Showing papers in "Ferroelectrics in 1997"

Lead-free piezoelectric ceramics based on (Bi1/2Na1/2)TiO3-NaNbO3

Abstract: Dielectric, ferroelectric and piezoelectric pŕoperties of a BNT-based solid solution, (1-x)(Bi1/2Na1/2)TiO3-xNaNbO3, that is, Bi(1-x)/2Na(1+x)/2 Ti1-x Nb x O3 (BNTN-100x), are studied from the viewpoint of a new group of lead-free piezoelectric ceramics. The conventional ceramic fabricating technique was used to prepare BNTN ceramics. D-E hysteresis loops could be easily observed with a relatively large remanent polarization, P r, of about 25∼33 μC/cm2 and a high coercive field of about 50 kV/cm near the composition of BNT side. The electromechanical coupling factors, k p, k t, and k 33 were about 0.108, 0.424 and 0.433 for BNTN-3. BNTN ceramics seem to be a candidate for lead-free piezoelectric ceramics.

On the potential energy of a piezoelectric inclusion and the criterion for ferroelectric switching

Abstract: The potential energy of an infinite homogeneous piezoelectric loaded body containinga single ellipsoidal transforming inhomogeneity is stated in terms of the fields in the inclusion. Explicit results are given for a piezoelectric inclusion in an isotropic matrix having the same elasticities and dielectric permittivity as the inclusion. The results are used to construct criteria for the first crystallite to switch in an unpolarized polycrystalline ferroelectric.

Ionic conductivity in ferroic CuInP2S6 and CuCrP2S6

Abstract: ac electrical measurements are reported for the paraelectric phases of the ferroelectric CuInP2S6 and antiferroelectric CuCrP2S6 A complex permittivity analysis confirms previous dielectric results for CuInP2S6 Arrhenius behaviors with nearly identical activation energies were observed for the dc conductivities and modulus relaxation frequencies A hopping mechanism for Cu(I) transport in these materials is thus suggested

Ab initio linear response study of SrTiO3

Abstract: The lattice instabilities of perovskite structure oxides are responsible for many of their interesting properties, such as temperature-dependent ferroelectric phase transitions. First-principles calculations using linear response theory provide an accurate means to determine the lattice dynamics throughout the entire Brillouin zone (BZ). Using the LAPW linear response method[1], we have previously carried out such a study on ferroelectric KNbO3 [2]. We present here the results of a similar investigation for cubic SrTiO3. While KNbO3 has only ferroelectric-type instabilities, SrTiO3 exhibits both ferroelectric (FE) and antiferrodistortive (AFD) instabilities. We correctly predict the known instability at the R-point in the Brillouin zone, which is responsible for the AFD phase transition to the tetragonal structure at about 105 K. Furthermore, the phase space of the ferroelectric instability is greatly reduced compared to KNbO3. Antiferrodistortive instabilities exist in one-dimensional cylindrica...

Domain formation in thin ferroelectric films: The role of depolarization energy

Abstract: Formulae for equlibrium stripe domain width Weq in a nonductive ferroelectric plate of thickness d are deduced, taking into account electrostatic interaction of surfaces. It is shown that the classical formula giving W eq ∞ d 1/2 is not applicable when the sample thickness decreases below the value dcrit which is a function of dielectric properties and domain wall energy density. For many ferroelectrics the value of dcrit lies in a range which can be easily reached far below the transition point by contemporary thin film techniques; it further increases as the transition point is approached. In the region d < dcrit the width Weq increases with decreasing d. For samples with thickness d << dcrit the domain structure becomes insensitive to electrical boundary conditions and will be primarily determined by other factors.

Surface effects in ferroelectrics: Periodic slab computations for BaTiO3

Abstract: Total energies, electronic structure, surface energies, polarization, potentials and charge densities were studied for slabs of BaTiO3 using the Linearized Augmented Plane Wave (LAPW) method. The depolarization field inhibits ferroelectricity in the slabs, and the macroscopic field set up across a ferroelectric slab is sufficient to cause electronic states to span the gap and give a metallic band structure, but the band shifts are not rigid and O p states tend to pile up at the Fermi level. There are electronic surface states, especially evident on TiO2 surfaces. The dangling bonds bond back to the surface Ti's and make the surface stable and reactive. The BaO surfaces are more ionic than the bulk.

