碱土氟化物中离子间相互作用势经验参数的确定

描述离子间非Coulomb短程互作用势的函数形式确定后，即可据此讨论离子晶体的一系列物理性质，势函数中出现的参数通过与实验数据的拟合而确定．基于电子壳模型对CaF₂，SrF₂，BaF₂离子晶体的讨论中，强调电子壳电量应一律取负值的基本前提，以经验参数化方法拟合势参数时注意引用近期实验数据．将F离子电量调整为0.97e之后，重新获得的模型参数集与上述碱土氟化物晶格形成能、弹性常数、长光学横波频率、Raman频率，以及极化率实验数据，有较满意的对应关系．     

碱土氟化物离子晶体中点缺陷形成能计算

基于以前通过经验参数化途径得到的碱土氟化物电子壳模型参数和离子间互作用势参数,计算了CaF₂,SrF₂和BaF₂晶体中点缺陷形成能．计算并对比了SrF₂和BaF₂的声子色散曲线与非弹性中子散射实验数据,再一次复验了经验参数集的质量. 关键词：     

Molecular Dynamics Simulations for Melting Temperatures of SrF2 and BaF2

利用壳层模型分子动力学方法,考虑萤石结构分子中的预熔化现象,对SrF₂和BaF₂的分子动力学模拟熔化温度进行修正,获得了高压下SrF₂和BaF₂的熔化温度. 同时给出了300 K、0.1 Mpa～7GPa和0.1 Mpa～3 GPa时SrF₂和BaF₂的状态方程,与已有研究结果的最大误差分别为0.3%和2.2%. 计算所得SrF₂和BaF₂常压下的熔点与已有的实验结果符合较好. 对于SrF₂和BaF₂分子体积变化和已有的熔化模拟的差别也做了比较和讨论.     

Ground state and lattice dynamical study of ionic conductors CaF2, SrF2 and BaF2 using density functional theory

The present paper reports a comprehensive and complementary study on structural, electronic and phonon properties of face centered cubic fluorites, namely CaF₂, BaF₂ and SrF₂, using first principles density functional calculations within the generalized gradient approximation. The calculated lattice constants and bulk modulus are in good agreement with available experimental data. The analysis of band structure and density of states confirms the ionic character for all the three fluorides. The phonon dispersion curves and corresponding phonon density of states obtained in the present work are consistent with the available experimental and other theoretical data. The LO-TO splitting is maximum for CaF₂, which confirms that the ionicity is maximum in the case of CaF₂. The phonon properties for SrF₂ have been calculated for the first time.     

Physical properties of multicomponent fluoride glasses for photonic and superionic applications

Heavy metal fluoride glasses are promising materials for ultra-low loss mid-infrared optical fibers. The fibers are applied in remote spectroscopy, laser surgery, and thermal imaging. Upon doping with rare earth ions, heavy metal fluoride fibers are suitable for a development of high power laser materials, up-conversion lasers, and optical amplifiers for telecommunications systems. As heavy metal fluorides are prospective fast fluoride ion conductors, fluoride glasses based on ZrF₄, BaF₂, LaF₃, AlF₃ and NaF (ZBLAN), PbF₂, InF₃, BaF₂, AlF₃, LaF₃ (PIBAL) or ZnF₂, BaF₂, InF₃, SrF₂, AlF₃, NaF (ZBISAN) are interesting for a development of glassy or fibrous ionic conductors. In this paper, the ionic conductivity and dielectric response of the abovementioned multicomponent fluoride glasses is studied. The influence of the glass composition on the glass transition temperature (T_g) and on the crystallization temperature (T_cr) is also reported. The optimum composition and drawing temperature for fluoride glass fibers is specified. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

A Study of Mechanochemical Doping of Fluoride Crystals with a Fluorite Structure by Er3+ Ions via Electron Paramagnetic Resonance Spectra

