Nb Atoms Research Articles

Atomic scale configuration of planar defects in the Nb-rich C14 Laves phase NbFe2

Laves phases belong to the group of tetrahedrally close-packed intermetallic phases, and their crystal structure can be described by discrete layer arrangements. They often possess extended homogeneity ranges and the general notion is that deviations from stoichiometry are accommodated by anti-site atoms or vacancies. The present work shows that excess Nb atoms in a Nb-rich NbFe2C14 Laves phase can also be incorporated in various types of planar defects. Aberration-corrected scanning transmission electron microscopy and density functional theory calculations are employed to characterize the atomic configuration of these defects and to establish stability criteria for them. The planar defects can be categorized as extended or confined ones. The extended defects lie parallel to the basal plane of the surrounding C14 Laves phase and are fully coherent. They contain the characteristic Zr4Al3-type (O) units found in the neighboring Nb6Fe7 µ phase. An analysis of the chemical bonding reveals that the local reduction of the charge transfer is a possible reason for the preference of this atomic arrangement. However, the overall layer stacking deviates from that of the perfect µ phase. The ab initio calculations establish why these exceptionally layered defects can be more stable configurations than coherent nano-precipitates of the perfect µ phase. The confined defects are observed with pyramidal and basal habit planes. The pyramidal defect is only ~1 nm thick and resembles the perfect µ phase. In contrast, the confined basal defect can be regarded as only one single O unit and it appears as if the stacking sequence is disrupted. This configuration is confirmed by ab initio calculations to be metastable.

Discovery of space aromaticity in transition–metal monoxide crystal Nb3O3 enabled by octahedral Nb6 structural units

An octahedral Nb6 structural unit with space aromaticity is identified for the first time in a transition–metal monoxide crystal Nb3O3 by ab initio calculations. The strong Nb–Nb metallic bonding facilitates the formation of stable octahedral Nb6 structural units and the release of delocalization energy. Moreover, the Nb atoms in continuously connected Nb6 structural units share their electrons with each other in a continuous space of framework, so that the electrons are uniformly distributed. The newly discovered aromaticity in the octahedral Nb6 structural units extends the range of aromatic compounds and broadens our vision in structural chemistry.

Unusual weak increase of Curie temperature and lattice parameters in Pr2Fe16.5Zr0.5

Crystal structure and magnetic and thermomagnetic properties of the Pr2Fe17-xMx compounds with M = Ti, V, Cr, Zr, Nb have been studied. The change of interatomic distances is not enough to explain the growth of the Curie point and it is necessary to assume the distribution of M = Ti, V, Cr, Nb atoms over the lattice positions, similar to that previously published for the Nd2Fe17-xMx compounds. It was established by neutron diffraction that Zr atoms substitute for Pr atoms in the 6c site and do not substitute for Fe atoms as might be expected from the original formula Pr2Fe16.5Zr0.5. So, the real composition of the substituted compound studied is Pr14.8(8)Zr0.52(8)Fe17. The magnetic moment of Fe atom and the magnetocaloric effect in the Pr2Fe17-xMx compounds decrease with decreasing Fe content.

Structural, electronic and magnetic properties of a ferromagnetic metal: Nb-doped EuTiO3

Experimentally, ferromagnetic and metallic behaviors of Nb-doped EuTiO3 were reported. Until now, first-principle computational study has not been carried out to investigate the origin of the FM interactions in doped EuTiO3. Here, structural, electronic and magnetic properties of Nb-doped EuTiO3 are investigated using hybrid density functional theory method. The calculated results show that the entire series of EuTi1−xNbxO3 (0 ≤ x ≤ 1) stabilize in a cubic perovskite structure. EuTi1−xNbxO3 is an antiferromagnetic insulator at x = 0 and a ferromagnetic metal at 0.125 ≤ x ≤ 1, which agrees well with available experiments. Nb doping induces itinerant electrons into the conduction band bottom at 0.125 ≤ x, and the Fermi level moves upward. As a result, the Ruderman-Kittel-Kasuya-Yosida (RKKY)-type interaction, between Eu2+ spins by the itinerant Ti 3d and Nb 4d electrons, can be responsible for ferromagnetism in EuTi1−xNbxO3. The present study provides a theoretical understanding of origin of ferromagnetism in EuTi1−xNbxO3 and is helpful to understand FM interactions of other doped EuTiO3. For EuTi0.5Nb0.5O3, arranging the Ti and Nb atoms into a 1 × 1 EuTiO3/EuNbO3 superlattice along the [0 0 1] direction results in interesting FM metallic properties, which are not present in the parent compound EuTiO3 but similar to those of EuNbO3.