On the birefringence of magnetoelectric BiFeO3

Abstract: The birefringence dispersion of a ferroelastic single domain of magnetoelectric BiFeO3 crystal has been measured at room temperature (24°C) in the rhombohedral phase. A thin (11μm) (110)cubic cut was used although it was not a principal cut. The optical axis (and spontaneous polarization) formed an angle of 35° with the normal to the cut. Due to anomalous interference colors, the right order was deduced with the help of conoscopy. The birefringence of this cut shows normal dispersion and its value is very large (dn′ = 0.13 at λ = 550 nm to 0.08 at λ = 850 nm). The principal birefringence (dn = 0.34 at A = 550 nm) has been evaluated with the help of the computed (Gladstone-Dale relationship) mean refractive index ([nbar] = 2.62). From dn′ (T), (8K < T < 820 K) the Neel temperature has been found for the first time optically, TN =653K (380°C), and a critical exponent has tentatively been derived near TN .

Optical detection of symmetry breaking on a nanoscale in SrTiO3:Ca

Abstract: Spontaneous and field-induced linear birefringence is measured at low temperatures on single-domained single crystals of weakly doped SrTiO3:Ca (x = 0·002 and 0·007) in order to investigate polar ordering processes near to Tc ≈ 4 and 18 K, respectively. In agreement with direct polarization measurements the size of mesoscopic polar nanodomains is determined within the framework of a nonlinear polarization optical response model. Quasi-first-order Raman scattering of soft and hard polar modes, TO1, TO2 and TO4, is observed on local precursor nanodomains above Tc . Below Tc the change of the global symmetry from D4h to C2v is reflected by selection rules imposed on first-order Raman lines in various scattering geometries.

Kinetics of ferroelectric domain structure: Retardation effects

Abstract: We review the experimental and theoretical investigations of the kinetics of ferroelectric domain structure in single crystals and thin films. The significant influence of the retardation of screening process on the evolution of the domain structure is pointed out and is demonstrated in various experimental situations.

High temperature quantum paraelectricity in perovskite-type titanates Ln1/2Na1/2TiO3 (Ln = La, Pr, Nd, Sm, Eu, Gd and Tb)

Abstract: The La3+ ion of a quantum paraelectric perovskite La1/2Na1/2TiO3 was substituted with the smaller tripositive lanthanide ions Ln3+. The X-ray structural study and Rietveld refinement of Ln1/2Na1/2TiO3 for Ln=Pr to Tb showed that they are synthesized as orthorhombic perovskite structure with the space group Pnma. Their lattice parameters and dielectric constants decreased from Pr to Tb with an increasing volume of TiO6-octahedron. A high temperature quantum paraelectricity was found for Ln = Pr to Eu. Because of negative extrapolated Curie temperature, the process of explanation used for the typical quantum paraelectric SrTiO3 is not applicable to the quantum paraelectricity in Ln1/2Na1/2TiO3 for Ln = La ∼ Eu.

Investigation of magnetoelectric interaction in composite

Abstract: Theoretical approach on the magnetoelectric effect in two-phase composite materials consisting of ferrite and piezoelectric components at various frequencies is developed. The mechanism responsible for the magnetoelectric effect is discussed. The generalised magnetoelectric sensitivity tensor is calculated. The estimates of magnetoelectric sensitivity at low and high frequencies are given. Some experimental results of the systems nickel-cobalt ferrite/piezoelectric ceramics of PZT-type and yttrium iron garnet/barium titanate are presented. The brief review of the previous work is given.

Pyromagnetic domain walls connecting antiferromagnetic non-ferroelastic magnetoelectric domains

Abstract: We describe a group-theoretical procedure that enables one to find necessary conditions for the appearance of spontaneous magnetization in domain walls. We illustrate the derivation of wall symmetries on example of Cr2O3 and present a brief summary of a systematic analysis of domain walls in antiferromagnetic non-ferroelastic and magnetoelectric phases which shows that in more than 50% of possible domain walls a spontaneous magnetization may appear.

