Paraelectric Materials Research Articles

Do we really need ferroelectrics in paraelectric phase only in electrically controlled microwave devices?

Typical paraelectric materials (e.g., SrTiO/sub 3/, KTaO/sub 3/, Ba/sub x/Sr/sub 1-x/TiO/sub 3/, x<0.5) and electrically tunable microwave devices based on these materials are briefly reviewed. The analysis shows that in spite of the recent year's extensive efforts, no considerable improvement in the microwave losses in thin paraelectric films has been achieved. Thin films, regardless of fabrication method and substrate type, have much lower dielectric permittivity than bulk single crystals, and the loss tangent at microwave frequencies (f>10 GHz) is of the order of 0.01 (at zero dc-bias field) at room temperature. Nevertheless, quite promising component and subsystem level devices are successfully demonstrated. Use of ceramic (bulk and thick film) ferroelectrics in tunable microwave devices, currently considered for industrial applications, offer cost reduction. In this paper, explicitly for the first time, we consider possibilities and benefits of using ferroelectrics in polar phase in electrically controllable microwave devices. Examples of using ferroelectrics in polar state (e.g., Na/sub 0.5/K/sub 0.5/NbO/sub 3/, SrTiO/sub 3/ in antiferroelectric phase) in electrically tunable devices are reported.

Precision microwave dielectric and magnetic susceptibility measurements of correlated electronic materials using superconducting cavities

We analyze microwave cavity perturbation methods, and show that the technique is an excellent, precision method to study the dynamic magnetic and dielectric response in the GHz frequency range. Using superconducting cavities, we obtain exceptionally high precision and sensitivity for measurements of relative changes. A dynamic electromagnetic susceptibility ζ̃(T)=ζ′+iζ″ is introduced, which is obtained from the measured parameters: the shift of cavity resonant frequency δf and quality factor Q. We focus on the case of a spherical sample placed at the center of a cylindrical cavity resonant in the TE011 mode. Depending on the sample characteristics, the magnetic permeability μ̃, the dielectric permittivity ε̃=ε′+iε″, and the complex conductivity σ̃=σ′+iσ″ can be extracted from ζ̃H. A full spherical wave analysis of the cavity perturbation indicates that: (i) In highly insulating samples with dielectric constant ε′∼1, the measured ζ̃H∼χ̃M, enabling direct measurement of the magnetic susceptibility. The sensitivity of the method equals or surpasses that of dc superconducting quantum interference device measurements for the relative changes in magnetic susceptibility. (ii) For moderate ε̃ and conductivity σ̃, ζ̃H∝ε̃+iωσ̃/ε0−1, thus enabling direct measurement of the sample dielectric constant ε̃, even though the sample is placed in a microwave magnetic field. (iii) For large σ we recover the surface impedance limit. (iv) Expressions are provided for the general case of a lossy dielectric represented by ε̃+iωσ̃/ε0. We show that an inversion procedure can be used to obtain ε̃+iωσ̃/ε0 in a wide range of parameter values. This analysis has led to the observation of new phenomena in novel low-dimensional materials. We discuss results on magneto dynamics of the three-dimensional (3D) antiferromagnetic state of the spin chain compound Sr2CuO3. In dielectric susceptibility measurements in Sr14Cu24O41, we directly observe a dielectric loss peak. Dimensional resonances in the paraelectric material SrTiO3 are shown to occur due to the rapid increase of dielectric constant with decreasing temperature. The cavity perturbation methods are thus an extremely sensitive probe of charge and spin dynamics in electronic materials.

