Photoelectron spectra of bis-cyclopentadienyl metal dihalides

He(I) and HE(II) photoelectron spectra are reported for (η-C₅H₅)₂MX₂ (M = Ti; X = F, Cl, Br, I: M = Zr, Hf; X = Cl, Br: M = Ta; X = Cl, Br) and (η-MeC₅H₄)₂MX₂ (M = Nb, X = Cl: M = Mo; X = Cl, Br, I). A substantial variation is found in the ordering of the halogen and cyclopentadienyl ionizations, the order being dependent on the metal as well as on the halogen. The compounds may be divided into three classes, namely, those in which the electrons in cyclopentadienyl e₁ orbitals ionize at a lower energy than those occupying halogen pπ orbitals, those in which halogen pπ electrons have lower ionization energy than cyclopentadienyl e₁ electrons and those in which the corresponding electrons arise from extensively delocalized molecular orbitals with significant contributions from both these categories of fragment orbital.     

掺杂对Ni51.5Mn25Ga23.5相变行为和磁性的影响

通过往母合金Ni_51.5Mn₂₅Ga_23.5掺入7种IVA, VA和VIA 过渡族元素得到系列掺杂合金Ni_51.5Mn₂₃M₂Ga_{23 .5}.M为掺杂元素.实验结果表明，掺杂效应一般引起马氏体相变温度的下降，其中，W 的掺杂是7种元素中唯一使相变温度升高的特例，且出现了中间马氏体相变.同时，在价电子 浓度不变的情况下，相变更敏感于原子的尺度效应.实验发现，Ti，Zr，Hf，V四种非磁性元 素的掺杂使Mn原子磁矩减小，而Nb，Ta，W三种非磁性元素的掺杂却可以明显地增大Mn原子 的磁矩.在考察掺杂效应时，不能忽略马氏体相变引起的晶格变化对材料磁性的影响. 关键词： NiMnGa 掺杂 马氏体相变 磁性     

Pressure-induced structural phase transformation and elastic properties of transition metal mononitrides

The high-pressure structural phase transition in six transition metal mononitrides (TMNs) (M=Ti, Mo, V, Nb, Hf, and Zr), have been studied using a two-body interionic potential theory which includes the effect of Coulomb screening due to the semi-metallic nature of these compounds. The present theoretical results have been compared with the corresponding experimental and predictions of LDA theory. These TMN compounds have been found to undergo NaCl (B₁) to CsCl (B₂) phase transition, at a pressure quite high as compared to other binary systems. We have also predicted the elastic constants. It is shown that these binary materials are partially ionic in nature and the structural transformation is analogous to several other ionic binary systems.     

Elastic properties, thermal expansion coefficients and electronic structures of Ti0.75X0.25C carbides

Elastic properties, thermal expansion coefficients and electronic structures of Ti_0.75X_0.25C carbides (X=W, Mo, Ta, Nb, V, Hf, Zr, Cr and Al) were systematically investigated using ab initio density functional theory (DFT) calculations. The calculated elastic moduli, electronic structures and thermal expansion coefficients α(T) of pure TiC are in good agreement with experimental data and other DFT calculations. Based on a phenomenological formula, the trends of elastic properties and ductile/brittle behavior of Ti_0.75X_0.25C were analyzed. It was found that alloying elements W, Mo, Ta, Nb, V and Hf can increase elastic moduli, while Zr, Cr and Al reduce moduli. The nearly free electron model and Debye approximation were applied in the evaluation of α(T). The anharmonic effect was taken into account by including volume-dependent elastic moduli and Debye temperature. Results show that alloying additions of 3d V, 4d Zr and Mo slightly reduce α(T), while 3d Cr increases α(T), Al, 4d Nb, 5d Hf and W almost keep α(T) unchanged in Ti_0.75X_0.25C at high temperatures. The electronic structures of Ti_0.75X_0.25C were calculated and analyzed, and the electronic density of states was used to interpret variations of elastic properties and ductile/brittle behavior induced by alloying additions.     

High-coercivity nanocomposite permanent magnet based on Nd–Fe–B–Ti–C with Cr addition for high-temperature applications

We have investigated influences of various elements (Ti, Nb, Mo, Ta, Hf, W, V, Cr) on magnetic properties of Fe–B/Nd₂Fe₁₄B-based Nd–Fe–B–Ti–C nanocomposite magnets in order to obtain larger coercivity required for high-temperature applications. As a result, addition of Cr was found to be most effective among additive elements investigated to enhance coercivity. Thermal flux losses of high-coercivity (H_cJ=1609 kA/m) Nd–Fe–B–Ti–C–Cr nanocomposite magnet at 200 °C are less than 2%.     

