VB Metals Research Articles

Jahn-Teller resonance states in the Vb metal hydrides: static and dynamic consequences

A Hamiltonian with a localised Jahn-Teller electron-phonon interaction plus a band-coupling term belonging to a metal cluster state adjacent to interstitial hydrogen in the Vb metals is presented. It is shown that in a low-frequency approximation, this problem is analogous to that of magnetic impurities in metals. The role of hydrogen in triggering the Jahn-Teller instability is clarified in the context of the Hartree-Fock solutions of the low-frequency Hamiltonian. Anelastic relaxation mechanisms are described and an elastic susceptibility expression is derived and compared with an observed elastic relaxation. The empirical connection of the hypothesised Jahn-Teller distorted metal cluster to the experimentally determined elastic dipole tensor is also discussed. Several experiments suggested by the model are proposed.

Electronic Energy States of HfSe2 and NbSe2 by Low Energy Electron Loss Spectroscopy Study

Low energy electron loss spectroscopy (ELS) study was performed on 1T-HfSe 2 (group IVB metal compound) and 2H-NbSe 2 (group VB metal compound) by using incident electron energies of 30–250 eV. From the loss data in the second derivative form, maxima in density-of-states in the conduction band of the compounds were deduced through the information on the filled core states by X-ray photoelectron spectroscopy. The conduction band of the transition-metal dichalcogenides could be devided into two parts. The results are discussed in relation to the previous work on WS 2 (group VIB metal compound), and also to proposals based on band calculations and experimental studies on the transition-metal dichalcogenides with constituent metals of group IVB, VB and VIB.

The preparation and surface structure of clean V(110)

The first quantitative determination of the surface structure of a group VB metal is reported. A procedure is described for the preparation of a clean, well-ordered surface of V(110), free from the major bulk impurity, oxygen. The clean surface exhibits the two-dimensional periodicity of the corresponding bulk plane. Experimental LEED intensity measurements are compared to the results of multiple-scattering model calculations, with very good agreement being obtained for a value of the first interlayer spacing d, close to the bulk value. Minimization of the r factor for the comparison of experimental and calculated intensity spectra leads to a value of d = 2.12 ± 0.02 A ̊ , compared to the bulk value of 2.14 Å.

Jahn-Teller resonance states in the group Vb metal hydrides: A model calculation

Jahn-Teller resonance states in the group Vb metal hydrides: A model calculation

Jahn-Teller resonance states in theVbmetal hydrides: A model calculation

A simple tight-binding model representing hydrogen in the Vb metals and including a special electron-phonon interaction is shown to give a molecular Jahn-Teller resonance state for physically reasonable values of the various energy parameters. This unusual resonance state is pinned to the shell of metal atoms adjacent to hydrogen by an effective Coulombic attraction that exists because of the nearly equal but opposite electron displacements on that shell, caused by the direct interaction of hydrogen on the one hand and by the Jahn-Teller interaction on the other. The mutual orthogonality of these two interactions implies that the Jahn-Teller state is an intrinsic feature of the host metal.

Quasimolecular Jahn-Teller resonance states in the bcc metallic hydrides of vanadium, niobium, and tantalum

