Ab initio study of the formation of structural kinks in carbon chains

Our theoretical studies using methods of electron density functional theory show that hydrogen, fluorine, and oxygen impurities must be present in the chamber during the synthesis of linear-chain carbon to form and stabilize experimentally observed kinks in carbon chains. The kink in the carbon chain forms during the adsorption of impurity atoms onto the chain. It is shown that it is energetically advantageous for impurity atoms to form kinks rather than to occupy other adsorption positions. The optimal kink angle of carbon chains is determined. The stability of bent carbon chains is estimated as a function of the length of linear fragments. It is shown that the formation of kinks leads to that the formation of a narrow band gap near the Fermi level: E _gH = 0.56 eV, E _gF = 0.59 eV, and E _gO = 1.04 eV.     

Comparative study between melted and mechanically alloyed samples of the Fe50Mn10Al40 nanostructured system

Nanostructured powders of melted and 48 h-mechanically alloyed (MA) samples of the Fe₅₀Mn₁₀Al₄₀ system were studied by XRD, SEM and Mössbauer spectroscopy (MS), and their properties were compared. The samples present BCC structure with similar mean lattice parameter (near 2.92 Å) and grain size (around 22 nm). The MA sample presents additionally the α-Fe phase. Mössbauer spectra of the samples present a hyperfine field distribution (HFD), a broad paramagnetic (P) site and a sextet showing that the BCC ternary phase is disordered with Fe sites in environments rich in Fe (HFD) and rich in Al and Mn (P), respectively. The mean hyperfine magnetic field vs. temperature curve of melted sample presents two kinks, one at 28 K and other at 210 K and a Curie temperature at 340 K. A similar curve is observed for the milled sample but the kinks occur at 65 and 265K. Mössbauer spectra at different temperatures with and without applied field permit to conclude that the low temperature anomaly corresponds to the freezing temperature of a re-entrant spin-glass phase (RSG) and that of the second one corresponds to a blocking temperature of a superparamagnetic (SP) phase. These phases are possible in the samples due to their disordered character induced by the preparation conditions and the competitive interactions of the Fe and Mn atoms. The enhancement of the magnetic behaviour of the MA sample is due its larger disorder induced by the preparation method that can also explain the increase of the RSG and SP transition temperatures.     

On the new universal possibility to detect phase transitions in correlated electron systems

We investigate temperature and concentration driven phase transitions (structural and reentrant phase transitions included) in magnetic and superconducting systems with the use of a wide class of model Hamiltonians applied to rare earth (Re) based compounds and alloys (integer and fluctuating valence systems). Studying the temperature or concentration dependence of the chemical potential we observe small but distinct and well localized kinks at all critical points as evidence for phase transitions. For systems with, at least, two kinds of interacting electrons the kinks at critical temperatures or concentrations occur also in the electronic average occupation numbers (critical electron redistribution). These observations suggest a direct and universal experimental application of the chemical potential as a detector of phase transitions for temperature and concentration driven phase transitions, as well as, for pressure-or external field-induced transitions in solids. The agreement between the calculated critical temperature behaviour of the chemical potential, presented in this paper, and experimental measurements for high-temperature superconductor YBa₂Cu₃O_7-δ entirely supports these general observations.     

Drag exerted on dislocation kinks in solid solutions

The theory of drag exerted on dislocation kinks by statistical density fluctuations of impurity atoms is generalized to the case of solid solutions. The conditions of the transition to anomalous kink drift, which is characterized by a nonlinear time (t) dependence of the displacement x (x ～ t ^δ, δ < 1), are obtained. The related consequences for the dislocation mobility and the plastic flow of a solid solution are derived. The theory is illustrated through calculating the concentration dependence of the deforming stress in a Ge_1?c Si _c solid solution.     

First principles study of the electronic and geometric structure of Cu(5 3 2)

Using density functional theory with the generalized gradient approximation and ultra-soft pseudo-potentials, we have calculated structural relaxations of the Cu(5 3 2) surface which contains steps and kinks. We find the relaxation pattern to oscillate dramatically for atoms in the first 10 layers before decaying rapidly in the bulk. The most striking feature is an outward expansion of the relative interlayer separation d₁₂ of 25%. We also find serious discrepancies with relaxation pattern and relaxation amplitudes calculated using embedded atom method potentials that may reflect the limitation of these potentials to accurately describe systems with complex geometries. Full potential calculations reveal a dispersionless surface state along a high symmetry direction in the surface Brillouin zone. Valence charge density along several planes show excess of charge around kink atom and the nature of bonding with other (5 3 2) atoms.     

