Calculations of the vibrational frequency and isotopic shift of UF6 and U2F6

Molecular structure, vibrational frequency and infrared intensity of UF6 are investigated by using the revised Perdew Burke-Enzerhof function with the triple-zeta polarized basis set. The calculation results are in good agreement with the experimental values and indicate the existence of a stable U2F6 molecule with a multiple bonded U2 unit. The calculation results also predict that the D3d symmetry of U2F6 is more stable than D3h. The optimized geometries, vibrational frequencies, and infrared intensities are also reported for U2F6 molecules in D3d symmetry. In addition, the isotopic shift of vibrational frequencies of the two molecules under isotopic substitution of uranium atom are also investigated with the same method. The U2F6 molecule is predicted to be better than UF6 for laser uranic isotope separation.     

An ultrasonic characterization of ferrofluid

Nanoparticles of Cr₂O₃ are prepared through hydrothermal synthesis process using CrO₃/PVA in aqueous solution using sucrose as a reducing agent. The calcination temperature is taken 300 and 350 °C. XRD and SEM of the powdered Cr₂O₃ particles are done for the characterization. The average particle size is found 30–80 nm. It is found that average particle size increases with calcination temperature. The UV–visible absorption spectra are taken for the study of photo-physical properties of ferrofluids. Ultrasonic velocity and absorption measurements are performed in Cr₂O₃ ferrofluid using variable path interferometer and pulse-echo techniques, respectively. The achieved results are discussed in correlation with the magnetic and other physical properties of Cr₂O₃.     

First-principles calculations of the structural,elastic, electronic and optical properties of Si2P2O and Ge2P2O at high pressures

The structures of Si₂P₂O and Ge₂P₂O with the space group Cmc21 are derived. The structural, mechanical, elastic anisotropy, electronic and optoelectronic properties are calculated by first principles calculations based on density functional theory (DFT) with generalized gradient approximation (GGA) at high pressure for Si₂P₂O and Ge₂P₂O. By using the elastic stability criteria, it is shown that their structures are all stable. The phonon dispersion spectra are researched throughout the Brillouin zone as implemented in the CASTEP code, which indicates that the optimized structures are stable dynamically. The brittle/ductile behaviors are assessed in the pressures from 0 GPa to 50 GPa. Our calculations present that the performances of them become ductile with pressure rise. Moreover, the anisotropies of them are discussed by the Young's moduli at different pressure, and the results indicate that the anisotropies of them are obvious. The direct band structures of Ge₂P₂O and the indirect band gap of Si₂P₂O show that Si₂P₂O and Ge₂P₂O present semiconducting character at 0 GPa and 50 GPa. The band structures of Si₂P₂O are changed obviously with the increase of pressure. The total DOS originate mainly from O ‘s’ states, O ‘p’ states, P ‘s’ states and P ‘p’ states and M ‘p’ states (M=Si, Ge). The trends of DOS for Si₂P₂O and Ge₂P₂O display many similarities, and the change of DOS is obviously affected by the pressure for Si₂P₂O. The optoelectronic properties of them are researched. The calculated static dielectric constants, ɛ₁(0), are 3.2 at 0 GPa and 6.4 at 50 GPa for Si₂P₂O, and the values of Ge₂P₂O are 10.2 and 9.2 at 0 GPa and 50 GPa.     

Pb-Mg-Al合金腐蚀机理的电子理论研究

为了了解Pb-Mg-Al合金腐蚀的物理本质, 本文采用基于第一性原理的赝势平面波方法系统地计算了Pb-Mg-Al合金中各物相的结合能、费米能级和局域态密度等电子结构参数, 分析了合金的电化学腐蚀机理. 计算结果表明：Pb-Mg-Al合金中各主要组成物相稳定性大小关系为 Mg₁₇Al₁₂>Mg₂Pb>Mg;Mg,Mg₂Pb和Mg₁₇Al₁₂的费米能级存在E_f(Mg)>E_f(Mg₂Pb)>E_f(Mg₁₇Al₁₂)的关系, 说明Mg最容易失去电子, Mg₂Pb次之, Mg₁₇Al₁₂最难;局域态密度表明, 在同样的外界条件下, 体系中Mg相和Mg₂Pb相对于Mg₁₇Al₁₂均处于不稳定的状态, 容易失去电子, 即容易发生腐蚀. Pb-Mg-Al合金体系中不同物相的费米能级差构成了电化学腐蚀的电动势, 导致电子从费米能级高的Mg相和Mg₂Pb相流向费米能级低的Mg₁₇Al₁₂相, 使Pb-Mg-Al合金发生腐蚀.     

