Average Number Of Valence Electrons Research Articles

Type-II superconductivity in W5Si3-type Nb5Sn2Al

We report the discovery of superconductivity (SC) in the ternary aluminide Nb5Sn2Al, which crystallizes in the W5Si3-type structure with one-dimensional Nb chains along the c-axis. It is found that the compound has a multiband nature and becomes a weakly coupled, type-II superconductor below 2.0 K. The bulk nature of SC is confirmed by the specific heat jump, whose temperature dependence shows apparent deviation from a single isotropic gap behavior. The lower and upper critical fields are estimated to be 2.0 mT and 0.3 T, respectively. From these values, we derive the penetration depth, coherence length and Ginzburg–Landau parameter to be 516 nm, 32.8 nm and 15.6, respectively. In contrast, the isostructural compound Ti5Sn2Al does not superconduct above 0.5 K. A comparison of these results with other W5Si3-type superconductors suggests that Tc of these compounds correlates with the average number of valence electrons per atom.

Structural evolution and electronic properties of Cu-Zn alloy clusters

Brass (Cu-Zn) is one of the typical Hume-Rothery alloys, which is extensively used in material science and industry. Earlier studies revealed that the structures of Cu-Zn alloys correlate with the average number of valence electrons or the electron-per-atom (e/a) ratio, but structural evolution rule still remains unclear. In this work, the Cu-Zn nanoalloy clusters (elementary cells of bulk brass) are studied to reveal the evolutional rule of their structures with sizes. Systematic unbiased global search is performed for structural prediction of CuxZny nanoalloys in a size range (x + y = 3–10) by using genetic algorithm with density functional theory. The global minimum and low-lying isomers of the series of nanoalloy clusters are obtained, and the structural phase diagrams are plotted depending on the relative energy. The results show that geometric structures of Cu-Zn nanoalloys are also determined by the total number of valence electrons (n*), just as in bulk brass. Cu-Zn nanoalloys with same n* have similar geometric motifs. When n* = 6, the clusters adopt planar motif, which have σ-aromaticity following the (4n+2) rule. When n* = 8, 18 and 20, the clusters keep spherical motifs, which satisfy the magic numbers of Jellium model and could be viewed as stable superatoms. In most of cases for n* = 10, 12 and 14, the clusters adopt prolate motifs, which have similar electronic structures to N2, O2, and F2 molecules, respectively, based on the super valence bond model. Moreover, in some cases for n* = 10 and 12, the clusters can be seen as an 8e-superatom combined with one (8e+2e) or two (8e+2e+2e) separate Zn atoms.

Schottky barrier engineering via adsorbing gases at the sulfur vacancies in the metal–MoS2 interface

Sulfur vacancies (S-vacancies) are common in monolayer MoS2 (mMoS2). Finding an effective way to control rather than abolish the effect of S-vacancies on contact properties is vital for the application of mMoS2. Here, we propose the adsorption of gases to passivate the S-vacancies in Pt–mMoS2 interfaces. Results demonstrate that gases are stably and preferentially adsorbed at S-vacancies. The n-type Schottky barriers of Pt–mMoS2 interfaces are reduced significantly upon the adsorption electron-donor gases, especially Cl2. The n-type transport character of the Pt–mMoS2 interface can be changed to p-type by the adsorption of electron-acceptor gases. As the adsorption concentration increases, both n- and p-type Schottky barriers are further reduced, and the lowest n- and p-type Schottky barriers are 0.36 and 0 eV, respectively. Note that the variations in Schottky barriers are independent of the oxidizing ability of gases but relative to the average number of valence electrons per gas atom. Analysis demonstrates that although gases at S-vacancies cannot cause gap states to vanish, and can even enhance Fermi level pinning, they modulate charge redistribution and the potential step at the interface region. Moreover, with increasing adsorption concentration, the valence band maximum of mMoS2 shows the opposite variation tendency to that of the potential step. Our results suggest that adsorption of gases is an effective way to passivate S-vacancies to modulate the transport properties of Pt–mMoS2 interfaces.

