Niobium Atoms Research Articles

Reactions of Group V Metal Atoms with Hydrogen Sulfide: Argon Matrix Infrared Spectra and Theoretical Calculations

The reaction of laser-ablated vanadium, niobium and tantalum atoms with hydrogen sulfide has been investigated using matrix isolation FTIR and theoretical calculations. The metal atoms inserted into the H−S bond of H2S to form the HMSH molecules (M=V, Nb, Ta), which rearranged to H2MS molecules on annealing for Nb and Ta. The HMSH molecule can also further react with another H2S to form the H2M(SH)2 molecules. These new molecules were identified on the basis of the D2S and H234S isotopic substitutions. DFT (B3LYP and BPW91) theoretical calculations are used to predict energies, geometries, and vibrational frequencies for these novel metal dihydrido complexes and molecules. Reaction mechanism for formation of group V dihydrido complex was investigated by DFT internal reaction coordinate calculations. The dissociation of HVSH gave VS+H2 on broad band irradiation and reverse reaction happened on annealing. Based on B3LYP calculation releasing hydrogen from HVSH is endothermic only by 13.5 kcal/mol with lower energy barrier of 16.9 kcal/mol.

Direct observation of niobium segregation to dislocations in steel

It is well known that niobium in solute solution retards the recovery of dislocations in steel. Segregation of niobium atoms to dislocations was observed for the first time by atom probe tomography in niobium added ferritic stainless steels for high temperature use. The observation results suggest that solute niobium atoms solely have strong attractive interaction with dislocations. We name it niobium-Cottrell atmosphere, and discuss the trapping site and interaction energy through a comparison with segregation to the grain boundary in the same steel.

A niobium-necked cluster [As3Nb(As3Sn3)](3-) with aromatic Sn3(2-) .

We describe here the synthesis and characterization of a ternary cluster compound [As3Nb(As3Sn3)](3-) (1), in which a niobium(v) atom is coordinated by an As3(3-) triangle and a bowl-type As3Sn3(5-) ligand. Cluster 1 was synthesized by dissolving K8NbSnAs5 (2) in the presence of [2.2.2]crypt in ethylenediamine solution, filtered and layered with toluene, then crystallized in the form of [K([2.2.2]crypt)]3[As3Nb(As3Sn3)]·en·tol. The flower-vase shaped compound 1 features a new structure type, rather different from the known Zintl phases. The stability and bonding of 1 are elucidated via extensive bonding analyses. The Sn3 ring is found to have σ-aromaticity featuring a delocalized Sn-Sn-Sn σ bond. Electronic structure calculations confirm the Nb(v) oxidation state and weak Nb-Sn and Sn-Sn bonding, in addition to the normal Nb-As and As-As bonds.

Charge Transfer Stabilization of Late Transition Metal Oxide Nanoparticles on a Layered Niobate Support.

Interfacial interactions between late transition metal/metal oxide nanoparticles and oxide supports impact catalytic activity and stability. Here, we report the use of isothermal titration calorimetry (ITC), electron microscopy and density functional theory (DFT) to explore periodic trends in the heats of nanoparticle-support interactions for late transition metal and metal oxide nanoparticles on layered niobate and silicate supports. Data for Co(OH)2, hydroxyiridate-capped IrOx·nH2O, Ni(OH)2, CuO, and Ag2O nanoparticles were added to previously reported data for Rh(OH)3 grown on nanosheets of TBA0.24H0.76Ca2Nb3O10 and a layered silicate. ITC measurements showed stronger bonding energies in the order Ag < Cu ≈ Ni ≈ Co < Rh < Ir on the niobate support, as expected from trends in M-O bond energies. Nanoparticles with exothermic heats of interaction were stabilized against sintering. In contrast, ITC measurements showed endothermic interactions of Cu, Ni, and Rh oxide/hydroxide nanoparticles with the silicate and poor resistance to sintering. These trends in interfacial energies were corroborated by DFT calculations using single-atom and four-atom cluster models of metal/metal oxide nanoparticles. Density of states and charge density difference calculations reveal that strongly bonded metals (Rh, Ir) transfer d-electron density from the adsorbed cluster to niobium atoms in the support; this mixing is absent in weakly binding metals, such as Ag and Au, and in all metals on the layered silicate support. The large differences between the behavior of nanoparticles on niobate and silicate supports highlight the importance of d-orbital interactions between the nanoparticle and support in controlling the nanoparticles' stability.

