X-ray absorption fine-structure study on the fine structure of lutetium segregated at grain boundaries in fine-grained polycrystalline alumina

Local atomic structures around the Lu³⁺ ion in a 0.1?mol%?LuO_1.5-doped fine-grained Al₂O₃, in which the doped Lu³⁺ ions segregate to the grain boundaries, was characterized by Lu L₃-edge X-ray absorption fine structure. Structural parameters in LuO_1.5-doped Al₂O₃ were determined by a curve-fitting method, and the results showed that six O^2? ions coordinate with the Lu ion in LuO_1.5-doped Al₂O₃. In addition, it was also revealed that the Lu–O interatomic distance in Lu-doped Al₂O₃ was close to that in Lu₂O₃, which was about 19% longer than the Al–O interatomic distance in undoped Al₂O₃. The present results indicated that the local atomic structures around Lu in Al₂O₃ are close to that in Lu₂O₃. It is thus supposed that atomic distances between Al³⁺ and O^2? ions in the vicinity of Lu-segregated grain boundaries are shortened in comparison with that in undoped Al₂O₃. A first-principles molecular orbital calculation was performed for the [Al₂O₉]^?12 model cluster, and the shortening of the Al–O interatomic distance was found to have an effect of increasing the ionicity in Al³⁺ ions.     

Improved theory of helium fine structure

Improved theoretical predictions for the fine-structure splitting of 2(3)PJ levels in helium are obtained by the calculation of contributions of order alpha5 Ry. New results for transition frequencies nu(01) = 29616943.01(17) kHz and nu(12) = 2291161.13(30) kHz disagree significantly with the experimental values, indicating an outstanding problem in bound state QED.     

Diffusion behavior of helium in titanium and the effect of grain boundaries revealed by molecular dynamics simulation

The microstructures of titanium(Ti), an attractive tritium(T) storage material, will affect the evolution process of the retained helium(He). Understanding the diffusion behavior of He at the atomic scale is crucial for the mechanism of material degradation. The novel diffusion behavior of He has been reported by molecular dynamics(MD) simulation for the bulk hcp-Ti system and the system with grain boundary(GB). It is observed that the diffusion of He in the bulk hcp-Ti is significantly anisotropic(the diffusion coefficient of the [0001] direction is higher than that of the basal plane),as represented by the different migration energies. Different from convention, the GB accelerates the diffusion of He in one direction but not in the other. It is observed that a twin boundary(TB) can serve as an effective trapped region for He.The TB accelerates diffusion of He in the direction perpendicular to the twinning direction(TD), while it decelerates the diffusion in the TD. This finding is attributable to the change of diffusion path caused by the distortion of the local favorable site for He and the change of its number in the TB region.     

High temperature embrittlement of metals by helium

The present knowledge of the influence of helium on the high temperature mechanical properties of metals to be used as structural materials in fast fission and in future fusion reactors is reviewed. A wealth of experimental data has been obtained by many different experimental techniques, on many different alloys, and on different properties. This review is mostly concentrated on the behaviour of austenitic alloys—especially austenitic stainless steels, for which the data base is by far the largest—and gives only a few examples of special bcc alloys. The effect of the helium embrittlement on the different properties—tensile, fatigue and, with special emphasis, creep—is demonstrated by representative results. A comparison between data obtained from in-pile (-beam) experiments and from post-irradiation (-implantation) experiments, respectively, is presented. Theoretical models to describe the observed phenomena are briefly outlined and some suggestions are made for future work to resolve uncertainties and differences between our experimental knowledge and theoretical understanding of high temperature helium embrittlement.     

Lamb shift, fine structure and hyperfine structure in helium like ions by fast beam laser spectroscopy

Using Doppler tuned fast beam laser spectroscopy with a continuous wave CO₂ laser the following energy intervals in helium like ions have been measured: 1s2s ¹S₀–1s2p ³P^1,F, ³P₀ in ^14,15N⁵⁺; 1s2p ³P_2,F–1s2p ³P_1,F' in ¹⁹F⁷⁺; and 1s2p ³P₀–1s2p ³P₁ in ²⁴Mg¹⁰⁺. The results test modern relativistic and QED atomic theory. This revised version was published online in August 2006 with corrections to the Cover Date.     

Finite-size effects on the structure of grain boundaries

We present a combined experimental and theoretical analysis of the structure of finite-sized Sigma 3 [112] grain boundaries in Au. High-resolution electron microscopy shows lattice translations at the grain boundary, with the magnitude of the translation varying along the finite-sized grain boundaries. The presence of this structural profile is explained using continuum elasticity theory and first-principles calculations as originating from a competition between elastic energy and the energy cost of forming continuous [111] planes across the boundary. This competition leads to a structural transition between offset-free and nontrivial grain boundary structures at a critical grain boundary size, in agreement with the experiments. We also provide a method to estimate the energy barrier of the gamma surface.     