The unusual properties of the group Vb bcc hydrides are shown to derive from a particular $d$-band quasimolecular resonance state localized near interstitial hydrogen. This state is schematically shown to be grounded in fundamentals. The point of departure is an Anderson-Newns approach to the problem of a single hydrogen atom in a transition metal, with the metal states in a mixed nearly-free-electron-tight-binding representation. Orthogonalization of pseudo-plane-wave states to the hydrogen $1s$ "core" state leads to a Hamiltonian with site-diagonal disorder (Anderson Hamiltonian) for the $d$ band. This gives the possibility of induced, localized $d$ states. Assuming that localized electron states derived from the $d$ band exist and are confined to the shell of metal atoms nearest the impurity, symmetrized linear combinations of atomic orbitals (-molecular orbitals) (LCAO-MO) are constructed for impurity polyhedra in the group Vb metals. The orientation of atomic $d$ orbitals used for this construction is that of the pure metal. One of the MO's so obtained is shown to be particularly well suited, in terms of availability and closeness to the Fermi level, for screening purposes. This state is degenerate and, by analogy with other well-known states (e.g., impurity-vacancy pairs in Si), should have a strong Jahn-Teller interaction. A particularly simple $〈111〉$ distortion proves to be key for understanding previously unexplained properties in the Vb metal-hydrogen systems, viz. cubic lattice distortion, hydrogen diffusion, and excess partial entropy. Following an argument that the Jahn-Teller distorted state should persist at higher concentrations because of its essentially molecular character, a blocking rule is formulated based on the assumed integrity of this state. The rule is shown to explain hydrogen partial entropy versus concentration, including a sharp decrease in entropy near [H]/[M]=1. It also predicts that hydrogen diffusion should become increasingly correlated at higher concentrations, agreeing with a recent experiment. Finally, exceptions to the blocking rule are discussed in terms of an additional Jahn-Teller state, and it is shown that observed superlattice structures can be understood in terms of one or the other of the two Jahn-Teller states.

Molten lithium carbonate—sodium carbonate—potassium carbonate eutectic: the reaction of group VB and VIB metal oxides and oxyanions

The reactions of seven acidic oxides and oxyanions of Group VB and VIB metals with molten alkali metal carbonate eutectic have been studied by thermogravimetry. Products have been identified by reflectance, infrared and Raman spectroscopy and stoichiometries have been established for the sequential reactions as oxide anions were progressively incorporated, ultimately to form the ortho-oxyanion.

Reduction of Organic Compounds with Low-Valent Species of Group IVB, VB, and VIB Metals

The present article gives a summary of reactions of low-valent species of titanium, vanadium, chromium, molybdenum, and tungsten with organic substrates. Preparations are given for some of the not yet commercially available reagents. The reactions are arranged in terms of the organic reagent. Reactions of chromium(II) and titanium(II)/titanium(III) are treated only briefly as separate reviews on these reagents are available. Mechanistic aspects of the reactions are not discussed in any detail. 1. Introduction 2. Alkynic and Alkenic Systems 3. Halides 4. Carbocations 5. Alcohols 6. Ethers 7. Carbonyl Compounds 8. Derivatives of Carbonyl Compounds 9. Nitro Compounds 10. Other Nitrogen Compounds 11. Sulfoxides. 12. Addendum

Secondary ion mass spectrometry and Auger electron spectroscopy investigations of Vb metal foils prepared for hydrogen permeation measurements

Surface corrosion layers on vanadium, niobium and tantalum foils are shown to prevent almost entirely the hydrogen interphase transfer into the metal at normal temperatures. These layers have been removed by outgassing the foils in ultrahigh vacuum at high temperatures, and their rebuilding in the atmosphere has been prevented by coating the clean surfaces with a vapor-deposited thin film of Pd, permeable only to hydrogen. The techniques of secondary ion mass spectrometry and Auger electron spectroscopy have been used to obtain information on the composition and thickness of these surface layers and on the sucess of the above mentioned purification and protective technique.

Superconductivity in hydrides of Nb-Pd and Nb-Rh

The hydriding properties of Nb--Pd and Nb--Rh alloys were investigated. Results show that hydrogen uptake under retention of the body centered cubic structure, characteristic of VB and VIB metals, can raise T/sub c/. This is the case for both the original Nb/sub 0/./sub 8/Pd/sub 0/./sub 2/H/sub 0/./sub 39/ as well as Nb/sub 0/./sub 8/Rh/sub 0/./sub 2/H/sub 0/./sub 10/. The situation is less clear in the case of the aged hydride of Nb/sub 0/./sub 8/Pd/sub 0/./sub 2/. So far hydrogen take up either in Nb as well as compounds such as Nb/sub 3/Sn has been proven to be deleterious to superconductivity. The increases of T/sub c/ upon hydriding of Nb--Pd and Nb--Rh alloys can, among other factors, be explained in a straightforward fashion on the basis of lattice parameter. Alloying Nb with Rh decreases the lattice parameter of Nb as well as T/sub c/. Hydriding increases the lattice parameters again. The concomitant increase in T/sub c/ should then not be surprising as it reestablishes the original spacial situation. A similar situation holds concerning straightforward electron rules. However, beyond simple spatial and electronic arguments there will certainly be some electronic rearrangements and modifications in bonding which will have a decisive influence onmore » superconductivity. In fact a relation can be expected to the respectable superconducting transition temperatures found in a variety of metastable compounds between Nb and noble metals at relatively high Nb concentrations. Results of the study open the possibility of achieving high values of T/sub c/ by modifying lattice parameters and electronic structure via the introduction of hydrogen.« less

