Showing papers in "Ferroelectrics in 1995"

Electric field induced cracking in ferroelectric ceramics

Abstract: Ferroelectric ceramics are susceptible to fracture under high magnitude cyclic electric field. Flaws concentrate the electric field, inducing a large incompatible strain, and thereby a large stress. Stable growth of cracks with either conducting or insulating interiors is observed in 8/65/35 lanthanum lead zirconate titanate samples. Indentations on the electroded surface are filled with distilled water or a water-salt solution. Under cyclic electric field, tree like damage grows from the indented electrode. Indentations on the surfaces 90° to the electrodes are filled with silicone oil. This results in stable crack growth perpendicular to the cyclic electric field. Nonlinear fracture models are presented for both conducting and insulating cracks. Tensile stress intensity factors are predicted for both cases.

X-ray diffraction study of the phase transitions in Na0.5Bi0.5TiO3

Abstract: X-ray investigations of Na05Bi0.5TiO3 crystals showed the coexistence of rhombohedral and tetragonal phases in the wide temperature range. The temperature dependence of phase concentrations can explain the anomaly of dielectric properties observed at 280°C and 320°C temperatures.

X-ray study of the electric field-induced phase transition in single crystal Pb(Mg1/3Nb2/3)O3

Abstract: High-resolution X-ray diffraction experiments were performed on a single crystal of PMN in the temperature range of 90-300 K, by applying an electric field (up to E = 3.0 kV cm-1) along the [111] direction. A first order electric field-induced phase transition, from the mean cubic phase to a rhombohedral Phase, was evidenced by discontinuous variation of the d222-spacing upon cooling with E ≥ 1.8 kV cm-1. Results suggest that a tricritical point exists in the (E, Tc) phase diagram of PMN for E ≥ 3.8 kV cm-1. Upon field heating no phase coexistence was detected, unlike the field cooling runs, and the rhombohedral-cubic phase transition appears to be of second order at least for E = 3.0 kV cm-1.

Pair-distribution functions of ferroelectric perovskites: Direct observation of structural ground states

Abstract: Experimental information about the microscopic structure and the changes in this structure at macroscopic phase transitions for the ferroelectric perovskites provide direct tests of theoretical descriptions of ferroelectricity. Unfortunately, the local ferroelectric displacements can differ from the average periodic structure and may not be correctly described using crystallographic techniques; however, the use of real-space pair-distribution functions, which can now be obtained from scattering data with unprecedented accuracy, provides an excellent opportunity for studying the local structure. We compare crystallographic and local structures for BaTiO3, and KNbO3 above and below the low-temperature phase transitions and for PbTiO3, at low temperatures, both derived from the same powder neutron scattering data. These results elucidate the nature of the ground-state structures in each of these materials and provide insight into the mechanisms for the phase transitions.

Piezoelectric properties of (Bi1/2Na1/2)TiO3 based ceramics

Abstract: In this paper, piezoelectric properties of (Bi1/2Na1/2)1–x (Sr1/2Ca1/2) x TiO3 system were studied from the viewpoint of lead-free piezoelectric materials To improve electromechanical properties in this system, MnO was added to this system In the composition of x = 013 which was close to morphotropic phase boundary (MPB), electromechanical properties such as kt and Qm were increasing with increase of the amount of MnO and took maximum value (kt ≊ 47%, Qm ≊ 367) at about 04 wt% On the other hand, dielectric constant ϵ r was decreasing with it and took comparatively lower value (ϵ33/ϵ0 = 518)

Recent development of the vibronic theory of ferroelectricity

Abstract: The vibronic origin of ferroelectricity and structural phase transitions first suggested three decades ago, has been given recently new theoretical foundation and strong experimental evidence. It has been shown that for any polyatomic system, including crystals, the vibronic mixing of the ground electronic state of the high-symmetry reference configuration with the excited states by low-symmetry nuclear displacements is the only source of instability of this configuration, and that visually the main contribution to this effect is due to the formation of new covalent binding by distortion. The proof of this statement allowed us to show that the driving force for ferroelectric distortions of the lattice, the vibronic coupling, is of local origin. The long-range Coulomb interactions are important in that they may soften the lattice, but they don't produce instability. It is shown that the interplay of local instability enhanced by long-range forces with the entropy factor that requires disorder at T ≠ 0, may...