Matrix Infrared Spectroscopic and Quantum Chemical Investigations of the Group 5 Transition Metal Atom and CX4 Molecule (X = H, F, and Cl) Reaction Products.

Laser-ablated vanadium, niobium, and tantalum atoms were reacted with CH2X2, CHX3, and CX4 (X = F and Cl) molecules in condensing argon, and the products were investigated by matrix isolation infrared spectroscopy. The major reaction products are new CH2-MX2, CHX-MX2, HC-MX3, and XC-MX3 complexes. These reactive species were identified by comparing their matrix infrared spectra with frequencies, intensities, and isotopic shifts from density functional theory calculations. Product structures and energies from these calculations are also presented. Results from previously studied Group 4 and 6 metal reaction products are compared. Little change is found in the calculated metal-carbon bond lengths in the early first row CH2═MF2 methylidene σ(2)π(2) series; however, the methylidyne complexes HC{}MF3 show considerable increase in bond strength for the nominally σ(2)π(1)π(1)(Ti), σ(2)π(2)π(1)(V), and σ(2)π(2)π(2)(Cr) carbon{}metal bonds left to right. The Group 5 HC{}MF3 complexes have only a plane of symmetry whereas the Group 4 and 6 analogues have 3-fold symmetry.

Gigahertz Dielectric Polarization of Substitutional Single Niobium Atoms in Defective Graphitic Layers.

We synthesize two Nb/C composites with an order of magnitude difference in the density of single niobium atoms substituted into defective graphitic layers. The concentration and sites of single Nb atoms are identified using aberration-corrected scanning transmission electron microscopy and density functional theory. Comparing the experimental complex permittivity spectra reveals that a representative dielectric resonance at ∼16 GHz originates from the intrinsic polarization of single Nb atom sites, which is confirmed by theoretical simulations. The single-atom dielectric resonance represents the physical limit of the electromagnetic response of condensed matter, and thus might open up a new avenue for designing electromagnetic wave absorption materials. Single-atom resonance also has important implications in understanding the correlation between the macroscopic dielectric behaviors and the atomic-scale structural origin.

Structure and phase composition of Nb/Ti system subjected to compression plasma flow impact

The results of structure, phase composition and mechanical properties of titanium alloyed with niobium atoms by means of compression plasma flow (CPF) impact on the “Nb coating (2.5μm)/Ti substrate” system are presented. The CPF treatment with the pulse duration of 100μs was carried out at the absorbed energy density of 13J/cm2 which is sufficient for melting of both niobium coating and some part of titanium substrate. The cross-section analysis by means of scanning electron microscopy revealed the formation of the alloyed layer with the thickness of 14μm and relatively uniform distribution of Nb along the depth (Nb concentration obtained by means of the EDX analysis was about 17at.%). Alloying of titanium with Nb atoms resulted in stabilization of a high-temperature β-Ti phase as a β-Ti(Nb) solid solution. In the near surface layer the niobium interacted with impurity atoms (N and C) providing carbonitride (Nb(C,N) and carbide and Nb2C) possible formation. Phase transformation in the surface layer of titanium led to microhardness increase up to 4.7GPa.

Structure, magnetic, and electric properties of bismuth niobates doped with d-elements: XI. Magnetic susceptibility of Bi3NbO7 of cubic and tetragonal modifications

Study of magnetic susceptibility and ESR spectra of two modifications of bismuth niobate Bi3NbO7, tetragonal and cubic, has shown that a significant difference in the diamagnetic susceptibilities is observed at a reversible phase transition, which is associated with the presence of tetravalent niobium atoms combined in aggregates in the tetragonal modification of the complex oxide.