First-principles investigation on the atomic structure, stability and electronic property of O(001)/B2(110) interface in Ti2AlNb alloys

In the present work, first-principles calculation is employed to investigate the adhesion, stability and electronic property of Ti2AlNb O(001)/B2(110) interface. It is found that the Ti2AlNb O(001) surface slab and B2(110) surface slab can reach bulk-like characteristics when the atomic layers are larger than seven. Four kinds of Ti2AlNb O(001)/B2(110) interfacial models are investigated in consideration of different interfacial atom stacking sites. The adhesive work, interfacial energy and electronic structure of these four interface models are calculated. The results show that the Ti2AlNb O(001)/B2(110) interface with the bridge-stacking site 1, i.e. the interfacial atoms of O phase are located either between Al and Nb atoms or between two Ti atoms on the first atomic layer of B2(110) surface slab, shows the smallest interface distance (2.34 Å), the largest adhesion work (3.38 J/m2) and the lowest interface energy (0.41 J/m2), which indicates that this interfacial configuration is the most thermodynamically stable and is preferred equilibrium structure for the Ti2AlNb O(001)/B2(110) interface. The calculated electronic properties reveal that the maximum thermodynamic stability of Ti2AlNb O(001)/B2(110) interface with the bridge-stacking site 1 is mainly contributed from Ti–Nb covalent and Ti–Al covalent interactions.

Sol-gel synthesis, spark plasma sintering, structural characterization, and thermal conductivity measurement of heavily Nb-doped SrTiO3/TiO2 nanocomposites

Heavily Nb-doped strontium titanate (SrTi1-xNbxO3) nanoparticles and SrTi1-xNbxO3/TiO2 nanocomposite powders were synthesized by a sol-gel method. Structural characterization of the obtained powders was performed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and UV–visible spectroscopy. The powders were densified by spark plasma sintering (SPS) method up to 98% of the relative density. Upon composite production, the thermal conductivity of the un-doped samples was effectively decreased for SrTiO3/TiO2 nanocpmposite from 12 to 8 W/m.K. On the other hand, thermal conductivity of the Nb-doped SrTi0.8Nb0.2O3/TiO2 composite was decreased by about 50% down to 3.4 W/m.K in comparison to SrTiO3/TiO2 due to the phonon scattering at the point defects originated from both Nb atoms and TiO2 nanoparticles.

Magnetic properties in Nb/Tc adsorbed gt-C3N4 monolayer

Abstract Basing on ab initio calculations, we investigate the electronic structures and magnetic properties of triazine-based graphitic carbon nitride (gt-C3N4) sheets with adsorbed by Nb and Tc. Both Nb and Tc atoms energetically prefer to be adsorbed on the cavity site. Adsorptions lead to total magnetic moments of 1.00 μB per Nb or Tc atom. The magnetic moments on Nb anti-ferromagnetically couples with those on C and N atoms, but the spins between Nb atoms strongly ferromagneticially couple to each other. The system therefore becomes ferromagnetic (FM) semiconductor with reduced band-gap, and the estimated Curie temperature (TC) is much higher than room temperature. The magnetic moments of Tc adsorbed system ferromagnetically couple with each other. The adsorption of Tc atom changes the semiconducting gt-C3N4 into half metallic material. These findings indicate adsorptions of Nb/Tc lead to various magnetic characters that has potential application in future spintronics.