Investigations of the phase transitions in Na0.5Bi0.5TiO3

Abstract: The dielectric, pyroelectric, electrostrictive, optical and thermal properties of Na0.5Bi0.5TiO3, in wide temperature range, particularly in the range of 200–340[ddot]C are presented and discussed. The results of investigations show the complicated nature of phase transition occurring in this material.

Improvement of piezoelectric properties of poly(vinylidene fluoride) and its copolymers by crystallization under high pressures

Abstract: We describe the results of our work performed to improve piezoelectric properties of poly (vinylidene fluoride) (PVDF) and its copolymers with trifluoroethylene (TrFE) and with tetrafluoroethylene (TeFE) by means of crystallization under high pressures. In addition, a short review is given on the related work reported in literature. Analysis of VDF concentration dependences of Curie temperature, melting point, and electromechanical coupling factor (kt ) of P (VDF/TrFE) revealed that, for the improvement, PVDF and VDF-rich P (VDF/TrFE) have to be crystallized into extended chain crystals in the hexagonal phase which exists only at high pressures. On the basis of the pressure-temperature phase diagrams obtained by differential thermal analysis, we studied high-pressure crystallization of these polymers, and evaluated piezoelectric properties of the resulting materials. A PVDF film, initially melted at 270°C under low pressure and then crystallized in the hexagonal phase by increasing pressure up to...

Local density and generalized gradient approximation studies of KNbO3 and BaTiO3

Abstract: Density functional total energy and electronic structure calculations for KNbO3 and BaTiO3 using a generalized gradient approximation (GGA) functional are reported. The results are contrasted with those of parallel local density approximation (LDA) calculations. For KNbO3 a larger equilibrium volume in considerably better agreement with experiment is obtained with the GGA functional. Neither the electronic structure, the energetics of the ferroelectric mode at fixed volume, nor the volume dependence of the ferroelectric instability were substantially changed from their LDA values. In BaTiO3 the GGA functional also yields an increased equilibrium volume relative to the LDA. However, in contrast to KNbO3 the volume is over-corrected, leading to little if any improvement. Calculations for BaO were performed in order to understand the origins of this overcorrection. A similar overestimation of the equilibrium volume in BaO was obtained.

Annealing effect on ferroelectric phase transitional behavior of vinylidene fluoride-trifluoroethylene copolymers: An interpretation based on the concept of domain and trans-gauche conformational disorder

Abstract: The annealing effect on the structure and ferroelectric phase transitional behavior of vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymer with VDF 73 molar % has been investigated by the thermal analysis, X-ray diffraction and Fourier-transform Raman scattering measurements. Heat treatment of the sample in the temperature region below 125°C increases the transition temperature largely, which correlates intimately with an increment of the domain size along the chain axis and the growth of the all-trans chain stems in a crystallite. When the sample is annealed in the temperature region between the phase transitional point and the melting point, the X-ray coherent domain size decreases and the gauche-type disordered region increases within the domain, resulting in the decrement of the transition temperature.

Light-induced absorption changes in reduced lithium niobate

Abstract: Reduced LiNbO3 crystals change their optical absorption under illumination with blue or green pump light. We measure the dependence of this effect on pump intensity I at different probe wavelengths. Steady-state absorption changes and build-up velocities are found to be proportional to the square root of I. Rates of relaxation towards equilibrium in the dark increase exponentially with temperature. Our experimental results can be well described by light-induced dissociation of bipolarons into small polarons unstable at room temperature.

Computer simulation of structure and ferroelectric phase transition of vinylidene fluoride copolymers (1) vdf content dependence of the crystal structure

Abstract: Force field parameters of fluorine polymers have been refined so as to reproduce well the X-ray analyzed crystal structures and the observed IR/Raman spectra of various crystalline forms of poly (vinylidene fluoride) (PVDF). By using the thus established force field parameters, the crystal structure of the low-temperature phase of vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymers has been simulated. The VDF content dependence of the crystal structural parameters and the vibrational spectra could be reproduced reasonably by this computer simulation.

