Ferroelectrics

A diatomic linear chain model is used to describe the dynamical properties of displacive type ferroelectric compounds. For these materials the non-bondingp-orbitals of the chalcogen ions play an essential role, which is taken into account by assuming a nonlinear fourth-order polarizability at the chalcogen-ion lattice site. Within the self-consistent phonon approximation soft modes, phonon dispersion curves, phonon anomalies and related quantities can be calculated for all temperatures. Going beyond the self-consistent phonon approximation, the model yields, in the continuum limit, interesting new solutions such as periodic non-linear waves, kinks, which describe the statics and dynamics of ferroelectric domain walls, and pulse solutions.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

Isofrequency Opalescence in Ferroelectrics

Isofrequency dependences of the spectral intensities of inelastic scattering in LiTaO₃, Pb₅Ge₃O₁₁, BaTiO₃, and KNbO₃ crystals are registered at a fixed frequency of the spectrometer close to that of the exciting radiation. Results from soft mode analysis are presented.     

Orientational Relaxation in Ferroelectrics of Tetragonal Symmetry

The energy absorption caused by rotation of the spontaneous and induced polarization vectors in external longitudinal alternating-sign stress field is calculated for the barium titanate ferroelectric of tetragonal symmetry. The acoustic absorption coefficient, the internal friction, and the E effect together with their frequency and orientational dependences are derived in terms of elastic and piezoelastic constants, components of the electrostriction tensor, and rotational dissipation coefficients. The given dissipation type can dominate in ferroelectric magnets with strong coupling of their elastic, electrical, and magnetic subsystems.     

Defect-Dipole Formation in Copper-Doped PbTiO3 Ferroelectrics

The defect structure of hard copper-modified polycrystalline PbTiO3 ferroelectrics is investigated by means of electron paramagnetic resonance and hyperfine sublevel correlation spectroscopy, as well as density functional theory calculations. Special emphasis is put on the 207Pb-hyperfine couplings, which are resolved up to the third coordination sphere. The results prove that copper is incorporated at the octahedrally coordinated Ti site, acting as an acceptor. Because of charge compensation the formation of Cu impurity-oxygen vacancy pairs is energetically very favorable. The corresponding (CuTi'-VO)x defect dipole is found to be orientated along the [001] axis.     

铁电物理的近期发展

介绍了铁电体物理学的基本概念、发展简史、近年来的主要进展以及当前的研究方向。指出铁电体物理学研究的核心是自发极化，发现历史可分为四个阶段。近年来的主要进展有四方面，即：第一性原理的计算，铁电相变的尺寸效应，铁电液晶和铁电聚合物，集成铁电体。当前的研究方向主要有两个，一是低维系统，二是调制结构。     

Ionization Cross Sections of Gas Molecules for Plasma Chemistry

Using the recently tested formula of Gryzinski electron impact ionization cross sections for several molecules are calculated. The additivity rule and the statistical approach by Fitch and Sauter are discussed. With new experimental ionization cross sections it is possible to derive the regression coefficients also for P, As, B, and Si, which are presented together with the previously obtained values as a function of the atomic number. It appears to be possible to obtain coefficients for other atoms by interpolation. First attemps to determine regression coefficients also for 20 and 35 eV electron energy are discussed.     

Cross-Sections,Rate Constants and Transport Coefficients in Silane Plasma Chemistry

This paper presents a critical review of the basic data concerning the physics and chemistry of low pressure SiH₄ glow discharges used to deposit hydrogenated amorphous silicon films (a-Si:H). Starting with an updated table of thermochemical data, we analyze the gas-phase elementary processes consisting of i) electron-molecule collisions, ii) ion-molecule collisions, iii) neutral-neutral collisions, iv) other electron and ion collisions involving electron-ion and ion-ion recombination, electron attachment on radicals and detachment of anions, and v) cluster growth kinetics in dusty plasmas. Experimental data or theoretical estimates are given and discussed in terms of cross-sections, collision and reaction rate constants, and transport coefficients. We also analyze the surface processes and reaction probabilities of ions, radicals and molecules.     

