Electronic Structure Of Nb Research Articles

Octahedral and trigonal-prismatic coordination preferences in Nb-, Mo-, Ta-, and W-based ABX2 layered oxides, oxynitrides, and nitrides

Crystallographic and electronic structures of Nb-, Mo-, Ta-, and W-based layered oxides, oxynitrides, and nitrides were analyzed to elucidate the structural relationship between layered oxides and nitrides consisting of octahedral and trigonal-prismatic layers. The electron density, as derived by synchrotron X-ray analysis of LiNbO2 and Ta5−x(O,N)6, showed orbital overlaps between Nb–Nb and Ta–Ta metals in the trigonal layers. Computational calculations based on DFT exhibited that these overlaps stabilized these structures by lowering the hybridization states composed of the dxy, dx2−y2, and dz2 orbitals below the Fermi level. Crystal structures and formation energies suggest that tuning the Fermi level through the substitutions and vacancies of the cation/anion sites determines the structural preferences of the coordination. The properties and syntheses of these compounds are briefly described. This study enhances the understanding of layered oxides, oxynitrides, and nitrides to further the development of new synthetic approaches, compounds, and applications.

Temperature effect on the electronic structure of Nb:SrTiO3 (100) surface**Project supported by the Funds from the Chinese Academy of Sciences (Grant No. 1G2009312311750101) and the National Natural Science Foundation of China (Grant No. 11375228).

The effect of temperature on the electronic structure of Nb-doped SrTiO3 (100) surface is investigated by high-resolution synchrotron radiation photoemission spectroscopy. According to the x-ray photoemission spectroscopy (XPS) results, at an annealing temperature of less than 700 °C, the adsorbed carbon and hydroxyl on the STO surface could be removed, to expose the fresh intrinsic surface with a constant ratio of Ti/O. It is obvious that the STO would be doped by Ca+ impurities of bulks and O vacancies in the surface after annealing at 920 °C for one hour.

Phase diagram, mechanical properties, and electronic structure of Nb-N compounds under pressure.

Niobium-nitrogen compounds, which are potential candidates for superhard multifunctional materials, may possess multiple stoichiometries and structures under pressure. Based on ab initio evolutionary structural searches, we predict three ground states (oP6-Nb2N, CW-NbN, and hP22-Nb5N6) and six stable high pressure phases (ε-NbN, AsNi-NbN, U2S3-Nb2N3, oC24-NbN2, mP8-NbN3, and mP20-NbN4) for Nb-N compounds at pressures up to 100 GPa. Among them, the oP6-Nb2N, oC24-NbN2, mP8-NbN3, and mP20-NbN4 have never been reported, and N-rich oC24-NbN2, mP8-NbN3, and mP20-NbN4 high pressure phases are recoverable to ambient pressure. We find that the structure of N-rich Nb-N compounds consists of NbNx polyhedral stacking configurations and connected with Nn (n = 2, 3, 4, and n) polymerizations, which can remarkably improve the elastic modulus. It is found that CW-NbN and mP20-NbN4 are two potential ultra-incompressible and hard materials with the hardness calculated to be 24.56 and 19.86 GPa, respectively, while other N-rich phases such as U2S3-Nb2N3, oC24-NbN2, and mP8-NbN3 are soft materials. Detailed electronic structure and chemical bonding analysis proved that the high hardness of CW-NbN and mP20-NbN4 stems from the strong covalent bonding and the fullfilled Nb-N bonding and antibonding states.

Electronic structures of Nb–W bulk and surface from first principles calculation

First principles calculation shows that the Nb and W atoms have a tendency to mix instead of phase separate in the Nb–W bulk within the entire composition range, and the interaction between Nb and W atoms has a nonlinear effect on the density of states of the Nb–W bulk at the Fermi level. Calculation also reveals that the surface segregation of Nb atoms is energetically favorable with an energy decrease of 0.4–0.55eV/atom, and the segregation has important effects on the electronic structures of Nb–W surfaces. Interestingly, the work function of Nb–W phases is insensitive to the composition as well as the Nb surface segregation when W is in the range of 0–60at.%, and over this composition range, the work function remains relatively stable.