Study of the thermal and dielectric behavior of P(VDF-TrFE) copolymers in relation with their electroactive properties

Abstract: On the basis of the pyroelectric spectra—pyroelectric coefficient p 3 versus temperature—of β-PVDF and P(VDF-TrFE) 75/25 mol%, recorded between -100 and 70°C, we have investigated transition and relaxation phenomena involving both the crystalline and amorphous phases of P(VDF-TrFE) copolymers for TrFE unit content in the range 0–50 mol% To this aim, complementary techniques, DSC and TSC were used The classical glass transition of the copolymers (Tgl) manifests by a clear change in slope in the p 3 versus T diagram From its variation in DSC as a function of the weight fraction of TrFE units, we deduced a Tg of-12°C for PTrFE An upper component of the glass transition, Tgu, has been found at ≈10–20°C in the pyroelectric spectrum It has been associated to the amorphous phase constrained by crystallites The pyroelectric coefficient increases strongly in the glass transitions range due to the large thermal expansion coefficient of the amorphous phase in the rubber-like state The crystalline α

Dielectric, ferroelectric and electric field-induced strain properties of (Pb1-xBax)(Zr1-yTiy)O3 ceramics

Abstract: Dielectric, ferroelectric and electric field-induced strain properties of (Pb1-xBax)(Zr1-yTiy)O3 ceramics with 0.15 ≤ x ≤ 0.45 and 0.16 ≤ y ≥ 0.70 were investigated. A wide range of compositions possessing relaxor phase characteristics was found to exist near the boundaries between the ferroelectric rhombohedral, tetragonal and paraelectric cubic phases. Within this compositional range, broadened peaks of dielectric constant as a function of both composition and temperature were identified as well as slim loop hysteresis and frequency-dependent relaxation behavior. Longitudinal field-induced strain for some selected samples and transverse strain for all the compositions studied were determined. Maximum total and differential strains were found in the vicinity of the rhombohedral-tetragonal phase boundary. Optimal values of the differential strain were close to (about 80%) those found in PLZT ceramics. Inside the relaxor phase region the relationship between the strain and electric field (or polar...

In situ observations of octahedral tilt transitions in strontium bismuth titanate layered perovskites

Abstract: In situ transmission electron microscopy has been used to study Bi4Ti3O12, SrBi8Ti7O27, SrBi4Ti4O15 and Sr2Bi4Ti5O18, layered perovskites. A cell-doubling phase transition has been found in all compounds at temperatures above the ferroelectric phase transitions. This transition is associated with tilting of the octahedra in antiphase around the c axis, causing doubling in the a,b plane. As the concentration of Sr increases in the compounds the temperatures at which the ferroelectric and cell-doubling transitions occur decrease. However, the rate of decrease for each transition is different and the temperature of the two phase transitions becomes almost coincident in the Sr2Bi4Ti5O18 compound.

Diffusion-induced domain inversion in ferroelectrics

Abstract: The basic property of ferroelectric crystals is switching of the direction of the macroscopic electric dipole moment under applying of an external electric field. Recently the polarization reversal effect without the external electric field has been discovered. It occurs under some treatments, namely, during proton-exchange. Ti,Rh-indiffusion in LiNbO3, out(in)-diffusion in LiNbO4, and LiTaO3, under Rb ionexchange and out(in)-diffusion at elevated temperature in KTiOPO4. The effect takes place at any polar surface and it is observed both far and near the Curie temperature. As a result a bidomain structure is formed. The diffusion-induced polarization reversal in ferroelectrics represents a new physical phenomenon and it is widely used in modern electronics and nonlinear optics. The studies of the effect are based on several traditional and new methods providing necessary data about the spontaneous polarization distribution and electrical parameters of these structures. In this paper, we review re...

Size driven phase transition and thermodynamic properties of nanocrystalline batio3

Abstract: Submicron powders of BaTiO3 with narrow size distribution were prepared by a modified sol-gel method. X-ray diffraction, Raman spectroscopy, calorimetry and Kurtz powder tests on nonlinear optical ...