A first principles calculation of site occupancy of the B2 phase in Ti2AlX (X=V,Cr, Fe,Mo, Ta,Nb, Zr,Hf and Re) intermetallics

The site occupancy of the B2 phase in Ti₂AlX (X=V, Cr, Fe, Mo, Ta, Nb, Zr, Hf and Re) intermetallics have been studied using first principles pseudo potential plane wave method.The Ti, Al and X atoms are arranged in five different ways, in the lattice sites corresponding to B2 structure of Ti₃Al. In Ti₃AlX, the X atoms are substituted at the Ti and / or Al sites. Further, the equilibrium lattice constants and the formation energy (E_for) of these intermetallics with different site occupancies in the B2 phase have been predicted. The formation energy values suggest that the B2 phase is stable in all alloys. Amongst the five cases in a particular alloy, stable configuration is identified with the minimum E_for and is further considered for the calculations of mechanical properties. All the alloys are mechanically stable in terms of Born stability criteria and show anisotropic behaviour. All the alloys display ductile behaviour in terms of G/B ratio.     

The superconductivity and structure of equiatomic ternary transition metal pnictides

Recent superconductivity and crystallographic data for the MM′X ternary compounds with M = Ti, Zr, or Hf, M′ = Ru or Os, and X = P or As are presented. Moderate to high superconducting transition temperatures (T _c 's)are exhibited by the ZrRuSi-type hexagonal phase which is found to be metastable at low temperatures. The low-temperature phase of ZrRuP has the TiNiSi-type orthorhombic structure and exhibits superconductivity at 3.9K. The low-temperature phase of HfRuAs has the TiFeSi-type orthorhombic superstructure and remains normal at 1.0K. TiRuAs exists only with the superstructure and remains normal at 0.35K. The details of these various structural modifications point to the importance of undistorted zig-zag chains of metal atoms for the occurrence of superconductivity in these compounds. The lowT _c for TiRuP and the absence of superconductivity for TiOsP above 0.35K are not accounted for by these structural modifications.     

Adsorption and solution of H2 and N2 by Ta and Nb

The condensation and desorption kinetics of H₂ and N₂ on polycrystalline wires of Ta and Nb are examined for solute concentrations between 0.1 and 100 monolayers. Accurate comparisons of adsorbate densities and sticking coefficients on Ta, Nb, Mo and W are obtained by making simultaneous measurements on all four substrates in the same vacuum system. Desorption exhibits second order kinetics for all substrates at all coverages. Desorption activation energies are higher on Ta and Nb than on W and Mo for both gases. Diffusion limited desorption is observed for H₂ at high heating rates. Solution of H₂ occurs at measurable rates (s ? 0.01) in Ta and Nb even at 78 °K while for N₂ no solution is observed for T < 600 °K.     

Surface energies of the solid solutions between Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W

The surface energy for different surface orientations of the solid solutions as a function of concentration formed by Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W is computed and analyzed using the BFS method for alloys. Similarities and differences among the different binary alloys are examined in terms of strain and chemical effects.     

Electronic structure and shearing in nanolaminated ternary carbides

We have studied shearing in M₂AlC phases (M=Sc,Y,La,Ti,Zr,Hf,V,Nb,Ta,Cr,Mo,W) using ab initio calculations. We propose that these phases can be classified into two groups based on the valence electron concentration induced changes in C₄₄. One group comprises M=V B and VIB, where the C₄₄ values are approximately 170 GPa and independent of the corresponding MC. The other group includes M=IIIB and IVB, where the C₄₄ shows a linear dependency with the corresponding MC. This may be understood based on the electronic structure: shear resistant bands are filled in M₂AlC phases with M=V B and VIB, while they are not completely filled when M=IIIB and IVB. This notion is also consistent with our stress-strain analysis. These valence electron concentration induced changes in shear behaviour were compared to previously published valence electron concentration induced changes in compression behaviour [Z. Sun, D. Music, R. Ahuja, S. Li, J.M. Schneider, Phys. Rev. B 70 (2004) 092102]. These classification proposals exhibit identical critical valence electron concentration values for the group boundary. However, the physical mechanisms are not identical: the classification proposal for the bulk modulus is based on MC-A coupling, while shearing is based on MC-MC coupling.     