Structural phase transitions in niobium oxide nanocrystals

Niobium oxide nanocrystals were successfully synthesized employing the green synthesis method. Phase formation, microstructure and compositional properties of 1, 4 and 7 days incubation treated samples after calcinations at 450 °C were examined using X-ray diffraction, Raman, photoluminescence (PL), infrared, X-ray photoelectron spectra and transmission electron microscopic characterizations. It was observed that phase formation of Nb2O5 nanocrystals was dependent upon the incubation period required to form stable metal oxides. The characteristic results clearly revealed that with increasing incubation and aging, the transformation of cubic, orthorhombic and monoclinic phases were observed. The uniform heating at room temperature (32 °C) and the ligation of niobium atoms due to higher phenolic constituents of utilized rambutan during aging processing plays a vital role in structural phase transitions in niobium oxide nanocrystals. The defects over a period of incubation and the intensities of the PL spectra changing over a period of aging were related to the amount of the defects induced by the phase transition.

Reduction of ((t)BuN═)NbCl3(py)2 in a Salt-Free Manner for Generating Nb(IV) Dinuclear Complexes and Their Reactivity toward Benzo[c]cinnoline.

The organosilicon reducing reagent 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene (1a) was used for the one-electron, salt-free reduction of ((t)BuN═)NbCl3(py)2 (2), resulting in the formation of a neutral, triply chloride-bridged dinuclear niobium(IV) complex, [((t)BuN═)ClNb(py)](μ-Cl)3[((t)BuN═)Nb(py)2] (3) in moderately high yield. Heating 3 in toluene at 80 °C caused a unique intramolecular rearrangement of 3 to another neutral dinuclear complex, [Cl2Nb(py)](μ-Cl)(μ-N(t)Bu)2[ClNb(py)2] (4), in which two niobium(IV) atoms were bridged by one chloride atom and two imido ligands. Reaction of complex 3 with benzo[c]cinnoline produced a benzo[c]cinnoline-bridged dinuclear niobium(V) complex 7 by an overall two-electron reduction of benzo[c]cinnoline through a disproportionation of 3 into a mixture of a niobium(V) complex 2 and a niobium(III) complex, the latter of which efficiently reduced benzo[c]cinnoline.

Modelling the influence of austenitisation temperature on hydrogen trapping in Nb containing martensitic steels

Hydrogen trapping behaviour is investigated by means of thermokinetic simulations in a martensitic steel. The heat treatment consists of austenitisation followed by quenching and tempering. The model prescribes a minimum in hydrogen trapping at an austenitisation temperature of 1050 °C. Below this temperature, austenite grain boundaries are the prevailing trap, whereas niobium atoms in solid solution are the main traps above 1050 °C. The model describes precisely the experimental results.

New energy levels of atomic niobium by laser induced fluorescence spectroscopy in the near infrared

Laser-induced fluorescence spectroscopy was applied in order to find new energy levels of the niobium atom. A continuous wave tuneable titanium–sapphire laser in the wavelength range from 750 to 865 nm and a hollow-cathode lamp were used. We discovered four energy levels of even parity, three lying levels below 19 000 cm−1 and one at much higher energy. Additionally hyperfine structure data of six levels of odd parity were determined.

Atomic reconstruction of niobium (111) surfaces

The {111} surface of niobium single crystals has been investigated by ultra-high vacuum scanning tunneling microscopy. An atomic reconstruction has been experimentally observed and has been characterized by a lattice constant two times larger than that expected for a body-centered cubic structure. Moreover, an upper incipient structure is observed on these (2×2) surfaces. The upper unit cell is 23 times larger than the bulk one and is also rotated by 30° with respect to the bulk structure. A model is proposed to explain the competition between a covering of the free surface by a non-dense atomic structure and an island formation of niobium atoms in the dense configuration. The experimental results are finally discussed with the help of ab initio calculations used to determine the surface energies of the associated atomic configurations.