Atomic Nb Anchoring on Graphdiyne as a New Potential Electrocatalyst for Nitrogen Fixation: A Computational View

AbstractThe exploration of effective active catalysts for the synthesis of ammonia NH3 is of vital importance to the development of the industrial dinitrogen‐fixation industry. Herein, a series of single transition metal atoms is screened anchoring on a monolayer graphdiyne (TM@GD, TM = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Nb, Mo, Ru, Rh, Pd, Ag, W, Ir, Pt, and Au) toward electrocatalytic N2 reduction reaction nitrogen reductive reaction (NRR) by density functional theory (DFT) calculations. Several TM@GD, including Ti@GD, V@GD, Nb@GD, Ru@GD, and W@GD, are computed to experience relatively lower free energy uphill for NRR than Ru(0001) surface. Specifically, the Nb@GD is demonstrated to be the most excellent for NRR with a limited potential of only 0.27 V, and also a high selectivity of 49.95% in spite of hydrogen evolution reaction (HER) competition simply estimated by Boltzmann distribution. Distal pathway is identified here without the formation of N2N4 species, ensuring a highly oriented product of NH3. Studies in this work would facilitate further experimental research in this field with a new idea and guidance to find out the active catalysts of high efficiency for NH3 synthesis at room temperature.

Atomic and electronic structure of an epitaxial Nb2O3 honeycomb monolayer on Au(111)

The experimental discovery and theoretical analysis of an epitaxial (2 \ifmmode\times\else\texttimes\fi{} 2) honeycomb $\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{3}$ monolayer on a Au(111) surface is reported. The oxide monolayer is grown by Nb deposition and subsequent annealing in an oxidizing atmosphere. Scanning tunneling microscopy (STM) images show that the films form a well-ordered honeycomb lattice, and low energy electron diffraction patterns confirm that the films adopt a (2 \ifmmode\times\else\texttimes\fi{} 2) periodicity with respect to the Au(111) substrate. Density functional theory (DFT) modeling shows that the Nb atoms are located in Au(111) threefold hollow sites and the O atoms are located in on-top positions. DFT also demonstrates the existence of a strong interfacial interaction characterized by a large electron transfer towards the Au substrate, an increase of the Nb oxidation state, and substantial film rumpling. High-resolution STM images, supported by simulations, are able to discriminate between Nb atoms adsorbed in fcc or hcp hollow sites on the Au(111) substrate.

Investigation on the structures of τ1 and τ2 compounds in the Zr–Fe–Cu system and phase equilibria of the Zr–Nb–Fe–Cu system

Two intermetallic compounds τ1 (i.e. Zr(Fe, Cu)) and τ2 (i.e. Zr2(Fe, Cu)) with unconfirmed structures in the Zr–Fe–Cu ternary system have been identified and the equilibrium phase relationship of the quaternary Zr–Nb–Fe–Cu system at 700 °C and 580 °C have been experimentally investigated by using optical microscopy, electron probe micro-analyzer coupled with wavelength-dispersive spectroscopy, transmission electron microscopy and X-ray diffraction on the equilibrated alloys. In the ternary Zr–Fe–Cu system, the results show that τ1 has the same structure as ZrCu phase with cubic CsCl-type structure, and the structure of τ2 phase is the same as Zr2Fe with tetragonal Al2Cu-type structure. The solution mechanism of alloying elements for the τ1 compound is that the Cu sites are occupied by Fe atoms to stabilize the ZrCu structure, in the meanwhile, the mechanism of τ2 is that the Fe sites are occupied by Cu atoms. For the quaternary Zr–Nb–Fe–Cu system, there is no new quaternary intermetallic phase detected in the present work. Only three intermetallic compounds, i.e. λ1, λ2 and τ2, presented in sub-ternaries were identified in the quaternary system at both 700 °C and 580 °C. It was found that Cu atoms dissolve in λ1 and λ2 phases forming cubic (Zr, Nb)2(Fe, Cu) and hexagonal (Cu, Fe, Nb)2Zr respectively, and Nb atoms dissolve in τ2 phase forming tetragonal (Zr, Nb)2(Fe, Cu).