Preparation of pure α″-phase titanium alloys with low moduli via high pressure solution treatment

A low-modulus Ti-30Zr-5Al-3V (wt%, TZAV-30) alloy with pure α″-phase was successfully prepared by high pressure solution treatment (HPST). The X-ray diffraction (XRD) results indicate a gradual structural change from hexagonal (α′) to orthorhombic (α″) martensite in the TZAV-30 alloys treated by various pressures from 1 GPa to 5 GPa. The Young's moduli of all treated TZAV-30 alloys were derived from the tensile stress-strain curves. Among them, the alloy with pure α″ martensite phase was found to have the lowest modulus of ∼34 GPa, near the modulus of human bone. To study the origination, we summarize an approximately linear relationship between the Young's modulus and average number of valence electrons per atom (i.e. e/a value) for all α″-phase Ti alloys. The TZAV-30 alloy with pure α″ martensite phase has the low e/a value, which determines the low Young's modulus. The current study suggests that HPST is a totally new and effective route to develop the low-modulus Ti alloys as biomedical applications.

Large enhancement of superconducting transition temperature of SrBi3 induced by Na substitution for Sr

The Matthias rule, which is an empirical correlation between the superconducting transition temperature (Tc) and the average number of valence electrons per atom (n) in alloys and intermetallic compounds, has been used in the past as a guiding principle to search for new superconductors with higher Tc. The intermetallic compound SrBi3 (AuCu3 structure) exhibits a Tc of 5.6 K. An ab-initio electronic band structure calculation for SrBi3 predicted that Tc increases on decreasing the Fermi energy, i.e., on decreasing n, because of a steep increase in the density of states. In this study, we demonstrated that high-pressure (~ 3 GPa) and low-temperature ( < 350 °C) synthesis conditions enables the substitution of Na for about 40 at.% of Sr. With a consequent decrease in n, the Tc of (Sr,Na)Bi3 increases to 9.0 K. A new high-Tc peak is observed in the oscillatory dependence of Tc on n in compounds with the AuCu3 structure. We have shown that the oscillatory dependence of Tc is in good agreement with the band structure calculation. Our experiments reaffirm the importance of controlling the number of electrons in intermetallic compounds.

Empirical design of single phase high-entropy alloys with high hardness

We collect the available basic properties of nearly 100 high-entropy alloys (HEAs) with a single face centered cubic (fcc) or body centered cubic (bcc) phase. HEAs crystallizing in the fcc structure are mainly composed of the late 3d elements (LTM-HEAs), whereas HEAs consisting of the early (refractory) transition elements and the LTM-HEAs containing an increased level of bcc stabilizer form the bcc structure. Guided by the solid solution theory, we investigate the structure and hardness of HEAs as a function of the valence electron concentration (VEC) and the atomic size difference (δ). The fcc structure is found for VEC between 7.80 and 9.50, whereas the structure is bcc for VEC between 4.33 and 7.55. High strength is obtained for an average valence electron number VEC ∼ 6.80 and for an average atomic size difference δ ≈ 6%. Based on these empirical correlations, one can design the high-entropy alloys with desired hardness.

Structural, Thermal and Magnetic Characterization of Ni-Mn-Ga Ferromagnetic Shape Memory Alloys

Ni-Mn-Ga Ferromagnetic Shape Memory Alloys (FSMA) are highly interested in industrial actuators and energy harvesters because of its uniqueness in microstructure and multiple phases of these materials at different temperatures. The type of crystal structure, lattice parameters and magnetic properties depend on the alloy composition. The transformation and crystallographic properties are usually mapped to average number of valence electrons per atom. The present work deals with the fundamental characterization of single crystal and polycrystalline nature of these alloys. Though, the polycrystal shows low magnetic field induced stain (MFIS), the easy preparation techniques and cost effective aspects compared with the single crystals makes an impressive interest in dealing with that. The structural, thermal and magnetic parameters are observed and compared for these single and polycrystals.