Quantum electrodynamical corrections to the fine structure of helium

Order α⁶mc² corrections to the fine structure splitting of the deepest triplet P state (2³P_0,1,2) of the He⁴ atom have been investigated. The investigation is based on the covariant Bethe-Salpeter equation including external potential to take account of the nuclear Coulomb field. All order α⁶mc² corrections which arise from Feynman diagrams involving the exchange of one, two, and three photons, as well as radiative corrections to the electron magnetic moment have been found. The results are presented in a form suitable for computerized numerical evaluation.     

Precision calculation of fine structure in helium and Li~+

The fine structure constant α can be extracted from high-precision spectroscopy of the 23 P J fine structure splittings in helium and light helium-like ions. In this work, the 23 P J fine structure splittings of helium and Li+ion are calculated,including relativistic and QED corrections of order mα4, mα4(m/M), mα5, mα5(m/M), and Douglas–Kroll operators of mα6and mα6(m/M), which provide an independent verification for the previous calculations performed by Drake [Can.J. Phys. 80 1195(2002)] and by Pachucki and Yerokhin [Phys. Rev. A 79 062516(2009); Phys. Rev. Lett. 104 070403(2010); Can. J. Phys. 89 1139(2011)]. The results of the three groups agree with each other.     

Radiation-induced changes in the atomic structure of grain boundaries in tungsten

Field-ion microscopy is used to study changes in the structure of the grain boundaries induced by intergrain adsorption of point defects created by ion bombardment of tungsten bicrystals. It is found that irradiation at temperatures below the threshold of grain-boundary relaxation causes a local expansion of the boundaries. Computer simulation using molecular dynamics shows that intergrain adsorption of vacancies can lead to the formation of three-dimensional grain-boundary structures. Fiz. Tverd. Tela (St. Petersburg) 41, 383–385 (March 1999)     

Nucleation and growth of helium bubble at(110) twist grain boundaries in tungsten studied by molecular dynamics

Migration of He atoms and growth of He bubbles in high angle twist grain boundaries(HAGBs) in tungsten(W) are investigated by atomic simulation method. The energy and free volume(FV) of grain boundary(GB) are affected by the density and structure of dislocation patterns in GB. The migration energy of the He atom between the neighboring trapping sites depends on free volume along the migration path at grain boundary. The region of grain boundary around the He bubble forms an ordered crystal structure when He bubble grows at certain grain boundaries. The He atoms aggregate on the grain boundary plane to form a plate-shape configuration. Furthermore, high grain boundary energy(GBE) results in a large volume of He bubble. Thus, the nucleation and growth of He bubbles in twist grain boundaries depend on the energy of grain boundary, the dislocation patterns and the free volume related migration path on the grain boundary plane.     

Changes in electrolyte structure in solid state cells during discharge: Extended x-ray absorption fine structure study

The first EXAFS study of a solid state battery system is reported. The working cell is based on a copper(I) iodide-sulphonium iodide electrolyte. The EXAFS results clearly indicate that cell modification occurs during cell discharge.     

氦原子自旋-其它轨道相互作用精细结构参数的理论计算

以多电子原子精细结构哈密顿的球张量形式和氦原子非相对论性能级结构理论为基础,借助不可约张量理论,建立了计算氦原子自旋-其它轨道相互作用精细结构参数的一种解析理论形式.完成了所有的角向积分和自旋求和计算,自旋-其它轨道相互作用精细结构参数最终用若干个径向积分来表示.以氦原子(1s2p)3P态为例,借用类氢形式的径向函数对这些径向积分进行了近似计算.计算结果表明:在氦原子的精细结构中,自旋-其它轨道相互作用与纯自旋-轨道相互作用的作用效果相反;在总自旋-轨道相互作用精细结构参数中,自旋-其它轨道相互作用起决定性作用,它决定着精细结构分裂的顺序.     

氦原子自旋-自旋相互作用精细结构参数的理论计

利用多电子原子的精细结构哈密顿算符的球张量形式,通过计算氦原子的自旋-自旋相互作用哈密顿算符在|LSJMJ〉表象中的矩阵元,导出了氦原子的自旋-自旋相互作用精细结构参数的理论计算式,并就氦原子(n1s)(n2p)组态具体计算参数B之值.     

Dul'yan-Faustov pseudopotential equation and the fine structure of the helium atom. I

Translated from Izvestiya Vysshykh Uchebnikh Zavedenii, Fizika, No. 10, pp. 45–50, October, 1989.     

Theoretical modeling of the structure of tilt grain boundaries in crystalline silicon

This paper reports on the results of the calculation of the structures of tilt grain boundaries in crystalline silicon. The calculation has been performed using the classical molecular mechanics method with the Tersoff potential. The calculation of the phonon spectrum has demonstrated that there are several frequency ranges corresponding to vibrations localized along the grain boundary.