Magnetoelectric microwave phase shifters

Abstract: We discuss magnetoelectric microwave devices. Combination of properties of ferrite and ferroelectric, and also the properties, induced by magnetoelectric (ME) interaction, find wide functional opportunities for creation of devices on the basis of ME materials. ME materials can be used for design of microwave phase shifters. Phase shifters will be most simply realised on effect of shift of a line of ferromagnetic resonance (FMR) under the action of a control electrical field. We shall consider an example of ME microwave through-passage microstrip phase shifter. As a working element a composite material, consisting of 95% from yttrium-iron garnet and of 5% from piezoceramics PZT is used. Phase shifters on ME materials have high performance, adaptability to manufacture, electromagnetic compatibility, radiating durability, thermostability, low power in control circuit.

Effects of annealing on the structure and switching characteristics of VDF/TrFE copolymers

Abstract: Structure-property relationship has been investigated in concern with the ferroelectric switching characteristics of a VDF(78)/TrFE(22) copolymer. Unoriented and oriented film samples prepared by melt-quenching and melt-extrusion, respectively, were subjected to annealing and resulting structural changes were characterized by SEM, X-ray, density and DSC measurements. It was found that annealing just above the Curie point (120–130°C) caused a marked increase in crystallinity, whereas annealing in the paraelectric state (130–145°C) induced a crystal growth into thick lamellae. Such structural changes resulted in the ferroelectric switching characteristics with larger remanent polarization and shorter switching time. The unoriented sample yielded a smaller remanent polarization than the oriented sample by a factor of π/4, which substantiated that polarization reversal occurred as a result of rotation of molecules rotation about their chain axis. The melt-extruded and well-annealed sample consisted o...

Effect of hydrostatic pressure on the structural phase transitions in CH3NH3PbX3 (X = Cl, Br, I)

Abstract: The dielectric properties and the effect of hydrostatic pressure on the successive phase transitions have been studied for single crystal CH3NH3PbX3 (X: Cl, Br, I). Anisotropic anomalies in dielectric constant at phase transitions are obtained. The pressure (p)-temperature (T) phase diagrams are shown for the three CH3NH3PbX3 crystals. The intermediate tetragonal phase of CH3NH3PbCl3 and the lower-temperature tetragonal phase (space group: P4/mmm) in CH3NH3PbBr3 become narrower as pressure increases and disappear at around 100 MPa. There appears a pressure-induced phase above about 360 MPa and 90 MPa in CH3NH3PbBr3 and CH3NH3PbI3, respectively. On the other hand, CH3NH3PbCl3 does not show a pressure-induced phase up to about 700 MPa and the p-T phase diagram is different from those of the Br- and I-salts. A common reduced p-T phase diagram is proposed for CH3NH3PbBr3 and CH3NH3PbI3 and discussed.

Charge transfer vibronic excitons as an origin of a new quantum paraelectric regime (Müller phase) and of polar microdomains in incipient perovskite-like ferroelectrics

Abstract: The unified mechanism of a new quantum paraelectric regime (Muller phase) as well as of polar microdomain forming in incipient perovskite-like ferroelectrics is considered. Muller phase (MP) is formed by charge-transfer vibronic excitons (CTVE) with intrinsic elastic and electric dipole moments. CTVE form order-disorder clusters with co-operative lowering of energy due to charge transfer ordering. Such CTVE clusters are characterized by long life time and can be considered as proper defects. The transition to MP (T = TQ ≈ 37 K in SrTiO3) corresponds to local ferroelectric-type transition in such CTVE-clusters. This local ferroelectric transition is induced by interaction between lattice soft TO-mode and soft polarization mode of CTVE cluster. This is the mechanism of polar microdomain appearing. Simultaneously the model gives a new explanation of soft TO-mode stabilization as a result of repulsion from local polarization modes of CTVE clusters. The coexistence of order-disorder and displacive typ...

Ferroelectric domains and domain boundaries observed by scanning force microscopy

Abstract: Splitting of ferroelectric crystals into domains is a bulk phenomenon. However, many investigation methods rely on observations of intersections of domains or domain boundaries with crystal surfaces. Among them scanning force microscopy promises to be one of the most powerful. In this paper we discuss the different modes of operating the scanning force microscope in order to obtain a contrast between domains with antiparallel polarization. TGS has been chosen as a model material both for its suitable cleavage properties and the wealth of previous knowledge about its domain properties. We have found that antiparallel domains - identified by their lenticular shape - are discernible in both the non-contact mode and the friction force mode of imaging. We offer explanation of the origin of contrast, taking into account forces of topographic origin and those connected with spontaneous polarization. Higher resolution studies made it even possible to evaluate the upper limit of the domain wall thickness ...