Crystal structure refinements of disordered pb(sc0.5nb0.5)o3 in the paraelectric and ferroelectric states

Abstract: The crystal structure of the relaxor ferroelectric Pb(Sc0.5Nb0.5)O3 has been refined from high-resolution neutron time-of-flight powder diffraction data in the paraelectric and ferroelectric states. In the ceramic used in this study, no evidence was found for unit cell doubling, associated with B-site cation ordering, consistent with the (NH4)3FeF6 structure in the paraelectric state. At 400 K, in the paraelectric state, Pb(Sc0.5Nb0.5)O3 is cubic, space group Pm3m, with lattice parameter 4.08153(1) A, and at 200 K, in the ferroelectric state, it is rhombohedral, space group Ram, with lattice parameters a = 4.08207(1) A and a = 89.872(1)°. Calculated absolute displacements in the ferroelectric state along the [111] ferroelectric axis are for Pb 0.094 &, Sc/Nb -0.148 A. and O -0.195 A. The 3.0 A coordination shell around the Pb cation shows evidence for a stereochemically active lone pair, whilst by contrast, the distortion in the SclNb octahedral site is small.

Dielectric aging and its temperature dependence in ferroelectric ceramics

Abstract: In acceptor doped perovskite type ceramics the decrease of the complex dielectric constant and the build-up of electric and elastic internal bias fields are caused by the slow orientation of dipola...

Magnetoelectric effect and low-temperature phase transitions in YMn2O5

Abstract: A possibility of ferroelectricity has been proposed in some low temperature phases of the oxides of the series RMn2O5 (R = a rare earth from Nd to Lu, Y or Bi). YMn2O5 is unusual in this series, because Y3+ is non-magnetic. We made dielectric, magnetic and magnetoelectric measurements on single crystals of this oxide to detect two electric transitions at 39 K and 19 K. These transitions are accompanied with anomalies in the dielectric constant and the magnetic and the magnetoelectric susceptibilities. Particularly, the sign of the magnetoelectric susceptibility is switched below 19 K, depending on the sense of the electric field applied during cooling the sample down to 4.2 K. This fact indicates that YMn2O5 possesses spontaneous polarization at least below 19 K.

Electromechanical properties of thin ferroelectric Pb(Zr0.53Ti0.47)o3-layers

Abstract: A method is described for improving the possibility and sensitivity of measurements of electromechanical properties of ferroelectric thin layers. This method is especially useful for investigating electrostrictive properties. A nonlinear model for the description of the electrical conductivity is presented. First results on the temperature, electric field and strain dependence of the high signal electrostrictive coefficient Q11 are given.

Order-disorder behavior in the phase transition of PbTiO3

Abstract: We have quantitatively measured the temperature dependence of the local distortions of PbTiO3, crystals below and above the structural ferroelectric phase transition using X-ray Absorption Fine Structure (XAFS) measurements. Both Pb and Ti edges were measured, providing quantitative determinations of the displacements of the atoms within the unit cell. These distortions vary little with temperature below the transition and decrease only slightly above the transition temperature. In the paraelectric phase, the Pb and Ti distortions are about 50% and 70% of the corresponding low temperature values respectively. These results show that an essential element of order-disorder is present even in this ferroelectric crystal which displays the soft mode behavior and a dielectric constant typical of displacive ferroelectrics. The presence of the local distortions suggests that the displacements should have at least two correlation length scales, one associated with the local distortions, the other with the order pa...

Ferroelectric phase transitions: A first-principles approach

Abstract: Fundamental aspects of the theoretical study of bulk ferroelectric phase transitions through the construction of model Hamiltonians from first-principles calculations are reviewed. It is demonstrated that density-functional-theory linear-response calculations and total-energy calculations with symmetry-constrained ionic relaxation are very useful in the determination of the local mode variable and of the parameters in an explicit expression for the model Hamiltonian. Results of first-principles pseudopotential calculations of the lattice constant, phonon frequencies, and distortion energies for the perovskite ferroelectric PbTiO3 are presented. From this information, the local mode variable is identified, a model Hamiltonian is constructed and its finite-temperature behavior is studied with mean-field theory and Monte Carlo simulation. Very rough agreement with the observed Tc is obtained and various improvements and refinements of the calculation proposed. A well-controlled method for investigation of th...