Periodic cation segregation in Cs0.44[Nb2.54W2.46O14] quantified by high-resolution scanning transmission electron microscopy.

The atomic structure of Cs0.44[Nb2.54W2.46O14] closely resembles the structure of the most active catalyst for the synthesis of acrylic acid, the M1 phase of Mo10V2(4+)Nb2TeO42-x. Consistently with observations made for the latter compound, the high-angle electron scattering signal recorded by scanning transmission electron microscopy shows a significant intensity variation, which repeats periodically with the projected crystallographic unit cell. The occupation factors for the individual mixed Nb/W atomic columns are extracted from the observed intensity variations. For this purpose, experimental images and simulated images are compared on an identical intensity scale, which enables a quantification of the cation distribution. According to our analysis specific sites possess low tungsten concentrations of 25%, whereas other sites have tungsten concentrations above 70%. These findings allow us to refine the existing structure model of the target compound, which has until now described a uniform distribution of the niobium and tungsten atoms in the unit cell, showing that the similarity between Cs0.44[Nb2.54W2.46O14] and the related catalytic compounds also extends to the level of the cation segregation.

Changes of the crystal structure at the relaxor ferroelectric phase transition of strontium barium niobate (SBN53)

The average crystal structures of the ferroelectric and the intermediate paraelectric phases of SBN53 were refined at various temperatures up to 200 °C from single crystal X‐ray diffraction data. Shifts of the atoms reduce the microscopic spontaneous electric polarization which, however, does not entirely vanish on heating at the ferroelectric (FE) phase transition near 95 °C. Most atoms show an unusual temperature dependence of the atomic displacement parameters (adp). At the FE transition, a very large anisotropic increase of the U33 adp of part of the niobium atoms indicates enhanced structural disorder in the intermediate high temperature phase. A similarly high anisotropic U11 adp of the large Me2 site can be assigned to static disorder caused by incomplete occupation by Ba2+ and Sr2+ which is not affected by the FE phase transition.

First-principles calculations of water dissociation on the oxygen-deficient (010) surface of Fergusonite-type LaNbO4 crystal

The water-containing atmosphere plays an important role in the improvement of proton conductivity of LaNbO4. The interaction between the water molecule and the oxygen-deficient (010) surface of LaNbO4 crystal has been investigated via first-principles calculations. The water molecule is set at different heights over the oxygen vacancy and the exposed niobium atom in Nb–O tetrahedron. The adsorption and dissociation behavior of the water molecule to the surface are illustrated and analyzed by the total and localized density of states (DOS) plots. By finely adjusting the heights of the water molecule from the surface, the relatively stable position for the water molecule is determined by free energies of the hydrated slabs. The water molecule prefers to adsorb onto LaNbO4 (010) surface and then to dissociate into one proton and one hydroxyl over the oxygen vacancy rather than over the exposed niobium atom in Nb–O tetrahedron.

A mixed-addenda Nb/W polyoxometalate containing dimeric Dawson subunit: Synthesis, structure, and characterization

A mixed-addenda Nb/W polyoxometalates, K4Na4[H6P2W12Nb4O59(NbO2)2]2·48H2O (1), has been isolated under acidic condition. The single crystal structural analysis showed that the six vacant sites of the lacunary {P2W12} precursor are occupied by six Nb atoms, two at the polar sites and four at the equatorial sites to form a {P2W12Nb6} mixed-addenda subunit. Two {P2W12Nb6} subunits are joined together in an inversion mode by two Nb–O–Nb bridges from the Nb atoms at equatorial sites to form a dimeric unit, which can be considered as a Nb12 core wrapped by two lacunary {P2W12} polyoxoanions. It is notable that the terminal peroxo groups bonded niobium atoms in the Nb12 core are only partially eliminated during the formation of the dimeric unit.