Relation of structure and magnetic properties in nickel substituted MgFe 2O 4

This study presents the structure-related magnetic properties of nickel substituted MgFe 2− x Ni x O 4 with x=0.01–0.20, synthesized using thermal decomposition of metal organic starting materials. The samples were annealed at temperatures ranging from 900 to 1100 °C in air. Magnetic and electric properties were characterized using vibrating sample magnetometer and an impedance analyzer, respectively. The fraction of inverse spinel structure was found to decrease with increase in nickel concentration. The samples exhibited soft magnetic behavior. The magnetization was found to depend on the substitution of Ni atoms on the octahedral sites and on the average number of valence electrons. The dielectric properties were found to have higher values at low frequency and decreased with increase in frequency. An enhancement of the dielectric constant was observed upon increase in the annealing temperature.

Modulated structure of β-brass CuZn compressed to 90 GPa

Cu–Zn is a classic example of an alloy system displaying a sequence of phases along an alloy composition, called Hume-Rothery phases. The crystal structure of these phases is determined by valence electron concentration (that is the average number of valence electrons per atom), and the lowering of the electronic energy is considered the key factor for the structure stabilization. Using powder x-ray diffraction, we study the β-phase of an equiatomic CuZn alloy in its body-centered cubic (bcc) phase in the pressure range up to 90 GPa and find a transformation to a modulated trigonal structure at around 40 GPa. We analyze the structural distortion of bcc CuZn by looking at the configuration of the Brillouin zone of the bcc and the trigonal structures and their interaction with the Fermi surface. We demonstrate that the stabilization of the complex high-pressure structure can be explained with the Hume-Rothery effect.

Saturation Magnetic Induction Prediction for Amorphous Magnetic Alloys by Using Support Vector Regression

This paper illustrates an application of support vector regression (SVR) approach in forecasting the saturation magnetic induction (Bs) of amorphous magnetic alloys. SVR was trained and tested with an experimental data set comprised of five input variables, comprising the average number of valence electrons of amorphous magnetic alloys, mixed entropy, ratio of radii, difference of electron density, and difference of work function. The prediction performance of SVR was compared with that of artificial neural networks’ (ANN) model. The results demonstrate that the prediction ability of SVR is superior to that of ANN. This investigation indicates that SVR-based modeling is a practically useful tool in prediction of the saturation magnetic induction of amorphous alloys. This study provides a novel methodology to foresee the saturation magnetic induction in sintering/development of novel amorphous magnetic alloys possessing high saturation magnetic induction.

The fcc–bcc Bain path in In–Sn and related alloys at ambient and high pressure

Experimental high-pressure structural studies on an In–Sn alloy containing8 at.% Sn reveal an isostructural transition of a face-centered tetragonalphase at pressures above 15 GPa with a switch of the axial ratio fromc/a>1 toc/a<1. Such tetragonal phases in binary alloys based on In and Sn are analyzed inrelation to the Bain path, i.e. a transformation between a face-centered cubic(fcc) and a body-centered cubic (bcc) structure. Variation of the axial ratioc/a in these phases correlates with the average number of valence electrons per atom in analloy. A common Bain path from fcc to bcc is discussed within a nearly-free-electron modelof Brillouin-zone–Fermi-sphere interactions.

The Average Valence Electron Number: Is it a relevant parameter for Ni2MnGa Alloys?

AbstractThe influence is investigated of the average valence electron number on the systematic changes of a Ni2MnGa based alloy series. The experimental investigation focuses on an isoelectronic alloy series Ni2Mnx(CrFe)1-x/2Ga for which the average valence electron number is unchanged for any value of x. Based on the changes of physical properties of alloys in this series compared to Ni2MnGa it is argued that local lattice distortions are more relevant for driving the change in alloy characteristics, such as the martensitic phase transition temperature or the ferromagnetic ordering temperature, than the band filling by valence electrons.