Using electron paramagnetic resonance (EPR) spectroscopy, we have shown that, upon mecha- noactivated doping of powders of compounds CaF₂, SrF₂, and BaF₂ with Er³⁺ ions, impurity centers of single erbium ions with cubic symmetry are formed. Investigations of dependences of EPR spectra intensities on the particle size show that the process of mechanochemical doping with Er³⁺ ions proceeds differently for CaF₂, SrF₂, and BaF₂ host matrices. In the case of CaF₂, impurity centers are localized in a very thin near-surface layer of CaF₂ particles, in SrF₂, the impurity is distributed over the volume of particles, while, in BaF₂, there is a layer of a finite thickness for which the probability of doping in the course of mechanosynthesis is very small and the impurity of the rare-earth element is localized in the core of large particles. These data can be explained assuming that the result of mechanosynthesis of particles of fluorides with a fluorite structure doped with Er3+ ions at room temperature is governed by two processes—mechanoactivated diffusion of rare-earth ions into particles and segregation of impurity ions at grain boundaries. In this case, the typical scales for compounds CaF₂, SrF₂, and BaF₂ considerably differ from each other.     

First-principles calculations of the electronic structure of fluorite-type crystals (CaF2, BaF2, SrF2, and PbF2) containing Frenkel defects. Analysis of optical and transport properties

The electronic structure of the alkali-earth fluorides CaF₂, BaF₂, SrF₂, and PbF₂ with Frenkel defects is investigated in the tight-binding approximation by the LMTO method. The defect formation and migration energies are calculated. The electronic structure and optical excitations of a H center in a defective fluorite structure are examined. It is shown on the basis of calculations of the binding energies that CaF₂, BaF₂, and SrF₂ are ionic compounds, while the chemical bond in PbF₂ is partially covalent. Possible methods of displacement of interstitial fluorine atoms that lead to the observed optical spectra of an H center are investigated. Fiz. Tverd. Tela (St. Petersburg) 40, 2019–2025 (November 1998)     

High pressure behavior of alkaline earth tellurides

We have predicted high pressure structural behavior and elastic properties of alkaline earth tellurides (AETe; AE = Ca, Sr, Ba) by using two body interionic potential approach with modified ionic charge (Z _m e). This method has been found quite satisfactory in case of the rare earth compounds. The equation of state curve, structural phase transition pressure from NaCl (B1) to CsCl (B2) phase and associated volume collapse at transition pressure of alkaline earth tellurides (AETe) obtained from this approach, so have been compared with experimentally measured data reveal good agreement. We have also investigated bulk modulus, second and third order elastic constants and pressure derivatives of second order elastic constants at ambient pressure which shows predominantly ionic nature of these compounds. First time, we have calculated the Poisson ratio, Young and Shear modulus of these compounds.      

Analysis of anharmonic and thermoelastic properties of non-centrosymmetric crystals

In the present paper an analytical potential form is used for overlap repulsive energy, derived by Harrison from quantum mechanical considerations, along with the composite effect of three-body forces and intersublattice displacement. The short-range overlap parameters in Harrison's potential form have direct correlation with the valence state energies for outermost electrons. The potential model is applied to calculate the third and fourth order elastic constants, first and second pressure derivatives of second order elastic constants, Grüneisen parameter and its volume dependence, Anderson parameter, and thermal expansion coefficient for three non-centrosymmetric crystals, viz. CaF₂, SrF₂ and BaF₂. The calculated values of various physical quantities are found to be in good agreement with experimental data.The authors are grateful to Dr. Mansour Khalef, the Head of Physics Department, TNRC, Tajura (Tripoli) for the facilities and encouragements.     

Time-resolved spectroscopy of self-trapped excitons in fluorides of alkaline-earth metals under pulsed electron irradiation

Nanosecond-resolution absorption spectroscopy at room temperature was used to study the laws governing the creation and evolution of the primary defect structure in CaF₂, SrF₂, and BaF₂ crystals exposed to an accelerated electron pulse. It is shown that the spectral-kinetic characteristics of self-trapped excitons created in undamaged parts of the crystal lattice are qualitatively similar. Partial polarization of the absorption of self-trapped excitons is observed in CaF₂. The structure of the transient absorption spectra becomes more complex in the sequence CaF₂, SrF₂, BaF₂ because of the formation of excitons trapped in phase inclusions of homologous cationic impurities. The spectral characteristics of excitons trapped in undamaged parts of the CaF₂ and SrF₂ lattice and in their phase inclusions in BaF₂ are the same although the latter have a considerably shorter relaxation time. Short-lived (τ⩽100 ns) absorption of unknown defects was observed in the spectral range ⩾5 eV. Fiz. Tverd. Tela (St. Petersburg) 40, 1228–1234 (July 1998)     