Isotope effects in solutions of hydrogen and deuterium in niobium

Hydrogen and deuterium equilibrium pressures in the region of single-phase solid solutions (α phase) with niobium were collected. These measurements were within the temperature and composition ranges of 535 to 773 K and 0.02 to 0.20 hydrogen atomic fraction. Good partial molar enthalpy and entropy values were calculated from these data. A ground-state hydrogen vibration frequency of 1800±50 K (3.8×1013 Hz) was derived by assuming a harmonic oscillator model. Finally, the results are compared with those obtained for the Group VB metals by other authors.

Stability of reactive and refractory metal bor1des in ternary chromium-base alloys

Dispersion hardening by the Group IVb and Vb metal carbides has been demonstrated to improve the high temperature strength of chromium significantly 1, and has created interest in the similar use of the borides of these metals. The purpose of this work was to characterize the elevated temperature formation and stability of these borides in chromium-base alloys, about which little is currently known.

Hot-Pressing of Tantalum Carbide (I)

Carbides of Groups IVb, Vb, and VIb metals have high melting point, high hardness, and good electrical conductivity as well as good thermionic emissivity. Moreover, they exhibit high mechanical strength at elevated temperatures.Among these, ZrC, HfC, TaC and their solid solutions are important as high temperature materials because of their high melting points. But, since they have poor sinterability, few precise details are known about the powdered mixture or solid solution. This study, in a series of studies on TaC base materials for incandescent filaments, is aimed at development of sintered materials resistant against thermal shock.In this report the sintering properties in the binary system TaC-ZrC were studied by using a vacuum hot-pressing apparatus, since it was considered that the acceleration of the densification seems to control the properties, especially the mechanical properties of the sintered material.

The external photoeffect and a simple band scheme of AVB 3 VII iodide crystals

The external photoeffect was measured on AsI3, SbI3 and BiI3 crystals grown in the gaseous phase from very pure parent materials. The energy necessary for transferring an electron from the top of the valence band to the zero energy level was determined from the position of the long wave-length threshold of photoemission in the first approximation. This energy was approximately the same for all Vb metal iodide crystals, i.e. 5.8 eV. The electron affinities of crystals were determined from the known forbidden band width and from the results of measurements, and a simple band scheme of the crystals was proposed.

Semiconducting Behavior of Substituted Tungsten Diselenide and Its Analogues

Tantalum substitutions up to 5 atomic per cent (a/o) of the cations have been made in the three Group VIB dichalcogenides, , , and . Tantalum‐substituted p‐type had the best thermoelectric properties with a figure of merit about . Substitution of a number of other elements was attempted in the compounds and , but only the Group VB metals were sufficiently soluble to give p‐type materials with electrical conductivities at useful levels. Hall coefficient measurements were made on all three of the above‐mentioned compounds, and was shown to have the highest carrier mobility (100 cm2/v‐sec); this explains its superior thermoelectric properties. An excellent correlation is shown between Seebeck coefficient measurements and Hall coefficient measurements for the family of compositions for an effective mass ratio of 1.0 with the assumption of an ionic lattice scattering mechanism.

New Solid Lubricants: Their Preparations, Properties and Potential for Aerospace Applications

A group of compounds are described whose physical and chemical properties make them attractive solid lubricant candidates for aerospace applications. The compounds are diehalcogenides of the Group VB and VIB metals niobium, tantalum, molybdenum and tungsten. Through a description and discussion of their frictional characteristics and other pertinent properties, the potentials that they possess for applications in unusual environments are illustrated.