Characteristic Ordering in Liquid Phase‐Change Materials

Ternary liquids containing approximately 50% Te are studied using neuron scattering. The local environment is quantified, based on the analysis of the total scattering function. In these tellurium-based alloys, a tetrahedral local order dominates for liquids with average number of valence electrons between 4.0 and 4.25, while a transition to octahedral ordering is induced for larger numbers of valence electrons.

Electronic structures of Al-based transition metal compounds and effective valence of transition elements

Abstract Electronic structures of Al-TM (transition metal) compounds with rather simple B2, L12 and A15 structures are studied. Focusing on the energy bands, which are not hybridized strongly with the TM-d bands, we estimate the number of electrons occupying the states below the gap induced by periodic potentials and hence obtain the average number of valence electrons per atom e/a. Although the values of e/a obtained for the different structures are scattered in spite of combination of the same chemical species, these values are obtained if one assumes similar negative values for effective valence of TM. Roles of the interference effect opening the gap in the nearly-free-electron bands are discussed.

A new orthorhombic approximant of the icosahedral Al–Cu–Fe quasicrystal

We report on the formation of a new crystalline approximant phase of the icosahedral (i-)Al–Cu–Fe quasicrystal. This phase is formed during sintering of Al-based composites reinforced with i-AlCuFeB quasicrystalline particles. The structure of this phase has been characterized by transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). TEM revealed that it is a B-centred orthorhombic phase with lattice parameters a = 1.166 nm, b = 1.195 nm and c = 3.44 nm. Its chemical composition, as determined by electron energy loss spectroscopy (EELS), is close to Al76.9Cu2.7Fe20.4, with an average number of valence electrons per atom e/a of 1.92, similar to the value in all other approximants of the i-phase discovered thus far. Initial results on local atomic arrangements along one of its pseudo-5-fold axes are also presented.

Optical properties of Au−Pt−Pd-based high noble dental alloys

The effects of the addition of various alloying elements (In, Sn, Zn) of up to 4 mass % on the optical properties of Au-Pt-Pd-based high noble dental alloys were investigated by means of spectrophotometric colourimetry. Spectral reflectance data from the mirror-polished flat samples were collected at 10 nm intervals in the wavelengths ranging from 360 nm to 740 nm under the CIE standard illuminant D65 and the observer of 10 degrees. Three dimensional colour coordinates in the CIEL *a*b* andL *C*h colour spaces were also obtained to specify the alloy colour. The alloying addition of a small amount of Sn and/or In increased the reflectance in the long-wavelength range and decreased the reflectance in the shortwavelength range. As a result, the maximum slope of the spectral reflectance curve at the absorption edge near 520 nm apparently increased with the additions of Sn and/or In. This change in shape of the spectral reflectance curve caused the increased chromaticity indices,a*, b*, and chroma,C*. On the other hand, the hue angle was not greatly affected by the alloying elements, with the exception of the alloy containing 4 mass % Sn showing a slightly lower hue angle. It was evidenced that in the single-phase structured alloys the average number of valence electrons per atom,e/a, in an alloy is a controlling factor of the colour of Au−Pt−Pd-based high noble dental alloys. That is, by increasinge/a-value,a*-, b*-, C*- coordinates systematically increased, giving a slight gold tinge to the parent Au-Pt-Pd alloy within the limitation that the structure of an alloy is a single phase. The addition of Sn of 2 mass % or more produced a small amount of the second phase of possible intermetallic compounds between Sn and Pd or Pt. The coexistence of a small amount of the second phase of possible intermetallic compounds further increased a gold tinge. However, the inclusion of 4 mass % Sn to the parent Au-Pt-Pd alloy gave a very light tint of red to the alloy. Results of the present study are expected to be useful in controlling colour of Au-Pt-Pd-based high noble dental alloys.