Ta对C—15相V2Hf和V2(Zr0.5Hf0.5)系列声子性能的影响

本文用中子飞行时间方法对C-15相的超导材料V₂Hf,V₂Ta和V₂Hf_0.8Ta_0.2以及V₂Zr_0.5。Hf_0.5和V₂Zr_0.5Hf_0.33Ta_0.17的热中子非弹性散射谱作了测量,并计算出相对的广义声子态密度。结果与早先发表的Nb对C-15相V₂Zr和V₂(Hf_0.5Zr_0.5)系列声子性能的影响一致:声子频率随超导转变温度T_c增加而软化,随T_c减小而硬化。这表明,对于此类材料弹性软化在一定程度上对提高T_c起了作用。结果还进一步表明V₂Zr或V₂Hf与V₂(Zr_0.5Hf_0.5)之间有着质的差别,V₂Hf加Ta后,T_c增加,声子频率软化,而V₂(Zr_0.5Hf_0.5)加Ta后,T_c减小,声子频率则略有硬化。这与V₂Zr和V₂(Hf_0.5Zr_0.5)加Nb的结果是一致的。此结果可以用角动量分波表象的能带论方法分析电-声耦合相互作用得出的杂化理论来定性解释。 关键词：     

MO2,M2O5和MO3氧化物对锂硼酸盐玻璃离子导电性的影响

本工作测量了45Li₂O·50B₂O₃·5M_mO_n(M=Al,Ti,Zr,P,V,Nb,Ta,Cr,Mo,W)玻璃的总电导率和电子电导率,以Raman光谱研究了玻璃结构,依据作为锂离子定域体的阴离子团的性质讨论了MO₂,M₂O₃和MO₃氧化物对玻璃离子导电性的影响。 关键词：     

Zur Zustandsdichte der Übergangsmetalle

X-ray isochromats of the bcc transition metals Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W have been measured with high resolution and under comparable conditions. These curves show, that the rigid-band-model is valid for these metals within one row of the periodic table. The width of the conduction band grows from the 3d- to the 5d-row. These results of the isochromat measurements are compared with conclusions about the density of states at the Fermi level, calculated from values of the electronic specific heat, magnetic susceptibility, and superconducting transition temperature. A comparison with theoretical band calculation shows, that a lowered value of the exchange term in the potential would bring into better agreement the widths of the conduction band, taken from calculation and from isochromat measurements.     

Plastic deformation properties of Zr–Nb–Ti–Ta–Hf high-entropy alloys

We have investigated the plastic deformation properties of single-phase Zr–Nb–Ti–Ta–Hf high-entropy alloys from room temperature (RT) up to 300 °C. Uniaxial deformation tests at a constant strain rate of 10^?4?s^?1 were performed, including incremental tests such as stress relaxations, strain-rate changes, and temperature changes in order to determine the thermodynamic activation parameters of the deformation process. The microstructure of deformed samples was characterized by transmission electron microscopy. The strength of the investigated Zr–Nb–Ti–Ta–Hf phase is not as high as the values frequently reported for high-entropy alloys in other systems. At RT we measure a flow stress of about 850 °C. We find an activation enthalpy of about 1 eV and a stress dependent activation volume between 0.5 and 2 nm³. The measurement of the activation parameters at higher temperatures is affected by structural changes evolving in the material during plastic deformation.     

Elastic properties of Nb-based alloys by using the density functional theory

A first-principles density functional approach is used to study the electronic and the elastic properties of Nb₁₅X(X = Ti, Zr, Hf, V, Ta, Cr, Mo, and W) alloys. The elastic constants c₁₁ and c₁₂, the shear modulus C', and the elastic modulus E_〈100〉 are found to exhibit similar tendencies, each as a function of valence electron number per atom (EPA), while c₄₄ seems unclear. Both c₁₁ and c₁₂ of Nb₁₅X alloys increase monotonically with the increase of EPA. The C' and E_〈100〉 also show similar tendencies. The elastic constants (except c₄₄) increase slightly when alloying with neighbours of a higher d-transition series. Our results are supported by the bonding density distribution. When solute atoms change from Ti(Zr, Hf) to V(Ta) then to Cr(Mo, W), the bonding electron density between the central solute atom and its first neighbouring Nb atoms is increased and becomes more anisotropic, which indicates the strong interaction and thus enhances the elastic properties of Nb-Cr(Mo, W) alloys. Under uniaxial 〈100〉 tensile loading, alloyed elements with less (more) valence electrons decrease (increase) the ideal tensile strength.     