A Novel Layered Niobium Oxychloride Compound Based on Nb2 Pairs and Nb6 Octahedral Clusters: Synthesis and Crystal and Electronic Structures of Nb10Cl16O7.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Theoretical study of electronic structure and superconductivity in Nb 1− xB 2 alloys

Using the Korringa–Kohn–Rostoker coherent-potential approximation in the atomic-sphere approximation we have studied the changes in the electronic structure and the superconducting transition temperature T c in Nb 1− x B 2 alloys as a function of x. We find that the variation in the electronic structure of Nb 1− x B 2 alloys as a function of x is consistent with the rigid-band model. However, the variation of T c, obtained using the Allen–Dynes equation within the Gaspari–Gyorffy formalism to estimate the electron–phonon matrix elements, does not follow the expected trend. We associate this disagreement to the use of a constant ω rms in the Allen–Dynes equation over the whole range of vacancy concentration, thereby indicating the importance of lattice dynamical effects in these systems.

A new quasi-one-dimensional ternary chalcogenide: synthesis, crystal structure, and electronic structure of Nb1+xV1-xS5 (x = 0.18).

The new low-dimensional ternary chalcogenide, Nb(1+x)V(1-x)S(5) (x = 0.18), has been prepared and characterized. This compound crystallizes in the monoclinic space group, C2(2h)-P2(1)/m with two formula units in a cell with dimensions a = 9.881(4) A, b = 3.329(1) A, c = 8.775(3) A, and beta = 114.82(3) degrees. The layer is composed of two unique chains of face-sharing Nb-centered bicapped trigonal prisms and edge-sharing M-centered octahedra (M = Nb or V). The electronic structures of the monomeric basic building units, NbS(8) and VS(6), and hypothetical and real one-, two-, and three-dimensional structures making up the compound are examined to understand the nature of inter- and intrachain interactions and orbital overlapping among metals and sulfur atoms. The electronic structure of Nb(1+x)V(1-x)S(5) is essentially given by superimposing those of the individual chains. V d orbitals are found to be crucial for the one-dimensional metallic conductivity along the chain axis.

Soft modes and superconductivity in the layered hexagonal carbidesV2CAs,Nb2CAs,andNb2CS

The electronic structure, selected phonon frequencies, and electron phonon coupling of the hexagonal layered compounds Nb 2 CS, Nb 2 CAs, and V 2 CAs are elucidated using density functional calculations. All materials are good three-dimensional metals with moderate values of the density of states, N(E F ). The electronic structure of Nb 2 CS, the only superconductor within this family of materials, consists of well-hybridized Nb d-S p-C p derived bands characteristic around the Fermi energy. The S is more ionic in this compound than the As in the related arsenides Nb 2 CAs or V 2 CAs. This results in a relative softness for phonon modes involving S. Rigid muffin tin approximation calculations show moderate electron phonon couplings consistent with low-temperature superconductivity. The key difference from the carbo-arsenides is the presence of soft moderately coupled S modes.

Energetics of NP and NB complexes in silicon

Using ab initio (Hartree–Fock and local density approximation) and semiempirical (Austin Model 1) calculations, we studied the energetics and electronic structures of NB and NP complexes. We found that these complexes are electrically inactive. The formation energies are 1.6 eV for the NB coupling and 2.4 eV for the NP pairing. The N–P and N–B interatomic equilibrium distances are about 3.5 Å for both complexes.

Alloying effect on the electronic structures of Nb and Mo

The alloying effects on the electronic structure were investigated for both Nb-based and Mo-based alloys in order to obtain useful information for alloy design. The electronic structure was calculated using the DV-X alpha cluster method, and two alloying parameters, the bond order BO and the d-orbital energy level MD, were obtained for a variety of alloying elements M in these metals. The binary phase diagrams of the Nb-M and Mo-M systems were found to be classified according to the location in the BO-MD map. Also, with respect to the nature of the chemical bond between atoms, Nb and Mo were compared with the other BCC metals such as Ti, V, Cr, Fe and Zr. Furthermore, it was shown that the activation energy for atomic diffusion, the heat of fusion and the melting temperature could be associated with the bond order. These results will indeed give some guide to the design of these refractory-metal-based alloys for high-temperature applications.