Composition and temperature dependence of the crystal structure of Ni–Mn–Ga alloys

The crystal structure of ferromagnetic near-stoichiometric Ni2MnGa alloys with different compositions has been studied at ambient temperature. The studied alloys, with five-layered (5M) and seven-layered (7M) martensitic phases, exhibit the martensitic transformation temperature (TM) up to 353 K. Alloys with these crystal structures are the best candidates for magnetic-field-induced strain applications. The range of the average number of valence electrons per atom (e/a) was determined for phases 5M, 7M, and nonmodulated martensite. Furthermore, a correlation between the martensitic crystal structure, TM and e/a has been established. The lattice parameters ratio (c/a) as a function of e/a or TM has been obtained at ambient temperature for all martensitic phases. That the paramagnetic-ferromagnetic transition influences the structural phase transformation in the Ni–Mn–Ga system has been confirmed.

Structural properties of d.c. magnetron sputtered B–C–N thin films and correlation to their mechanical properties: a new empirical formula

B–C–N thin films of a wide composition range were deposited by reactive d.c. magnetron sputtering from targets with different B/C ratio in an Ar/N 2 atmosphere. The films were investigated by electron diffraction (ED) and their mechanical properties hardness and Young's modulus were determined from nanoindentation. The diffraction pattern of most of the films gave evidence of an amorphous structure and the number and distance of nearest neighbours were calculated. The mechanical properties hardness and Young's modulus are correlated to the structural parameters. An empirical formula could be established that allows the calculation of hardness and Young's modulus when the composition of the film and the number and distance of nearest neighbours are known. In this formula, the percentage of each element (B, C and N) is included in a weighted sum that is close to the average valence electron number.

Multiscale origin of the magnetocaloric effect in Ni-Mn-Ga shape-memory alloys

We have analyzed magnetization measurements in a series of composition-related Ni-Mn-Ga shape-memory alloys. It is shown that the magnetocaloric effect in the vicinity of the martensitic transition mainly originates from two different contributions: (i) magnetostructural coupling on the mesoscopic scale between the magnetic moments and the martensitic variants, which is also responsible for the magnetic shape-memory effect and (ii) the microscopic spin-phonon coupling which gives rise to the shift of the transition temperature with the applied magnetic field. The relative importance of these two contributions has been shown to vary with composition, which is suitably expressed through the average number of valence electrons per atom $e/a.$ In alloys with a large difference between the Curie and martensitic transition temperatures $(e/a\ensuremath{\simeq}7.5),$ mesoscopic coupling is dominant and a negative giant magnetocaloric effect (increase of temperature by adiabatic demagnetization) is induced at moderate applied fields. In contrast, in alloys when these temperatures are very close to one another $(e/a\ensuremath{\simeq}7.7),$ the microscopic coupling is the most relevant contribution and gives rise to a positive giant effect.

Electronic structure and stability of the pentlanditesCo9S8and(Fe,Ni)9S8

First-principle electronic structure investigations of transition-metal sulfides ${\mathrm{Co}}_{9}{\mathrm{S}}_{8}$ and related alloys with the unique structure of pentlandite are carried out using density-functional theory within the local-density approximation. The total-energy calculations for ${\mathrm{Co}}_{9}{\mathrm{S}}_{8}$ and $(\mathrm{F}\mathrm{e},\mathrm{N}\mathrm{i}{)}_{9}{\mathrm{S}}_{8}$ alloys have been computed and we predict equilibrium lattice parameters that are on average 1% smaller than in the experiment. The heats of formation have been calculated, the theoretical prediction for ${\mathrm{Co}}_{9}{\mathrm{S}}_{8}$ being in excellent agreement with that available in the Thermocalc database. The predicted heat of formation for the ${\mathrm{Fe}}_{5}{\mathrm{Ni}}_{4}{\mathrm{S}}_{8}$ alloy is very close to ${\mathrm{Co}}_{9}{\mathrm{S}}_{8},$ reflecting the fact that the Fermi level is found to fall in a pseudogap for an average number of valence electrons per atom $e/a=7.58.$ Furthermore, we determined the individual bond energies for ${\mathrm{Co}}_{9}{\mathrm{S}}_{8}$ and ${\mathrm{Co}}_{8}{\mathrm{S}}_{8}$ to stress the contribution of the octahedral metal cobalt to the stability of the ${\mathrm{Co}}_{9}{\mathrm{S}}_{8}$ phase.