飞秒脉冲激光在铁电体中的冲击受激拉曼散射

将冲击受激拉曼散射应用于某些铁电体中 ,在时间域内分析物质晶格振动。τωα <2π是应用冲击受激拉曼散射的必要条件。使用钛宝石激光再生放大系统作为激发源 ,在LiTaO3 晶体内获得了声子电磁激元振荡模。将同种方法用于KDP ,在其铁电体相位转换点以上 5 0K的温度范围内 ,仅发现了波矢 q =6848cm- 1 ,弛豫时间常数τ0 =1 33× 10 - 1 3 s的弛豫振动软模 ,显示出KDP铁电体相位转换的弛豫特性。     

Mathematical Modeling of the Polarization of Polycrystalline Ferroelectrics

General patterns of the polarization models of Rayleigh, Preisach, and Jiles–Atherton are found. The limiting dependence of the polarization is confirmed. A way of constructing it based on the energy criterion for domain switching for polycrystalline ferroelectrics with perovskite structure is proposed. Differential equations are constructed for determining the irreversible components of polarization and deformation, and large loops of dielectric hysteresis are calculated.     

Er-Yb Codoped Ferroelectrics for Controlling Visible Upconversion Emissions

Under a 980 nm laser pumping, quenching of green upconversion (UC) emission accompanied with enhancement of red UC emission observed was dominated by the energy back-transfer (EBT) process in Er³⁺ and Yb³⁺ co-doped PbTiO₃, BaTiO₃, and SrTiO₃ polycrystalline powders. The efficiency of the EBT process depends not only on Yb³⁺ concentration but also on level match of the doped Er³⁺ and Yb³⁺ ions caused by the crystal fields with different symmetries. Our UC emission spectra and X-ray diffraction confirm that the centrosymmetric crystal field arising from reducing tetragonality causes level match of transition of Er³⁺ and of Yb³⁺. This level match is responsible for enhancing red UC emission.     

Ferroelectrics in the para phase in a homogeneous,intense vhf field

Conclusion Investigation of the behavior of ferroelectrics in an intense variable electric field has revealed a number of significant features. The choice of models explaining the observed effects requires that the interaction between the electric field and the acoustic vibrations of the crystal lattice of the material be taken into account, demonstrates the role of static charged defects, and reveals features of charge-carrier kinetics.The main interest centers on investigations in the vhf range, which is distinguished by high information content. Here, primarily, the small wavelength of the acoustic vibrations ( l m) at frequencies of the intense excitation comparable with the film thickness and the crystallite size must be noted, together with the small duration of the vibration period, comparable with the charge-carrier relaxation time.The next problem is the creation of a model combining both the nonlinear effects and the charge-carrier kinetics determining the redistribution of the static internal field and the charge transfer. The development of such a model will have a decisive influence in improving the characteristics of ferroelectric devices.The development of microscopic models explaining the noted features of the ferroelectric properties is also very urgent. One of the interesting trends in this field is the use of the interband theory of ferroelectricity.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 59–80, August, 1981.     

Phase Transitions of KIO3 Ferroelectrics in Al2O3-Based Nanoporous Matrices

Physics of the Solid State - Temperature dependences of the linear permittivity ε' and the third harmonic amplitude γ3ω of composites prepared by introducing ferroelectrics KIO3...     

Ferroelectrics with composition gradient: On the nature of hysteresis loop shift

Possible reasons of the difference in polarization characteristics of thin-film and bulk ferroelectrics with a composition gradient have been considered.     