Fabrication of L10 FePtAg nanoparticles and a study of the effect of Ag during the annealing process

L1₀ ferromagnetic phase FePt nanoparticles containing Ag atoms (FePtAg) were synthesized by means of a liquid phase process, followed by annealing. The addition of Ag to FePt nanoparticles permits annealing to be conducted at a lower temperature (350 °C). This is further accompanied by a subsequent transformation in the crystal phase from the FCC superparamagnetic phase to the FCT (L1₀) ferromagnetic phase. The effects of annealing temperature and the Ag atoms inside the nanoparticles on the magnetic properties of the FePt nanoparticles have been studied. Using electron spectroscopy for the chemical analysis (ESCA), Ag atoms in the L1₀ phase FePtAg nanoparticles were found to be localized on the surface region of the annealed nanoparticles. The Ag atoms function to inhibit the oxidation of FePt, causing the particles to become more stable and to have ferromagnetic properties.     

XAFS和XRD研究高能球磨对Fe70Cu30合金结构的影响

利用XRD和XAFS方法研究机械合金化Fe₇₀Cu₃₀二元金属合金随球磨时间的结构变化.XRD结果表明，球磨2 h后，部分金属Fe与Cu生成Fe-Cu合金；球磨20h后，金属Fe与Cu已完全合金化生成Fe-Cu合金，并只在2θ=44°处出现一个宽化的弱衍射峰，认为是在球磨20h后的Fe₇₀Cu₃₀合金中共存着fcc和bcc结构的Fe-Cu合金相.XAFS结果进一步表明，在球磨的初始阶段(2h)，fcc结构的Cu颗粒的晶 关键词： XAFS XRD 70Cu₃₀合金')" href="#">Fe₇₀Cu₃₀合金 机械合金化     

Propagation and collision of compacton-like kinks in Klein—Gordon lattice system

We study the propagation and collision of the compacton-like kinks in the system of an anharmonic lattice with a double well on-site potential by a direct algebraic method and numerical experiments. It is found that the localization of the compacton-like kinks is related to the nonlinear coupling parameter Cnl and the potential barrier height V0 of the double well potential. The velocity of the propagation of the compacton-like kinks is determined by the linear coupling parameter Cl, the nonlinear coupling parameter Cnl and the localization parameter q. Numerical experiments demonstrate that appropriate Cl is not detrimental to a stable propagation of the compacton-like kinks. However, the collision of compacton-like kinks and anti-kinks in the lattice with comparatively small Cl leads to the emergence of a discrete stationary breather and small amplitude nonlinear oscillation background, while moderate Cl results in the emergence of two deformed kinks with radiating oscillations and lower propagation velocities.     

铝-0.5%铜合金中反常振幅效应内耗峰的进一步实验和位错弯结气团模型

用低频扭摆进一步研究了在Al-0.5％Cu合金中观察到反常位错内耗峯的条件。结果指出,对于完全退火的试样来说,需要有适当的冷加工量,但是对于高温淬火试样则不需要冷加工。用位错气团模型定性地解释了过去所观测到的表现反常振幅效应的时效内耗峯和温度内耗峯;同时指出,简单的气团模型在作定量的解释时,遇到了下述的困难:(i)在测量内耗所用的交变应力的作用下,位错线所能够拖着气团移动的距离太短。(ii)为了气团能够被位错拖着移动,组成气团的溶质原子必须具有比通常大很多个数量级的扩散系数。(iii)根据气团模型,从理论上计算出来的使位错拖着气团以临界速度而移动时,所需的临界应力比观测值大几百倍。提出了溶质原子沿着位错弯结而扩散的气团模型,这个改进模型能够初步解决上述困难,并能定性地解释所观测的结果。这个模型所依据的基本假设是,要观测到反常内耗现象,位错线上必须具有一定数目的弯结。要得到这种弯结,可以对于退火试样进行适量的冷加工,或者把试样从高温淬火。带着弯结的位错线能够通过弯结的沿边运动而实现垂直于位错线方向的移动。可以假定,气团只在弯结两端的直位错段处形成,在弯结本身上并不形成气团。在弯结的沿边振动的过程中,聚集在弯结两端的溶质原子可以沿着位错管道进行来回的短程扩散。已知沿着位错管道的扩散具有比在正常晶体点阵中扩散时大得多的扩散系数。     

Analysis of hydrogen adsorption in the bulk and on the surface of magnesium nanoparticles