Charge Transfer Luminescence in Ce-Doped Fluorides

New emission bands were found in Ce^3? doped SrF₂ and BaF₂ crystals under excitation into the charge transfer region. The bands at 4.85 eV in BaF₂ and 4.35 eV SrF₂ were found in all crystals with Ce-concentration from 0.001 to 1 mol.%, most prominent in 0.01% samples. Decay of luminescence was simple exponential in BaF₂, decay time decreased from 1.3 ms at 65 K to 0.41 μs at 523 K. The decay curve in SrF₂ shows t^?0.3 dependence at room temperature. All experimental results were naturally explained by assumption that new emission belongs to radiative charge transfer recombination in Ce^2?—F_i^o pairs with different distances between them.     

5d-4f luminescence of Nd3+, Gd3+, Er3+, Tm3+, and Ho3+ ions in crystals of alkaline earth fluorides

The vacuum ultraviolet emission spectra of alkaline-earth fluoride (CaF₂, SrF₂, BaF₂) crystals with rare earth impurity ions (Nd, Gd, Er, Tm, Ho) have been investigated. The main luminescence bands are described well by the transitions from the lowest excited 5d state to different 4f levels of rare earth ions.     

Concentration dependences of the elastic constants of the two-dimensional graphene-silicene system

The concentration dependences of the elastic constants of the two-dimensional Si _x C_{1 − x} system have been investigated with the use of the Harrison bonding-orbital method and the Keating model. The central and non-central force constants and the Grüneisen parameter have been considered by means of the bonding-orbital method. All quantities under consideration have been shown to exhibit a nonlinear behavior during the transition from graphene to silicene. A nontrivial role of the short-range repulsion has been discussed. The second-order and third-order elastic constants, the pressure dependences of the second-order elastic constants, as well as the Poisson’s ratio and Young’s modulus have been investigated in the Keating model. It has been found that the elastic constants and Young’s modulus change almost linearly upon the transition from graphene to silicene, whereas the other quantities under consideration exhibit nonlinearity.     

Change of hyperfine and transferred hyperfine interaction of Eu2+ in CdF2, CaF2, SrF2, and BaF2

ENDOR measurements of the hyperfine and transferred hyperfine interaction of Eu²⁺ in CdF₂, SrF₂, and BaF₂ were performed. Compared with measurements on CaF₂, there is a radial shifting of the next fluorine ligands in CdF₂∶ +5.4%, CaF₂∶ + 3.5%, and BaF₂∶ ?3.9%. Since the SrF₂ and EuF₂ lattice constants are approximately equal, a shifting in SrF₂ was not assumed. In both europium isotopes (151 and 153) a change of hyperfine fields at the nucleus was observed. This can be explained qualitatively by the difference in radial distribution of the different europium orbitals. Also noticed was a small change of the hyperfine anomaly, which indicates contributions of the zero-point vibration.     

Effect of electron irradiation on fluorides of alkali-earth elements (CaF2, SrF2 and BaF2)

The effect of electron irradiation on fluorides of alkaline-earth elements CaF₂, SrF₂, and BaF₂ in the column of an electron transmission microscope is studied by means of elemental and diffraction analysis. It is established that when the current density of the electron beam exceeds the threshold value, we observe the desorption of fluorine and the binding of liberated metal ions by the oxygen of the residual vacuum to form oxide phase MeO (Me = Ca, Sr, and Ba) with the structure of rock salt NaCl.     