Electronic Structures and Dielectric Properties of Substituted BaTiO3

Electronic structures were investigated on BaTiO3 substituted with other elements, such as Ca, Sr, Sc, Y, Ce and Gd for Ba, using a molecular orbital treatment that yielded net charge, bond order, density of states, energy levels and contour maps of electron density. By this method, the transformation into paraelectric material by ferroelectric material was discussed with respect to relative permittivity as measured experimentally. Furthermore, X-ray photoelectron spectroscopy (XPS) at the valence energy level was used on the Sr-substituted cluster, and the experimental spectrum was compared to the calculated one to evaluate the appropriateness of the cluster model. The overlap population between the A site in BaTiO3 and oxygen showed either a covalent or ionic nature depending on the elements. In addition, the relative permittivity related to the overlap population between site A and Ti resulting in lower permittivity for the trivalent elements such as Y and Gd. The computer simulation can be applied to design properties of the ceramics in the microwave range.

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

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 temperature range. In order to show our strategy to obtain the titled materials for microwave resonator applications, dielectric properties of several solid solution systems, SrTiO3-LaAlO3, Sr2TiO4-SrLaAlO4 and BaTiO3-Ba(Mg1/3Ta1/3)O3 were exemplified. Concerning about relaxor ferroelectric materials, it is pointed out that a drastic broadening of ferroelectric phase transitions in the KNbO3-BaTiO3 and BaTiO3-Ba(Fe1/2Ta1/2)O3 solid solution systems is caused by a strong local field fractuation come from the randomly oriented permanent electric dipoles produced by the heterogeneous cationic charge distribution in the solid solution.

Analysis of C-V and I-V data of BST thin films

Thin films of Ba0.7Sr0.3TiO3 have been fabricated using Enhanced Metal Organic Decomposition (EMOD) process on Si/SiO2/Ti/Pt substrates. The C-V characteristics of these films reveal a relation of log(1/Cm) ∝ VA where Cm is the measured capacitance at a frequency of 10 KHz and VA is the applied voltage. This voltage variable capacitance is characterized by an inverse quadratic distance proportinal charge distribution at the boundary layer between metal and paraelectric material. Above a threshold voltage of 3–5 V the leakage current density exhibits a modified Schottky emission, which states that log (JL) ∝ V 1/4. Below this threshold voltage, the origin of the leakage current which is operative in normal circuit applications has been discussed and identified from its temperature variation.

Optical second-harmonic generation at interfaces of ferroelectric nanoregions in SrSiO3:Ca.

Optical second-harmonic generation (SHG) is observed in ${\mathrm{Sr}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{TiO}}_{3}$ with x=0.007 within the ferroelectric domain state below ${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}18 K. Surprisingly, its dependence on the polarization angle is incompatible with global ${\mathit{C}}_{2\mathit{V}}$ symmetry. The appearance of maximum intensity for ${\mathit{E}}_{\mathrm{\ensuremath{\omega}}}$\ensuremath{\parallel}${\mathit{E}}_{2\mathrm{\ensuremath{\omega}}}$ parallel to the nonpolar c axis suggests that SHG is primarily due to symmetry breaking at the interfaces between ferroelectric nanoregions and the paraelectric host material ${\mathrm{SrTiO}}_{3}$. The observed angular dependences are satisfactorily modeled by assuming the dominant component of the nonlinear surface polarizability to lie parallel to the surface normal of ellipsoidally shaped nanoregions. The unusual electric-field dependence of the SHG intensity recorded at various temperatures and its temporal relaxation after steplike changing the field are described in terms of nanoregion nucleation and condensation in the presence of dipolar random fields. \textcopyright{} 1996 The American Physical Society.

Optimization of energy storage density in ceramic capacitors

A theoretical treatment, based on the Devonshire theory of ferroelectrics, is presented to describe the storage of electrostatic energy in ferroelectric and paraelectric materials at very high field strengths. In all cases, optimal energy density is achieved by using compositions with Curie temperatures well below the operating temperature. The theory is applied to barium - strontium titanate ceramics and optimal compositions are deduced for energy storage at given working fields. The theory is supported by experimental data showing energy densities up to 8 J at 100 kV .