The stability of magnesium hydride (MgH _x ) nanoparticles (x = 0.5, …, 2) is investigated using ab initio calculations. It is shown that for a nanoparticle diameter of D ～ 5 nm, the internal pressure becomes lower than 3 kbar; for this reason, the structure of hydride nanoparticles coincides with the structure of this hydride in crystalline form. It is found that the phase of partly saturated MgH _x hydrides (x < 2) must decompose into the phase of pure hcp magnesium and the α phase of MgH₂. The frequencies of jumps of hydrogen atoms within the hcp phase of magnesium and in the α phase of MgH₂ are calculated; it is shown that slow diffusion of hydrogen in magnesium is due to the large height of potential barriers for motion of hydrogen within MgH₂. To attain high diffusion rates, the structures of Mg₅₃Sc and Mg₅₃Ti crystals and their hydrides are calculated. It is found that the frequency of jumps of H atoms in Mg₅₃ScH₁₀₈ near the Sc atoms does not noticeably change as compared to the frequency of jumps in the α phase of MgH₂, while the frequency of jumps in Mg₅₃TiH₁₀₈ near Ti atoms is higher by approximately a factor of 2.5 × 10⁶. This means that diffusion in manganese hydride with small admixtures of titanium atoms must be considerably eased. Chemical dissociation of hydrogen molecules on the (0001) surface of hcp magnesium, on the given surface with adjoined individual Ti atoms, and on the surface of a one-layer titanium cluster on the given surface of magnesium is investigated. It is found that dissociation of hydrogen at solitary titanium atoms, as well as on the surface of a Ti cluster, is facilitated to a considerable extent as compared to pure magnesium. This should also sharply increase the hydrogen adsorption rate in magnesium nanoparticles.     

Nonlinear conductivity and narrow band noise in NbSe3

A charge density wave system near commensurability and with strong damping is considered, as a model for NbSe₃. The observed threshold field is associated with depinning of a commensurate part of the charge density, while the excess charge, in form of phase kinks, contributes just to the ohmic conductivity. The characteristic length associated with the frequency generation is one lattice constant.     

Atomic simulation of the dynamic properties for a double period structure with 90° partial dislocation in Si

The dynamic properties of a double period (DP) structure with a 90° partial dislocation were investigated by atomic simulation methods in Si. By the use of molecular dynamics (MD) method, the motion sequences of kinks and reconstruction defect (RD) of DP structure were obtained. Based on the conjugate gradients (CG) results and tight-binding (TB) potential, the formation energies E_f of kinks were computed. In addition, the migration barriers W_m of kinks were also calculated via nudged elastic band (NEB) method with TB potential. The results show that E_f is smaller than W_m, which means that the migration barrier of kink dominates the motion of DP structure. According to the activation energies of short dislocation segments (2.17 eV) and long dislocation segments (1.61 eV), we predict that the experimental results may be between these two values.     

Thermal evolution of the first stage Cs(THF)1.75C24 graphitide,an example of segregation and decorrelation at low temperature

The thermal variation of the first stage ternary compound Cs(THF)_1.75C₂₄ has been investigated by powder neutron diffraction. Two phase transitions have been observed. The first one occurs at 248 K and can be interpreted as a three-dimensional segregation within the graphitic host. The second one starts at about 180 K and is continuous down to 50 K. This corresponds to a decorrelation of the graphen planes at low temperature. The origin of this transition could be the elastic constraint due to the kinks of the planes.     

MAGNETIC PROPERTIES OF BALL MILLED FexCu1-x (x=0.4, 0.5) ALLOY

The samples Fe_0.4Cu_0.6 and Fe_0.5Cu_0.5 ball milled for 50 h are investigated by X-ray diffraction, M?ssbauer spectra, as well as magnetic measurement. The experiments show that the structure of the samples is fcc, with lattice constant 0.361 nm and there are fcc Fe-rich phase and fcc Cu-rich phase in the samples. Most of Fe atoms (91%) are in the fcc Fe-rich phase, which is a ferromagnetic phase. The M-H curve at 1.5 K shows the saturation magnetization of the samples are 80.5 emu/g and 101.6 emu/g for Fe_0.4Cu_0.6 and Fe_0.5Cu_0.5 respectively. The average magnetic moment of Fe atoms is deduced to be 2.40 μ_B . Compared with theoretical predication, the Fe atoms in the fcc phase are in high spin state.     

Statistical description of the motion of dislocation kinks in a random field of impurities adsorbed by a dislocation

A model has been proposed for describing the influence of impurities adsorbed by dislocation cores on the mobility of dislocation kinks in materials with a high crystalline relief (Peierls barriers). The delay time spectrum of kinks at statistical fluctuations of the impurity density has been calculated for a sufficiently high energy of interaction between impurities and dislocations when the migration potential is not reduced to a random Gaussian potential. It has been shown that fluctuations in the impurity distribution substantially change the character of the migration of dislocation kinks due to the slow decrease in the probability of long delay times. The dependences of the position of the boundary of the dynamic phase transition to a sublinear drift of kinks x ∝ t^δ (δ σ 1) and the characteristics of the anomalous mobility on the physical parameters (stress, impurity concentration, experimental temperature, etc.) have been calculated.     