Chemistry in space

Summary Molecules have been observed in widely disparate astronomical objects, from comets to supernova remnants. Interstellar space appear to be a real chemical laboratory, able to produce a large number of molecules, some of them relatively complex. The abundances show a very high sensitivity to local physical properties and dynamical history. This sensitivity renders molecular observations and astrochemical modelling very flexible tools for investigating the properties and evolution of the interstellar medium. A brief outline of morfology of interstellar medium is given, together with a discussion of the basic chemical processes leading to the molecular formation. Since interstellar-dust particles play a crucial role in the chemical evolution of interstellar medium, physical and chemical properties of dust are reviewed. Finally, by way of an example of modelling exercise, a toy model of the chemical evolution of interstellar gas is presented. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Incorporating Analytical Chemistry into an Introductory Course in Chemistry

This paper highlights a new introductory level course entitled “Thermodynamics, Equilibria, and Kinetics” that provides students with an early introduction to incorporating analytical chemistry into their laboratory work. The rationale for creating the course and the course's format are outlined. Examples highlighting the use of spectroscopy illustrate the development of a student's ability to think as an analytical chemist.     

On the Plasma Chemistry of an RF Discharge Containing Aluminium Tri‐Isopropoxide Studied by FTIR Spectroscopy

In Ar and Ar/N₂ radio frequency (RF) discharges with admixtures of aluminium tri‐isopropoxide (ATI) the fragmentation of this metal‐organic precursor was studied by means of Fourier Transform Infrared (FTIR) spectroscopy using an optical long path cell providing an optical length of l = 17.2 m. The experiments were performed in an asymmetric capacitively coupled process plasma at a frequency of f = 13.56 MHz and at pres‐sure values in the range of p = 1–10.5 Pa. The discharge power was chosen between P = 10–100 W. Using FTIR spectroscopy the evolution of the concentrations of ATI and of six stable molecules, CH₄, C₂H₂, C₂H₄, C₂H₆, CO and HCN, was monitored under flowing conditions at gas flows of Φ_total = 0.5–14.5 sccm in the discharge. The concentrations of the reaction products were measured tobe between 2 x 10¹² molecules cm^–3 as e.g. found for C₂H₄and C₂H₆, and 5 x 10¹³ molecules cm^–3, as e.g. in the case of CO. In the plasma a complete dissociation of the precursor ATI was found at a power value of about P = 80 W independent on the admixture of Ar or N₂. The fragmentation efficiency (F_E) of the reaction products which originate from the ATI molecules ranges between 0.2 and 4 x 10¹⁶ molecules J^–1 while the fragmentation rate (F_R) reached up to 2.5 x 10¹⁸ molecules s^–1. The multi component detection ability of the spectrometer served to analyse the carbon balance of the by‐product formation. For all experiments, the carbon balance never exceeded 25%. Therefore, in the plasmas the majority of the provided carbon is most likely deposited at the reactor walls or forms dust particles or higher molecular C_xH_y. The conversion efficiencies (C_E) of the produced molecular species ranges between 0.1 x 10¹⁵ molecules J^–1 for C₂H₄ and 5 x 10¹⁵ molecules J^–1 for C₂H₆ depending on the discharge conditions of the RF plasma. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Phase Diagrams of Solid Solutions of Relaxor Ferroelectrics from the Dielectric Spectroscopy Data

The dielectric spectra of Pb(_1–z)Ba _z (Mg_1/3Nb_2/3) _m (Zn_1/3Nb_2/3) _y (Ni_1/3Nb_2/3) _n TixO₃ (x = 0.25–0.4, y = 0.1130–0.0842, m = 0.4844–0.1298, n = 0.1266–0.4726, z = 0–0.15) ceramics with substitution in both A and B crystallographic positions of the perovskite structure are studied. The system demonstrates a transition from the relaxor state to the normal ferroelectric state in both cases: when the concentration of lead titanate grows and the concentration of barium is reduced. On the basis of experimental results, the x–T and z–T phase diagrams are plotted. Despite different crystal chemical reasons of the relaxor state emergence in the investigated solid solutions, their diagrams demonstrate an evident similarity. We have revealed the disappearance of the temperature hysteresis at the transition to the relaxor state in both cases, which has allowed us to make an assumption of the existence of tricritical points on the corresponding diagrams.