Theoretical calculations of vibrational modes in endohedral metallofullerenes: La@C82 and Sc2@C84

Density functional theory at the B3LYP level is applied to representative mono- and dime-tallofullerenes, La@C₈₂ and Sc₂@C₈₄, to calculate the metal-cage vibrational modes that are currently of considerable interest in far-infrared and Raman spectroscopic studies. The La based vibrations calculated for La@C₈₂ are in good agreement with those assigned experimentally. The vibrational frequencies are little changed upon reduction and oxidation. The Sc based vibrations calculated for Sc₂@C₈₄ are also compared with the available experimental data. In addition, the structures of the isomers of Sc₂@C₈₄ are discussed. Electrostatic interactions play an important role in endohedral metallofullerenes.     

Measurements of K-shell ionization cross sections of35Br,37Rb,39Y using low energy protons

K-shell ionization cross section measurements are reported for₃₅Br,₃₇Rb and₃₉Y targets caused by protons over 300–400 keV energy range in 20 keV increment. The K-shell ionization cross sections (σ _k ^l ) at different energies were deduced from the K_α and K_β X-ray production cross sections which were obtained from X-ray yields of the K_α and K_β transitions. The experimental values are compared with the calculated values of ECPSSR theory and empirical reference cross sections. The resultant K-shell ionization cross sections are found to be in reasonable agreement with the ECPSSR theory. The K_α/K_β intensity ratios are also presented and compared with other experimental values and also with the theoretical one-hole values given by Scofield.     

Cationic nonstoichiometry of oxides of the Ba-Cu-O and Y-Ba-Cu-O systems

The structure and cationic composition of oxides of the Ba-Cu-O system with cubic BaCuO₂ and tetragonal BaCu₂O₂ structure and YBa₂Cu₃O_{6 + ??} oxide of the Y-Ba-Cu-O system are studied. The oxides are found to be nonstoichiometric in cationic composition when the composition differs noticeably from the corresponding formula relation with conservation of the crystalline structure??s type.     

Possible scheme of synthesis-assembling of fullerenes

A new scheme of fullerene formation is proposed on the basis of the similarity between the experimentally detected carbon structures. According to experimental data, the microclusters of C₂ and C₁₀ are synthesized first and then either an intermediate nucleus cluster or an obtainable lower fullerene is assembled from them. A high-symmetry fullerene can be assembled with a high probability from a nucleus cluster with a “good” symmetry. The atomic and electronic structures of molecules such as C₃₆, C₆₀, C₇₀, and C₇₆ are analyzed. For C₃₆, the NMR spectra are calculated and compared with the experimental data.     

Microwave absorption coefficients of atmospheric pollutants and constituents

Calculations of the transition frequencies and absorption coefficients of microwave rotational transitions are given for a number of atmospheric pollutants and constituents. New measurements of the absorption coefficients are made in the vicinity of 70 GHz. The apparatus used in these measurements is briefly described. The calculated absorption coefficients are compared with these measurements and with existing measurements at other frequencies where available. Transitions with frequencies up to about 200 GHz are considered for the molecules and radicals SO₂, O₃, H₂O, NO₂, H₂S, H₂CO, NH₃, CO, OCS, N₂O, NO, OH, O₂, SO. Also discussed are criteria for the selection of appropriate transitions for the development of high sensitivity monitors to be used in air pollution and combustion research.     

Far-infrared optical properties of YVO4 single crystal

Near-normal incident infrared reflectivity spectra of a (001) YVO₄ single crystal have been measured at different temperatures in the frequency region between 100 and 6000 cm^-1. The reflectivity spectra are analyzed with the factorized form of the dielectric function, and the dielectric properties and optical conductivity of the YVO₄ crystal are obtained. From the TO/LO splitting, effective charges at different temperatures are calculated to study the ionicity of YVO₄. The internal modes of the VO₄^3- ion and the external modes of the Y(VO₄) lattice are compared with SiO₄^4- in zircon and with other rare-earth vanadates.     