The quantum kinks of two-dimensional theories of the φ4 type

We apply a recently established method of kink quantization, to two-dimensional theories of the φ⁴ type possessing either a Z(4) × O(N) or an SU(N) symmetry and containing N complex scalar fields. Correlation functions of kinks are estimated through a 1/N expansion. Quantum kinks are interpolated by a local field, whenever a broken symmetry phase occurs. These kinks are massless. This result holds up to all orders in 1/N.     

Crystal-quasicrystal local structural transition in Al-Cu-Fe

The rearrangement of the local environments of copper and iron in the ternary alloy Al₆₅Cu₂₂Fe₁₃ in transition from a crystalline phase to a quasicrystalline phase has been studied by combined extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) analysis. It has been found that the nearest environment of copper retains symmetry characteristic of a crystal; however, a turn and small shifts of copper matrix atoms causes a considerable rearrangement of aluminum atoms around iron. As a result, icosahedral clusters with pentagonal symmetry are formed around iron atoms, and translational symmetry breaking is accompanied by the transition of Al₆₅Cu₂₂Fe₁₃ to a quasicrystalline state.     

Structural, elastic, and electronic properties of orthorhombic NH3BH3: An ab initio study

At the generalized gradient approximation (GGA), the elastic constants of the orthorhombic phase of NH₃BH₃ were calculated with plane-wave pseudo-potential method. Our calculation showed that the orthorhombic phase NH₃BH₃ is a loose and brittle material, as well as hard to be deformed, also we calculated the elastic anisotropies and the Debye temperatures from the elastic constants. And from the band structure and density of state (DOS), we concluded that NH₃BH₃ is a wide-gap semiconductor and the band gap is almost 6.0 eV. The bonds between N atoms and H atoms show a strong covalent characteristic, B atoms and H atoms form ironic bonds, and so as to the B-N bonds. Electrons from the B atoms are absorbed by the H atoms around the B atoms, and the H atoms display electronegativity.     

Enhanced alignment and magnetic properties of die-upset nano-crystal Nd2Fe14B magnets with Nb addition

It is difficult to obtain the crystallographic alignment for stoichiometric Nd₂Fe₁₄B alloys by applying the melt-spun and subsequent hot-pressing and hot-deformation techniques. However, the enhanced alignment and magnetic properties of die-upset nano-crystal Nd₂Fe₁₄B magnets have been obtained by Nb addition in the present paper. The magnetic properties studies show that Nb addition leads to the remarkable increase of remanence B_r and intrinsic coercivity H_ci, which is due to the improvement of c-axis texture and refinement of microstructure. Microstructure studies using transmission electron microscopy (TEM) and X-ray diffraction (XRD) reveal that Nb atoms are enriched at grain boundary and the NbFeB phase is observed with increasing Nb content. Since some Fe atoms in the Nd₂Fe₁₄B phase participate in the formation of NbFeB phase, the excessive Nd atoms may be enriched at grain boundary, which may improve the physical property of grain boundary and provide a mass transport pass for preferential growth of oriented Nd₂Fe₁₄B grains, thus leading to the enhanced alignment and magnetic properties.     

机械合金化Fe100-xCux体系的X射线吸收精细结构研究

利用X射线吸收精细结构(XAFS)方法研究机械合金化制备的Fe_100-xCu_x(x=0,10,20,40,60,70,80,100,x为原子百分比)合金中Fe和Cu原子的局域环境结构.对于Fe_100-xCu_x(x≥40)二元混合物,球磨160h后,Fe原子的近邻配位结构从bcc转变为fcc,但Cu原子的近邻结构保持其fcc不变.与之相反,在Fe₈₀Cu₂₀和Fe₉₀Cu₁₀(x≤20)合金中,Fe原子的近邻配位保持其bcc结构而Cu原子的近邻配位结构从fcc转变为bcc结构.XAFS结果还表明,fcc结构的Fe_100-xCu_x样品中Fe的无序因子σ(0.0099nm)比bcc结构的Fe_100-xCu_x中的σ(0.0081nm)大得多;并且在机械合金化Fe_100-xCu_x(x≥40)样品中Fe原子的σ(0.0099nm)比其Cu原子的σ(0.0089nm)大.这表明机械合金化Fe_100-xCu_x样品中Fe和Cu原子可以有相同的局域结构环境但不是均匀的过饱和固溶体,而是由Fe富集区和Cu富集区组成的合金.我们提出互扩散和诱导相变机理来解释在球磨过程中Fe_100-xCu_x合金产生从bcc到fcc和从fcc到bcc变化的结构相变 关键词： XAFS 100-xCu_x合金')" href="#">Fe_100-xCu_x合金 机械合金化