Perturbation of the decay rate of90m Nb by implantation in metals

The perturbation of the decay constant of^90m Nb implanted in various transition metals (Zr, Nb, Hf, Ta, Ag and Au) has been determined by direct half-life measurements. Particular care was taken to evaluate and minimize possible systematic errors. The maximum relative change found is ν_Au?ν_Nb/ν_Nb=(4.5±0.8)%     

Alloying effects and site occupancies of Re in the C14 Cr-based Laves phases: a first-principles study

First-principles calculations based upon the density functional theory have been carried out to investigate the alloying effects and site occupancies of Re in the C14 XCr₂ (X?=?Nb, Ta, Ti, Zr, Hf) Laves phases. The calculated results indicate that Re tends to facilitate and participate in the formation of the Laves phases, generating X₈Cr₁₅Re accordingly. The partial density of states and charge density di?erence were analysed to re?ect the bonding characteristics. For X₈Cr₁₅Re, the bonding between the doped Re and its nearest neighbour Cr atoms all show covalent characteristics, which contribute to the phase stability. The substitution of Re on X sites is energetically unfavourable due to the weak bonding between Re and its nearest neighbour X atoms.     

Structural features of triple equilibrium diagrams of the systems based on Ni-Al

We present the results from our search for common regularities between the number of compounds on the diagrams of state in the Ni-Al-Me triple systems (Me = Ti, V, Cr, Mn, Fe, Co, Cu, Zr, Nb, Mo, Ru, Hf, Ta, Re) in dependence on the thermodynamic and dimensional parameters of the third component in compounds based on Ni-Al. We determine the correlation between the number of electrons outside the shell of the inert gas of the third element in the Ni-Al-Me system and the number of compounds in the corresponding triple diagrams.     

Magnetic and magneto-optical properties of heusler alloys based on aluminium and gallium

The authors have studied a number of alloys of the type X₂YZ, where X represents a 3d transition metal (or Cu), Y a second 3d transition metal or one of the elements La, Th, Zr, Hf, Ta, Nb, W or Mo, and Z either Al or Ga. It was found that only a limited number of these combinations leads to the cubic L2₁ Heusler-type compounds. The lattice constants of these compounds were determined. The formation of the Heusler L2₁-type phase was compared with model predictions. A study to the magnetic properties and the room temperature Kerr rotation of the L2₁-type Heusler compounds showed that there is hardly any correlation between the Kerr rotation in the ferromagnetic Heusler alloys and the corresponding magnetization. In the Co₂YAl and Co₂YGa series the moment per Co atom and the electronegativity difference between Co and the Y component showed a linear dependence.     

Complete suppression of metastable phase and significant enhancement of magnetic properties of B-rich PrFeB nanocomposites prepared by devitrifying amorphous ribbons

The effect of refractory element addition on phase transformation, crystallization behavior and magnetic properties of Pr_8.5Fe_81.5B₁₀ (addition-free) and Pr_8.5Fe_81.5M₂B₁₀ (M=V, Cr, Nb, Zr, Ti) ribbons has been investigated. The annealed addition-free ribbon as well as the samples with V or Cr additions are mainly composed of the metastable Pr₂Fe₂₃B₃ phase, whereas annealed ribbons with Nb, Zr or Ti additions primarily consist of Pr₂Fe₁₄B and a minor amount of Fe₃B/boride. The complete suppression of the metastable Pr₂Fe₂₃B₃ phase due to Nb, Zr or Ti additions leads to a significant enhancement of the magnetic properties. For example, the remanence, the coercivity and the energy product are remarkably increased from 2.5 kG, 0.4 kOe and 0.2 MG Oe for the addition-free material to 9.2 kG, 4.7 kOe and 7.6 MG Oe for the specimens with Nb addition. The successful elimination of the metastable Pr₂Fe₂₃B₃ phase is believed to profit from two factors: (a) Nb, Zr or Ti atoms substitute the Pr site, comparatively increase the Pr content, and thus inhibit the nucleation of Pr-lean Pr₂Fe₂₃B₃ phases, and (b) the formation of Nb, Zr, or Ti borides consumes some part of B, which hinders the generation of the B-rich Pr₂Fe₂₃B₃ phase.