Lattice dynamics and ferroelectric instability of barium titanate

Abstract: Using first-principles calculations, we investigate the lattice dynamics of cubic and rhombohedral BaTiO3 and rye discuss the origin of the structural instability of this ferroelectric material. First, we report results on the Born effective charges and the dielectric tensor and we emphasize the important reduction of these quantities in the ferroelectric phase, Then, the phonon frequencies at the Gamma point are calculated. We point out the similarity of theoretical eigenvectors in the cubic and rhombohedral phases. We examine the interaction of the vibration modes with the electric field and in particular the giant LO-TO splitting of the ferroelectric mode. Finally, separating the dipole-dipole interaction from the remaining short-range forces, we quantify the balance of forces leading to an unstable phonon in the cubic phase and we demonstrate its sensitivity to tiny effective charge changes. Within our decomposition, the stabilization of the unstable mode in the rhombohedral phase is produced by a reduction of the Born effective charges, while its stabilization under isotropic pressure is associated with a modification of the short-range forces.

Spin-flop and incommensurate structures in magnetic ferroelectrics

Abstract: Incommensurate structures in uniaxial magnetic ferroelectrics are theoretically investigated. Two different phases called “easy-axis like” and “easy-plane like” have been found. The transition between them is compared with the xylspin-flop” in antiferromagnets. The critical field is obtained. The second derivative of the susceptibility with respect to the magnetic field is shown to have the jump at the critical field. The effect of higher order anisotropy is discussed.

Origin and characteristics of internal fields in LiNbO3 crystals

Abstract: We report the presence of an in-built internal field in Z-cut LiNbO3 crystals of 3.35KV/mm in the direction of the polarization in a virgin crystal. Upon polarization reversal under an electric field, the internal field tends to realign along the new polarization direction. At room temperature, this realignment process was incomplete even after many days, while on annealing at 250°C for 2 min followed by cooling down, ∼ 80% recovery was observed (from + 3.35 KV/mm to −2.14 KV/mm). A change in the infrared absorption spectrum of OH− ions in LiNbO3 crystal was also observed upon polarization reversal at room temperature. Upon annealing above 200°C for a few minutes followed by cooling to room temperature, the shape of the spectrum partially recovers to that of the virgin crystal at room temperature, indicating a strong correlation between OH− ions and internal field in LiNbO3 crystals. We also show that 180° domains in a polarization reversed LiNbO3 crystal can be observed by light microscope, sugg...

Progress in single crystal growth of LiNbO3 using double crucible Czochralski method

Abstract: In order to grow single crystals with high quality and high homogeneity from off-congruent melts, a double crucible Czochralski method has been developed. In this method, the melt was divided into two parts by a double structure (two chamber system) crucible. The crystal was grown from the inner melt, and powder with the same composition as the growing crystal was supplied to the outer melt. The powder was supplied smoothly at the rate of weight increase of the growing crystal. This rate was automatically controlled to match the weight increase of the growing crystal as monitored by a load cell. In combination with an automatic diameter control system, a fully automatic double crucible Czochralski method was established. Using this method, LiNbO3 single crystals with a composition close to stoichiometric were grown, and their photorefrative properties were compared with those of Nb-rich congruent melt LiNbO3 crystals. Two beam coupling experiments show that both non-doped and Fe-doped stoichiomet...

Local atomic structure of relaxor ferroelectric solids determined by pulsed neutron and x-ray scattering

Abstract: The origin of relaxor ferroelectric behavior is complex, and is likely to include both compositional as well as structural order and disorder. Non-crystallographic methods of structural study by pulsed neutron and synchrotron X-ray scattering applied to these solids reveal mas-sive local displacive disorder and short range compositional order in these solids. Examples of such local structural complexities in PMN and PLZT are discussed.