Phase transition in hydrogen-bonded ferroelectric compounds — Quantum fluctuations versus thermal fluctuations

We have performed incoherent scattering (NIS) measurements to investigate proton dynamics in several ferroelectric or paraelectric materials, KH 2PO 4, KD 2PO 4, K 3H(SO 4) 2 and Rb 3H(SeO 4) 2. In KH 2PO 4, a typical material with three-dimensional hydrogen-bond network, strong quasi-elastic scattering due to proton fluctuation is observed at T > T c ( = 122 K). In K 3H(SO 4) 2 and Rb 3H(SeO 4) 2, typical materials composed of isolated hydrogen-bonds, we find low-energy excitations around ɛ = 6 meV along with a quasi-elastic component for T > T c ( = 0 K). The experimental results have been analyzed in the framework of ‘tunnelling proton system coupled to phonons’. The characteristic features of the observed neutron spectra are understood in terms of phonon-assisted tunnelling between the proton self-trapped states. The ferroelectric phase transition mechanism, in particular the large isotope effect, is discussed based on the obtained results.

Nonlinearity of ferroelectric capacitors on dram R/W operations

Abstract The effects of the nonlinearity and polarization relaxation of storage capacitor on DRAM R/W operations are explained using a simple model and PSPICE simulation. For a given line-voltage design and pass device, the voltage signal and the READ/WRITE times depend on the exact shape of the Q-V curve and on the amount of polarization relaxation. It has been found that typical high dielectric constant paraelectric materials provide smaller voltage signals than the linear capacitor, and exhibit slower READ but faster WRITE compared to the linear capacitor.

Pressure-temperature phase diagrams of betaine-arsenate, -phosphate, and -phosphite

Abstract Betaine arsenate (BA), betaine phosphate (BP), and betaine phosphite (BPI) are paraelectric materials at ambient temperature, displaying structural phase transitions to ferro- or antiferroelectrically ordered low-temperature states. The crystals are closely related with respect to their chemical compositions, their crystal structures, and due to the types of intermolecular chemical bonds. Hydrogen bonds play a decisive role. By measuring the dielectric constant ε(p, T) in the temperature interval 40 K ≤ T ≤ 250 K we have observed linear shifts of the transition temperatures Tc on applying hydrostatic pressure p in the range p ≤ 450 MPa. In all three materials the low temperature ordered phase is destabilized with increasing p, dTc/dp < 0, however with different slopes. These results are closely related to similar findings in the KH2PO4-group. In BP the two transitions into the antiferroelectric phase (Tc 2 = 86 K) and into the cell-doubled phase (Tc 3 = 81 K) coincide at p > 8 MPa, i.e., the cell-doubled transition is triggered by the anti-ferroelectric transition. In BA a characteristic dielectric dispersion with frequency is observed at low temperatures, accompanied by considerable dielectric losses, which indicates a dipole-glass behavior, presumably due to pressure induced lattice defects.

Real-time Metrology with BSO Crystals

Electro-optical crystals are used for real-time holography, optical data storage, optical information processing and more recently for speckle applications. Many ferro-electric materials were investigated, the Bi12 SiO20 (BSO), a cubic, para-electric and electro-optic material was found to be very useful for real-time holographic interferometry and speckle applications. For the application of the BSO in real-time metrology, the crystals is usually biased with a transverse electric field in the 110 or 100 crystallographic direction. Illuminating the crystal with spatially structured information in the 110 direction as a result of a hologram or speckle pattern, a space charge field is built up, leading to a refractive index variation in the crystal. Double-exposure techniques and time average exposure will be presented for real-time deformation displacement and vibration analysis and for contour line holography using a two-wavelengths method. Young's fringes of double-exposed, or in the time-average recorde...