Theoretical study of the ground-state structures and properties of niobium hydrides under pressure

As part of a search for enhanced superconductivity, we explore theoretically the ground-state structures and properties of some hydrides of niobium over a range of pressures and particularly those with significant hydrogen content. A primary motivation originates with the observation that under normal conditions niobium is the element with the highest superconducting transition temperature (${T}_{c}$), and moreover some of its compounds are metals again with very high ${T}_{c}$'s. Accordingly, combinations of niobium with hydrogen, with its high dynamic energy scale, are also of considerable interest. This is reinforced further by the suggestion that close to its insulator-metal transition, hydrogen may be induced to enter the metallic state somewhat prematurely by the addition of a relatively small concentration of a suitable transition metal. Here, the methods used correctly reproduce some ground-state structures of niobium hydrides at even higher concentrations of niobium. Interestingly, the particular stoichiometries represented by NbH${}_{4}$ and NbH${}_{6}$ are stabilized at fairly low pressures when proton zero-point energies are included. While no paired H${}_{2}$ units are found in any of the hydrides we have studied up to 400 GPa, we do find complex and interesting networks of hydrogens around the niobiums in high-pressure NbH${}_{6}$. The Nb-Nb separations in NbH${}_{n}$ are consistently larger than those found in Nb metal at the respective pressures. The structures found in the ground states of the high hydrides, many of them metallic, suggest that the coordination number of hydrogens around each niobium atom grows approximately as 4$n$ in NbH${}_{n}$ ($n$ $=$ 1--4), and is as high as 20 in NbH${}_{6}$. NbH${}_{4}$ is found to be a plausible candidate to become a superconductor at high pressure, with an estimated ${T}_{c}$ \ensuremath{\sim} 38 K at 300 GPa.

Bonding, stability, and electronic properties of the BC3honeycomb monolayer structure on NbB2(0001)

Two-dimensional graphenelike BC${}_{3}$ honeycomb structures, which have been grown on the NbB${}_{2}$(0001) surface, possess intriguing properties that offer new opportunities for applications in nanoelectronics, mechanical materials, and superconductivity. The bonding configuration of BC${}_{3}$ on the substrate has not yet been determined, however. We report ab initio thermodynamics calculations and analysis that lead to a prediction about the most stable bonding configuration of a monolayer BC${}_{3}$ on NbB${}_{2}$, namely the C-top configuration in which carbon is bonded directly on the top of the niobium atoms. We also find that the BC${}_{3}$ monolayer on NbB${}_{2}$ is thermodynamically stable, accounting for the experimental observation that the BC${}_{3}$ sheet can be grown on NbB${}_{2}$ by carbon substitution and surface segregation of carbon impurities. We further provide detailed information about the atomic structures and electronic properties of the BC${}_{3}$-bonded NbB${}_{2}$ surface.

Oxygen Atom Adsorption on and Diffusion into Nb(110) and Nb(100) from First Principles

To understand the dynamics of oxidation of Nb, we examine the adsorption, absorption, and diffusion of an oxygen atom on, in, and into Nb(110) and Nb(100) surfaces, respectively, using density functional theory. Our calculations predict that the oxygen atom adsorbs on the threefold site on Nb(110) and the fourfold hollow site on Nb(100), and the adsorption energy is −5.08 eV and −5.18 eV, respectively. We find the long and short bridge sites to be transition states for O diffusion on Nb(110), while the on-top site is a rank-2 saddle point. In the subsurface region, the oxygen atom prefers the octahedral site, as in bulk niobium. Our results show also that the O atom is more stable on Nb(110) subsurface than on Nb(100) subsurface. The diffusion of oxygen atoms into niobium surfaces passes through transition states where the oxygen atom is coordinated to four niobium atoms. The diffusion barriers of the oxygen atom into Nb(110) and Nb(100) are 1.81 eV and 2.05 eV, respectively. An analysis of the electronic density of states reveals the emergence of well-localized electronic states below the lowest states of clean Nb surfaces due to d–p orbital hybridization.