The electronic structure of bcc-based random solid solutions of transition metals

The electronic structure of Nb x Zr1−x, V x Nb1−x and Mo x V1−x random solid solutions with the bcc lattice structure is investigated in the frame of the first-principles tight-binding muffin-tin orbital method and the coherent potential approximation. The total and component densities of states as well as the Bloch spectral densities are determined over a broad concentration range.

Growth and electronic structure of Nb and Ta films on Pd and their interaction with CO

The growth of Nb and Ta films on Pd(111) and their modification of CO chemisorption are studied using ultraviolet photoelectron spectroscopy (UPS), low-energy electron diffraction, and low-energy ion scattering (LEIS). At 300 K, LEIS shows significant amounts of Pd in the top layer even after more than a monolayer of Nb or Ta is deposited. This is in contrast to Pd on Nb or Ta(110) where a Pd monolayer completely covers the substrate. UPS shows that the surface electronic structure at monolayer Nb or Ta coverage resembles that of a Pd monolayer on Nb or Ta(110) and is characterized by strong Pd 4d emission 1–4 eV below Ef and a low density of states at Ef. Molecular CO adsorption is observed at submonolayer coverage; there is little CO adsorption at monolayer coverage and dissociative CO adsorption starts at greater than monolayer coverage. It is concluded that the deposited Nb or Ta monolayers intermix with the Pd(111) substrate and it is also shown that this behavior can be understood by examining the energetics of the system.

Ab initio calculations of the electronic structure and effective magnetic moment of Nb 4

The electronic structure of Nb 4 clusters has been calculated within the spin-polarized local density approximation using the LCAO method. The calculations were performed for a pyramidal and a planar structure with equal bond lengths extending from the value in the bulk, to the value for the dimer. The position and symmetry of the HOMO and LUMO levels depend strongly on the bond length in the pyramidal as well the planar structure. All clusters are diamagnetic except for the planar structure with a bond length longer than 4.85 au for which a net spin polarization is obtained.

Electron band structure, resistivity, and the electron-phonon interaction for niobium under pressure

Accurate measurements of the electrical resistance of Nb are presented as a function of temperature and pressure in the region 0-40°C and 0-1 GPa. From these measurements and published results for the compressibility and the pressure dependence of the superconducting transition temperature Tc we evaluate the pressure dependence of the electron-gas plasma frequency ω(p). The electronic structure of Nb is calculated from a self-consistent linear muffin-tin orbital method and results are obtained for the pressure dependence of the density of states at the Fermi surface, the root mean square over the Fermi surface of the Fermi velocity, the optical mass, and the plasma frequency. The experimental and calculated results for the pressure dependence of ω(p) are both close to dlnω2 / dlnV=-1.9. This agreement suggests that measurements of the electrical resistance as a function of temperature and pressure provide a new test of band-structure calculations. From our measurements of the resistance and calculations of ω(p) and from published results for the compressibility we obtain the pressure dependence of the electron-phonon interaction λ(p). With p given in gigapascals, the result is dlnλ / dp=-0.0047.

Self-consistent pseudopotential calculation of the electronic structure of Nb

The electronic structure of Nb is calculated using a self-consistent pseudopotential method. The calculated band structure and density of states are in good agreement with augmented-plane-wave calculations and with photoemission measurements. The total electronic charge density and partial charge densities for specific energy regions are presented and discussed.

Electronic structure of Nb, NbC, and NbN studied by X-ray emission

New high-resolution X-ray fluorescence measurements of the Nb L3 emission band and Nb and NbN are presented. The Nb L3 emission of Nb, NbC and NbN are compared with available density-of-states calculations. It is shown that only the NbC band can be described satisfactorily by an APW model. A complex satellite structure appears on the high-energy side of the parent Nb L alpha 1,2 emission lines, and it is interpreted as due to L3M-M4,5M double-vacancy distribution. No Nb L alpha 1,2 line shift is observed, in agreement with earlier results on NbC.