Computation of collective modes and acoustic investigations at different temperatures of vitrous silica

This paper is mainly concerned with elastic and acoustic properties of vitrous silica besides the computation of phonon frequencies. Thus the phonon frequencies of vitrous silica have been calculated assuming the electronic bulk modulus,K _e, as equal to zero. New equations have been derived to relate the pressure derivatives of second order elastic constants to the acoustic Gruneisen’s parameters using both Bhatia-Singh’s parameters and Schofield’s equations. The calculated longitudinal and transverse Gruneisen’s parameters and the predicted absorption band spectra from Nagendranath’s equation and Bhatia Singh’s parameters are in good agreement with experiment. The calculated mean acoustic mode Gruneisen’s parameter evaluated from the pressure derivative of Nagendranath’s equation is also in good agreement with experiment. An erratum to this article is available at .     

Phase relations for SrF2 TO 50 Kbars and 1900°C and its compression to 40Kbars at 25°C

The phase diagram of SrF₂ to 50 Kbars and 1900°C, obtained primarily by differential thermal analysis (d.t.a.), is presented. In analogy to the isostructural compound CaF₂, three solid phases, α, β, γ-SrF₂, as well as the liquid field, have been located. Thermochemical data (dT/dP, ΔV, ΔS) of the α-β and α-γ boundaries, and the melting curve are given. A tentative phase diagram for the isostructural compound BaF₂ is proposed. In addition, the first compression data for SrF₂ to 40 Kbars are reported. The bulk modulus, its pressure derivative, and the Grüneisen constant of SrF₂, CaF₂ and BaF₂ are compared.     

Elastic properties and spectroscopic studies of lithium lead borate glasses

Elastic properties of xLi₂O — 20PbO — (80-x)B₂O₃ glasses have been measured at a frequency of 10 MHz using X-cut and Y-cut quartz transducers. The trends in the variation of elastic moduli, Poisson’s ratio and Debye temperature have been studied. The elastic moduli namely longitudinal and young’s modulus show strong linear dependence while bulk and shear modulus vary marginally as a function of Li₂O concentration. The Poisson’s ratio is found to be almost constant and Debye temperature increases with the increase of Li₂O concentration. IR, MAS-NMR and glass transition temperature studies have been also carried out. Glass transition temperature is found to increase with increase of Li₂O concentration. IR and MAS-NMR spectra show characteristic features of borate network and systematic change as a function of Li₂O concentration. The variation in the elastic properties and structural features of IR and MAS-NMR indicate that Pb²⁺ ions are likely to occupy network forming positions in this glass system. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Elastic anomalies in strontium titanate

The anomalies of the second and third-order elastic constants have been considered for the phase transition of strontium titanate within the framework of Landau’s theory. All the anomalies of the second-order elastic constants have been obtained in a single formula using Kronecker delta functions and relations among them have been established. The real parts ofC*₁₁ andC*₄₄ decrease steeply across the transition temperature and thereafter flatly tend to their asymptotic values in the low temperature phase agreeing qualitatively with experimental observations. We have also derived expressions for the third-order elastic anomalies and discussed the temperature variation of the real part ofC*₁₁₁. We have derived expressions for the attenuation of the longitudinal and transverse waves along certain simple symmetry directions and have shown that there is nearly good agreement with experimental observations.     

Thermal decay of photochromic color centers in CaF2, SrF2, and BaF2 crystals doped by La and Y impurities

The absorption spectra of photochromic centers in CaF₂, SrF₂, and BaF₂ crystals doped by La and Y impurities and thermal decay of the centers in the temperature range 80–600 K are investigated. Under low-temperature x-ray irradiation, ionized photochromic color (PC⁺) centers are generated in La- and Y-doped CaF₂ crystals and in a La-doped SrF₂ crystal. It is revealed that, upon heating of the CaF₂-LaF₃ crystal, PC⁺ centers are transformed into photochromic color (PC) centers. In the SrF₂-YF₃ crystal irradiated at room temperature, photochromic color centers are generated as well. All color centers decay at a temperature of approximately 600 K. After irradiation of the BaF₂-YF₃ crystal at a temperature of 80 K, absorption bands are observed at energies of 2.25 and 3.60 eV, which are related to neither PC centers nor PC⁺ centers.