0.10. Similarities of the AO–FR transition in PBZ with nonthermoelastic martensitic transformations are pointed out. The FR to PC transition is also shown to be first order but with a small thermal hysteresis (∼10 °C) and a small discontinuous change in the cell volume (∼0.5%).

ELECTRO-OPTIC AND DIELECTRIC PROPERTIES OF AN ANTIFERROELECTRIC LIQUID CRYSTAL MATERIAL

Results reported in this paper on the electro-optic properties and dielectric relaxation in the SmCA* and SmC* phase of a liquid material as a function of sample thickness. The measurements were carried out in 3μm, 6μm, and 10μm thick cells. In the whole SmC* phase two modes were obtained in the low frequency region of the dielectric spectra in each of the three cells without bias electric field. The relaxation strength of both the modes sharply decreases with the applied bias dc electric field. One of the modes is assigned as Goldstone mode, arises due to phase fluctuation and the origin of the other mode may be related to the space charge accumulated at the interface between liquid crystal and the polymer coating. In antiferroelectric liquid crystal (SmCA*) phase two switching current peaks were observed only in a 3μm thick cell by applying triangular wave of frequency around 30 Hz-40 Hz. Two antiferroelectric modes were obtained in the dielectric spectra of SmCA* phase in the low frequency region and the results have been dicussed. Both the results of spontaneous polarization and dielectric studies reveal that the antiferroelectric behavior in the SmCA* phase is squeezed in each of the three cells.

Phase diagram of mixed ferroelectrics

The integral equations for calculation of ferroelectric and antiferroelectric transition temperatures, order parameters and critical concentrations of mixed system components have been derived. The electric dipoles randomly distributed in mixed system were considered as random field sources. This field distribution function with nonlinear and spatial correlation effect contribution and with different orientations of the components electric dipoles has been written. The concentration dependence of A1−xBx mixed system transition temperature was calculated for the cases when A and B materials are antiferroelectric and ferroelectric respectively and when A and B materials are ferroelectric and paraelectric. The numerical calculation were carried out for PbTixZrxO3 and BaZrxTii-xO3. The obtained results describe rather good experimentally observed phase diagrams of these mixed systems. The physical reasons of strongly different behaviour of the systems (in particular the relaxor properties of BaZrxTi1−xO3 at x>0.25) and peculiar role of lead ions in appearance of ferroelectric and antiferroelectric phase transitions are discussed.

Ferroelectric and antiferroelectric transitions in random copolymers of vinylidene fluoride and trifluoroethylene

Copolymers of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) containing 50–80% former undergo a ferroelectric-to-paraelectric transition and then melt. The transition entropies determined by thermal measurements are shown to be consistent with the conformational entropies of the paraelectric and molten phases that are comprised of statistical combinations of TT, TG and TG' bonds. As the VDF-content exceeds 80%, melting overtakes the transition into the paraelectric phase. TrFE-rich copolymers containing 30–50% VDF exhibit an additional phase that is shown to be antiferroelectric-like on the basis of nonlinear dielectric investigation. Ferroelectricity tends to be obscured with increasing TrFE content and disappears at PTrFE. The dependence of the transition behavior on the composition of VDF and TrFE is discussed phenomenologically. It is shown that the most essential role is played by the cohesive energy of the ferroelectric phase that decreases with increasing TrFE content due to its asymmetry and bulkiness. The resulting increase in the Gibbs energy induces appearance of the paraelectric as well as antiferroelectric phases as the VDF content is further decreased.

Ferroelectric and antiferroelectric transitions in random copolymers of Vinylidene fluoride and trifluoroethlene

Abstract Copolymers of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) containing 50–80% former undergo a ferroelectric-to-paraelectric transition and then melt. The transition entropies determined by thermal measurements are shown to be consistent with the conformational entropies of the paraelectric and molten phases that are comprised of statistical combinations of TT, TG and TG' bonds. As the VDF-content exceeds 80%, melting overtakes the transition into the paraelectric phase. TrFE-rich copolymers containing 30–50% VDF exhibit an additional phase that is shown to be antiferroelectric-like on the basis of nonlinear dielectric investigation. Ferroelectricity tends to be obscured with increasing TrFE content and disappears at PTrFE. The dependence of the transition behavior on the composition of VDF and TrFE is discussed phenomenologically. It is shown that the most essential role is played by the cohesive energy of the ferroelectric phase that decreases with increasing TrFE content due to its as...

μ–H Interaction in Hydrogen Bonding Systems

Muon spin rotation (μSR) experiments were performed on single crystal samples of KH2PO4(KDP) and KD2PO4(dKDP) to study the dynamics of hydrogen in hydrogen bonding systems. At low temperature, the nuclear dipole interaction of muon and proton was confirmed from the angular dependence of precession frequency of the muon spin under zero magnetic field. The muon occupation site was also determined. A clear change in μSR spectra was observed at the antiferroelectric transition temperature (123 K). At 90 K well below the transition temperature, the muon spin starts to relax, possibly due to muon dynamics.

An investigation on the structural evolution of Pb 2YbTaO 6

The structure of the complex perovskite Pb 2YbTaO 6, reported to undergo two phase transitions at 285 and 180°C, respectively, was analysed within the temperature range 30–320°C by DSC measurements and X-ray diffraction. The occurrence of just one thermal effect was revealed in DSC curves, corresponding to the first-order paraelectric–antiferroelectric transition, while no signal was detected at the reported antiferroelectric–ferroelectric transition. The structure of the high-temperature phase was confirmed to be elpasolite-type, with complete ordering of Yb and Ta on octahedral sites. The structure at 220°C was refined in space group Pbnm (n. 62), with lattice parameters derived from the cubic one as a o=( a c+ b c)/2, b o=2(− a c+ b c) and c o= c c. The shifts of all atoms with respect to the cubic positions occur along the (100) orthorhombic direction, which corresponds to an original 〈110〉 cubic one. The two independent Pb atoms are shifted in opposite directions. A mode analysis shows that the shifts of heavy atoms can be described by the modulation associated with the condensation of the lattice mode Σ 3 of wavevector q:(3/4,3/4,0).

Pyroelectric spectrum in Pb(Zr,Sn,Ti)O3 antiferroelectric-ferroelectric ceramics

The pyroelectric effect of phase transition induced with temperature in Nb-modified Pb(Zr,Sn,Ti)O3 antiferroelectric-ferroelectric ceramics is studied. Experimental results reveal that the phase transitions are accompanied with marked pyroelectric peaks, there exists the close relation between the type of phase transition and the shape of pyroelectric peak. Because of the variations of phase transition, various pyroelectric spectra result. The pyroelectric spectrum can display the polarization effect and some inferior phase transitions with temperature variations, such as antiferroelectric AFEA-AFEB or ferroelectric FEL-FEH transition, which are not detected by the conventional dielectric measurement.