Lattice instabilities, anharmonicity and phase transitions in PbZrO3 from first principles

Abstract: An effective Hamiltonian for the antiferroelectric transition in PbZrO3 is constructed from first-principles density-functional-theory total-energy and linear-response calculations through the use of a localized, symmetrized basis set of “lattice Wannier functions''. The effective Hamiltonian consists of three-component vector degrees of freedom at the Pb sites and two-component vector degrees of freedom at the oxygen sites. This effective Hamiltonian allows us to analyze the structural energetics and identify the role of various physical contributions. We find that there exists a strong competition between sets of coupled unstable modes in producing low energy states. Two such groups of instabilities found from the effective Hamiltonian are seen to be relevant to the experimental ground and intermediate states.

On the perovskite-related materials of high dielectric permittivity with small temperature dependence and low dielectric loss

Abstract: Dielectric properties of quantum paraelectric materials, SrTiO3, CaTiO3 and (La1/2Na1/2)TiO3, were presented and discussed since they show the titled dielectric properties in the lowest te...

Origin of ferroelectricity in LiNbO3 and LiTaO3

Abstract: The origin of the ferroelectric materials LiNbO3 and LiTaO3 was studied using extensive LAPW calculations. The coupled displacements of the oxygens and lithiums give a deep double well whereas oxygen alone displacements result in a shallower double well as a result of the oxygen-lithium overlap. Density of states calculations show hybridization between the transition metals and the oxygens, and the origin of the ferroelectric instability in these systems is similar to the ferroelectricity in perovskites. The electronic structures of the two materials is compared with the related perovskite ferroelectrics KNbO3 and KTaO3.

Dynamic behavior of polar nanodomains in PbMg1/3Nb2/3O3

Abstract: Temporal relaxation of the electric field-induced polarization in single crystalline PbMg1/3Nb2/3O3 has been measured at temperatures 180 ≤ T ≤ 230 K and times up to 104 s via polarization-optically induced linear birefringence, Δn 1/2. The data are excellently described within the framework of Chamberlin's dynamically correlated domain theory. Whereas activated Curie-von Schweidler-type cluster-flip behavior prevails in the paraelectric regime, T 210 K, Kohlrausch-Williams-Watts-type domain wall relaxation characterizes the ferroelectric nanodomain state at low temperatures.

Time evolution of the phase transformation in Na0.5Bi0.5TiO3

Abstract: For Na0.5Bi0.5TiO3 single crystals the time dependencies of dielectric and structural properties were investigated. The obtained results reveal that phase transformation in Na0.5Bi0.5TiO3 between tetragonal and rhombohedral symmetries disclosed a time dependent character. This transformation was completed after about 300 minutes.

Grain size dependence of relaxor characteristics in La-modified lead zirconate titanate

Abstract: Studies have been performed on a series of La-modified lead zirconate titanate (PLZT) specimens with grain sizes between 0.6 and 6 μm by dielectric spectroscopy and transmission electron microscopy. These investigations have revealed an increase in the degree of relaxor characteristics with decreasing grain size for tetragonal-structured PLZT. In addition, small grain sizes were found to favor tweed-like structures and polar nanodomains, over normal micron-sized ferroelectric domains.

Ferroelectric phenomena in holographic properties of strontium-barium niobate crystals doped with rare-earth elements

Abstract: We report on a relation between the two-beam coupling gain factor Γ in Sr0·61Ba0·39Nb2O6 (SBN:0·6) crystals and their ferroelectric properties. In SBN:0·6 doped with rare-earth-metal impurities Ce and Tm we found a hysteresis dependence of Γ versus the external dc-field E applied after recording which is due to the ferroelectric P-E hysteresis. Particularly, Γ-E hysteresis manifests itself in an alteration of the sign of Γ, i.e., of the direction of the energy transfer between the coupling beams, under applying of E reversing Ps. The value of Γ at room temperature essentially increases with impurity concentrations. It is caused by an increase of the dielectric permittivity, which in its turn is due to a significant lowering of the phase-transition temperature Tc produced by a rare-earth doping.

Some properties of NaO0.5Bi0.5TiO3

Abstract: The specific heat of Na0.5Bi0.5TiO3 crystal in the temperature range 300–650 K and the changes of spontaneous polarization of Na0.5Bi0.5TiO3 ceramic in the temperature range 4.2–90 K were measured. The calorimetric measurements showed that the anomaly of specific heat exists in the temperature range where the phase transition rate from the rhomhedral phase to the tetragonal phase is utmost. From low-temperature pyroelectric effect a phonon wave number equal to 128 cm−1 was found to have the main share in the temperature changes of spontaneous polarization.