Holographic grating formation in photorefractive crystals with arbitrary electron transport lengths

The time development of optically induced refractive-index hologram gratings in photrefractive electro-optic crystals such as lithium niobate and bismuth silicon oxide is analyzed allowing arbitrary electron-transport lengths and nonsinusoidal grating shapes. In this analysis, diffusion and drift in applied and space-charge fields as well as the bulk photovoltaic effect (when applicable) are included. Results for the photoinduced space-charge field are presented for a range of realistic experimental conditions. It is shown that the spatial phase of the space-charge field and therefore the beam coupling is strongly dependent on the electron-transport lengths, whereas the magnitude of the field and therefore the diffraction efficiency is not dependent. It is shown that for paraelectric materials (such as BSO) the results of short- and long-transport-length analyses must converge for long recording times (saturation).

Effect of pressure on the dielectric properties of a paraelectric material

Employing a self-consistent renormalization procedure for the treatment of the frequency spectrum of a model paraelectric, the essential features of the pressure dependence of the static dielectric constant of the incipient ferroelectric KTaO 3 are reproduced.

Dielectric susceptibility of a paraelectric material

Abstract An expression is obtained for the dielectric susceptibility of a paraelectric material from a Hamiltonian propounded by Silverman and Joseph using the double-time Green function technique. Using the temperature dependence of the low-lying transverse optic mode frequency, it is shown that the static dielectric susceptibility obeys a Curie-Weiss law.

Classical Diagram Technique for Calculating Thermostatic Properties of Solids; Application to Dielectric Susceptibility of Paraelectrics

A diagram technique suitable for calculating equilibrium classical statistical properties of solid dielectrics is developed. The method is applied to the temperature-dependent dielectric susceptibility and vibration spectrum of an anharmonic crystal. The techniques of "diagram summation" and "frequency renormalization" are used to obtain a Curie-law formula for the susceptibility of paraelectric materials. The method is extended to obtain a new formula for the susceptibility in the presence of a biasing field. Some possible applications of the theory to relevant experiments are mentioned.

Measurements of Large Dielectric Constants and Loss Tangents at 55 Gc/s

Measurements of dielectric constant and loss tangent have been made on several paraelectric and ferroelectric materials in the region of 35 Gc/s. The measurement technique is appropriate for materials with large dielectric constant and large loss tangent; typical values measured are in excess of 1500 for dielectric constant and 0.1 for loss tangent. Preliminary considerations indicate an accuracy of approximately 5 per cent. Previously reported techniques are not suitable for making such measurements at 35 Gc/s, and this aspect is discussed briefly. The technique consists of measuring insertion loss and phase shift of a signal upon passing through a slab of the test material filling the cross section of the waveguide. These data are measured as a function of slab thickness. The insertion loss (in dB) will undulate with increasing thickness for sufficiently thin samples, but it quickly becomes proportional to thickness for material having the afore-mentioned properties. In the region of proportionality the equations relating insertion loss and phase shift to the dielectric constant and loss tangent become rather simple, allowing data to be reduced easily. The paper gives a derivation of the equations required for data reduction and also reports measured dielectric constants and loss tangents of various barium-strontium titanates at 35 Gc/s. Experimental problems are discussed including the very real problem of mounting the sample within the waveguide so as to eliminate air gaps and erratic data.

Temperature Dependence of the Frequency Spectrum of a Paraelectric Material

The temperature-dependent frequency spectrum of a paraelectric material is discussed with use of an effective-harmonic-lattice equation of motion. For a given wave vector, the effective couplings between atoms are written as a sum of a temperature-independent part, arising from harmonic interactions, and a part linear in the temperature, arising from fourth- and third-order anharmonic interactions. The conditions required for the observed Curie-Weiss behavior of the dielectric constant and associated temperature dependence of certain long-wavelength transverse optical frequencies are examined. It is shown that the salient features of the temperature-dependent low-lying optical branch in SrTi${\mathrm{O}}_{3}$ can be understood by considering the effects of anharmonic interactions at constant volume.

Nonlinear Dielectric Constant of a Paraelectric Material

An expression is obtained for the nonlinear dielectric constant of a paraelectric material. This is given in terms of the coupling parameters of the Born-von K\'arm\'an lattice dynamical theory. Deviations from the Devonshire phenomenological theory are discussed.