Effect of nanosizing on lattice and magnon modes of hematite

Broadening of Raman lines and a decrease of frequencies relative to the bulk material are observed for the lattice modes of nanoparticles of the hematite phase of Fe₂O₃. The magnon mode frequency is observed to shift up with reduced particle size in contrast to the lattice modes. The effects are suggested to be a result of phonon confinement and represent the first observation of phonon confinement effects on magnon modes. The Fe₂O₃ nanoparticles are produced by laser decomposition of Fe₃O₄ nanoparticles. X-ray diffraction and Raman frequency shifts indicate the Fe₂O₃ nanoparticles are smaller than the parent Fe₃O₄ nanoparticles.     

Analysis of the conditions favoring the onset of long-range order in disordered relaxors of different types

The possibility of realizing a stable relaxor behavior in PbSc_1/2Nb_1/2O₃ (PSN) compounds and the conditions favoring the formation of a phase with long-range order in PbZn_1/3Nb_2/3O₃ (PZN) are studied using optical methods. It is found that the replacement of only 6% of Pb ions with Ba ions (PBSN-6) freezes the relaxor state in PSN compounds, while adding even 7% of PbTiO₃ to the PZN relaxor (PZN-7PT) leads to the formation of a phase with long-range order and a phase transition occurring at T _maxε through the percolation mechanism. Common features in the formation and destruction of the long-range order phase in relaxors of different types are revealed. The differences associated with the presence of spontaneously polarized ferroelectric regions in the cubic nonpolar matrix of PSN-type compounds are determined.     

Ab initio study of phonon dispersion and thermodynamic properties of pure and doped pyrites

Pyrites (FeS₂) are solid minerals that are found abundantly in Nigeria and are easy to prepare in laboratories. In this work, FeS₂ is studied extensively in its pure state as well as when iron is substitutionally doped with zinc and calcium at concentrations of 0, 0.25, 0.5, 0.75 and 1. Using density functional theory, the eectronic, dynamic and thermodynamic properties were calculated. The results revealed that the lattice parameters and bulk modulus increases with increasing concentration and the obtained values are in agreement with available experimental and theoretical values. Though pyrite, when doped with zinc, obeys Vegard’s law, doping with calcium revealed pronounced deviation from this law. The calculated band structures showed that FeS₂ has an indirect band gap whose size decreases after introducing zinc while doping with calcium increases the band gap. The phonon dispersion of the end members FeS₂ and ZnS₂ indicate that the systems are dynamically stable while CaS₂ is dynamically unstate. Also, the thermodynamic properties of the pure and doped pyrites were calculated and the ranges of temperature at which the lattice and electronic degrees of freedom contribute to the specific heat capacity are presented.     

A new family of ternary intermetallic superconducting/magnetic stannides

A new family of rare earth-rhodium-tin intermetallic compounds, with the representative formula (RE)Rh_xSn_y, has been synthesized in single crystal form. The compounds containing the heavier rare earths are superconducting and those with the lighter rare earths are generally magnetic. The compound ErRh_1.1Sn_3.6 exhibits reentrant superconductivity with T_c = 0.97 K and T_m = 0.57 K as determined from ac magnetic susceptibility measurements. The synthesis and X-ray characterization of the series are described and the results of electrical resistivity, upper critical magnetic field, magnetic susceptibility, specific heat and neutron scattering measurements on the Er compound are given.     

Electron-beam charging of dielectrics preirradiated with moderate-energy ions and electrons

The effects of charging of dielectric targets irradiated with moderate-energy electrons in a scanning electron microscope are examined. Considerable differences in the kinetics of charging of the reference samples and the samples preirradiated with ions and electrons are reported. These differences are attributed to the processes of radiation-induced defect formation in Al₂O₃ (sapphire) and SiO₂ that are, however, dissimilar in nature. The contributions of surface structure modification and changes in the electrophysical parameters of the surface (specifically, the charge spreading effect) are revealed. Critical doses of irradiation with Ar⁺ ions and electrons inducing active defect formation in dielectric targets and critical values of internal charge fields producing a significant contribution to the temporal parameters of Al₂O₃ and SiO₂ charging are determined.     