On problems of search for novel Bi-containing layered perovskite-like high-Tc ferroelectrics

Abstract: A crystallochemical analysis of structural parameters of Bi-containing layered perovskite-like oxides (BLPO) of the type Am-1Bi2BmO3m + 3 is given. The morphotropic series, the technique of calculations of unit cell parameters and existence conditions are considered for the first time.

Polarized Raman spectra of Cs5H3(SO4)4· H2O single crystals

Abstract: The Raman spectra of the superprotonic conductor Cs5H3(SO4)4·H2O have been investigated in the 300-500 K range. An assignment of bands due to internal and external vibrations in term of approximate type of motions is proposed. The spectroscopic results are used for discussing crystal structures of different phases, the type of structural disorder and phase transitions.

A quasielastic neutron scattering study of protonic transport in hydrogen-bonded alkali metal hydrogen sulphates and selenates

Abstract: The diffusion of protons in the superprotonic phases of CsHSO4, Cs3H(SeO4)2, Rb3H(SeO4)2 and Cs5H3(SO4)4-H2O have been studied by quasielastic neutron scattering on the IRIS spectrometer at the ISIS pulsed neutron spallation source The data were analysed on the basis of the jump diffusion model via unfilled proton positions in the superprotonic phases Quantitative characteristics of the diffusion process (mean residence time, jump length, activation energy) were found to be consistent with the structural features of the superionic phases and with the experimental data obtained by other methods Some common characteristic features of the mechanism of protonic transport in these compounds are discussed

Very high pressure behavior of precisely-poled PVDF

Abstract: Ferroelectric polymers with well defined and precisely known electrical properties are now routinely available from commercial sources Electrical processing with the Bauer cyclic poling method can routinely produce individual samples with a range of remanent polarizations up to 9 μC/cm2 Such well defined samples provide an unusual opportunity to carry out experiments which study the structure and physical properties of the materials In the present case, the behavior of PVDF has been studied over a wide range of pressures with the destructive, but precise, method of very-high-pressure shock loading The method of loading provides accuracy and precision of pressure of about 1% In studies to 10 GPa, the nonlinear piezoelectric behavior of PVDF is carefully studied Studies to pressures of 30 GPa are available which show that the piezoelectric behavior is linearly dependent on volume compression to a close approximation

Dielectric, electro optic and X-ray studies of a room temperature ferroelectric mixture

Abstract: Dielectric spectra for the SmA* and SmC* phases of a room temperature ferroelectric liquid crystalline mixture FELIX-008 have been studied in the frequency range from a few Hz to 13 MHz. It has been found that in the vicinity of the SmA*-SmC* transition there is a pronounced soft mode dielectric spectrum, the dielectric parameters of which obey a Curie-Weiss law given by the mean-field theory. In the SmC* phase a distinguished Debye-type dielectric spectrum of the Goldstone mode has been detected, and upon increasing the bias field to 35 V it is suppressed and a new non-Debye-type dielectric spectrum-originating most probably from the so-called domain mode-shows up. Spontaneous polarization and tilt angle measurements show that the transition SmA*-SmC* is of the second order type. The dielectric and electro optic studies make it possible to compute the rotational viscosity, which is of great importance for practical applications. It turns out that the energy barrier for rotational viscosity (19.6...

Ferroelectricity in A 2 BX 4-type halide compounds

Abstract: A review was given for our search for halide ferroelectrics with the chemical formula A 2 BX 4-type, having structure of β-K2SO4-type and Sr2GeS4-type.

Random fields influence on dynamic properties of disordered ferroelectrics

Abstract: The theoretical model for description of dielectric relaxation in highly polarizable dielectrics with dipole impurities like K1–x Li x TaO3 (KTL) is suggested. In this model the distribution function of relaxation times is expressed through distribution function of random electric fields of impurity dipoles, which in turn calculated on the base of dipole-dipole interaction law only. The derived expressions for dynamic dielectric permittivity together with equation for long-range order parameter determine completely the relaxational properties of the system under consideration in whole range of temperatures and impurity content: in dipole glass state, in mixed ferroglass phase, in paraelectric and ferroelectric phases. It is shown that in dipole glass state relaxation has long-time logarithmic character, while in mixed phase the crossover between glassy and purely Debye dynamics, peculiar to paraelectric and ferroelectric phases occurs. It is shown also that Cole-Cole diagram in mixed phase has tw...