Laser synthesis of nanopowders

YSZ, YSZ + Al₂O₃, Ce_0.78 Gd_0.22O_2−δ, and 5NdY₂O₃ nanopowders are obtained using target evaporation with a repetitively pulsed CO₂ laser and subsequent vapor condensation in the flow of a carrier gas. The design of the laser complex for producing the nanopowder and the block diagram and the characteristics of the repetitively pulsed CO₂ laser pumped by a combined discharge are presented. The size distribution of the nanoparticles is studied and the x-ray data are reported. It is demonstrated that a nanopowder output rate of 15–75 g/h linearly increases with the mean laser power. Under equal conditions, the size distribution of the particles is weakly affected by the type of the target material. The results obtained are interpreted. Original Text ? Astro, Ltd., 2006.     

5d过渡金属二硼化物的结构和热、力学性质的第一性原理计算

利用基于密度泛函理论的第一性原理系统地研究了5d过渡金属二硼化物TMB₂ (TM=Hf–Au) 的结构、热学、力学和电学性质. 我们考虑了三种结构, 分别为AlB₂, ReB₂和WB₂结构. 计算得到的晶格常数与先前的理论和实验研究符合得很好. 通过计算生成焓预测了化合物的热力学稳定性; 从HfB₂到AuB₂, 生成焓的整体趋势是逐渐增加的. 在所考虑的结构中, 对HfB₂和TaB₂, AlB₂结构是最稳定的; 对WB₂, ReB₂, OsB₂, IrB₂和AuB₂, ReB₂结构是最稳定的; 对PtB₂, WB₂结构是最稳定的. 在所考虑的化合物中, ReB₂结构的ReB₂具有最大的剪切模量(295 GPa), 是最硬的化合物, 与先前的理论和实验结果相符. 计算得到的总态密度显示所有结构都具有金属特性. 讨论了系列化合物的变化趋势. 关键词： 弹性性质 热力学性质 第一性原理 5d过渡金属二硼化物     

Hydrogen Isotope Effects in Electrochemical Reductions of Organic Chloro Compounds

Abstract

The kinetic hydrogen isotope effects in electrochemical reductions of CCl₃COOD, CDCl₂COOD, CCl₄, CDCl₃, benzyl chloride, 1-chloronaphthalene and 4-chlorobenzonitrile in deuterated reaction media were determined. The aliphatic chloro compounds are reduced with rather small isotope effects k_H/k_D = 1.2…1.7, as expected if anionic intermediates are formed and protonated. Benzyl chloride is reduced with an apparent k_H/k_D = 2.1, which is probably too high due to radical reactions. Aromatic chloro compounds are reduced with significant participation of a second mechanism via radical intermediates which abstract carbon-bound hydrogen atoms from the components of the reaction system. The resulting isotope effects are k_H/k_D = 1.2 for the part of the reduction which proceeds via anions and k_H/k_D ～ 2.5 for the competing radical reaction.     

Numerical test of approximate theories of the superconducting transition temperature

Various equations for t_c obtained from approximations to the Eliashberg theory are numerically solved and T_cvs λ curves are drawn. Specifically, the weak coupling Kirzhnits, Maksimov and Khomskii (KMK) equation, Eliashberg equation and BCS equation with Bardeen Pines interaction are considered with the α²F function of Nb. The KMK equation gives quite a high value or T_c and the BCS equation overestimates T_c by an even larger factor when compared with the results of the Eliashberg equation.     

Luminescence properties of “relatively” trap-free crystals of anhydrous salts containing divalent manganese

Crystals of RbMnCl₃, Rb₂MnCl₄, CsMnCl₃, RbMnBr₃, CsMnBr₃ and CsMnI₃ which are grown from melts in contact with elemental manganese luminescence strongly throughout the 10K to 300K range. At room temperature the emission intensities from these crystals are larger by orders of magnitude than those observed from crystals of the same salts prepared without metallic manganese. Emission lifetime and intensity measurements made as a function of temperature are consistent with the view that the contact of the molten salt with metallic manganese during crystal growth yields materials which are relatively free of quenching traps. The effect appears to be quite general.