Site Occupation of Nb in γ-TiAl: Beyond the Point Defect Gas Approximation

Microalloying is an effective approach to improve the mechanical properties of γ-TiAl intermetallic compound. Knowledge about the site occupancy of the ternary alloying element in the crystal lattice of γ-TiAl is highly demanded in order to understand the physics underlying the alloying effect. Previous first-principle methods-based thermodynamic models for the determination of the site occupancy were based on the point defect gas approximation with the interaction between the point defects neglected. In the present work, we include the point defect interaction energy in the thermodynamic model, which allows us to predict the site occupancy of the ternary alloying element in γ-TiAl beyond the point defect gas approximation. The model is applied to the γ-TiAl–Nb alloy. We show that, at low temperature, the site occupancy of Nb atoms depends on the composition of the alloy: Nb atoms occupy the Al sublattice for the Ti-rich alloy but occupy Ti sublattice for the Al-rich alloy. The fraction of Nb atoms occupying Al sublattice in the Ti-rich alloy decreases drastically, whereas the fraction of Nb atoms on the Ti sublattice in the Al-rich alloy decreases slightly with increasing temperature. At high temperature, Nb atoms occupy dominantly the Ti sublattice for both the Ti-rich and Al-rich alloys. The interaction between the point defects makes the Ti sublattice more favorable for the Nb atoms to occupy.

Charge density wave and spin 1/2 insulating state in single layer 1T-NbS2

In bulk samples and few layer flakes, the transition metal dichalcogenides NbS2 and NbSe2 assume the H polytype structure with trigonal prismatic coordination of the Nb atom. Recently, however, single and few layers of 1T-NbSe2 with octahedral coordination around the transition metal ion were synthesized. Motivated by these experiments and by using first-principles calculations, we investigate the structural, electronic and dynamical properties of single layer 1T-NbS2. We find that single-layer 1T-NbS2 undergoes a star-of-David charge density wave. Within the generalized gradient approximation, the weak interaction between the stars leads to an ultraflat band that results to be located at the Fermi level and isolated from all other bands. The spin-polarized generalized gradient approximation stabilizes a total spin 1/2 magnetic state with a magnetic moment localized on the central Nb in the star and the opening of a 0.15 eV band gap. In the GGA + U framework, the magnetic moment on the central Nb is enhanced to and a larger gap occurs. Most important, this approximation gives a small energy difference between the 1T and 1H polytypes (only +0.5 mRy/Nb), suggesting that the 1T-polytype can be synthesized in a similar way as done for single layer 1T-NbSe2. Finally we compute first and second nearest neighbors magnetic inter-star exchange interactions finding J1 = 9.5 K and J2 = 0.4 K ferromagnetic coupling constants.

Strain-induced N–N bonding and magnetic changes in monolayer intrinsic ferromagnetic TmN2 (Tm = Tc and Nb)

The modulation of magnetic property in two-dimensional (2D) intrinsic ferromagnets is important for their future application in spintronic devices at the nanoscale. In this work, using first-principles calculation, we investigate the effects of strain on the structures, electronic structures and magnetic properties of monolayer 1H-NbN2 and 1H-TcN2. The results show that both the unstrained monolayer 1H-NbN2 and 1H-TcN2 are 2D intrinsically ferromagnetic (FM) metal, in which the magnetic moment of the 1H-NbN2 and 1H-TcN2 comes mainly from the d orbitals of Nb atom and the p orbitals of N atom, respectively. Remarkably, two neighboring N atoms in the unstrained 1H-NbN2 form N–N bond, while those in the 1H-TcN2 do not. When lattice constant a increases to 3.17 Å, monolayer 1H-TcN2 undergoes N–N nonbonding-bonding transition at which the distance between the N atoms dN–N suddenly drops by almost 25%. In particular, due to the bonding between two neighboring N atoms, the magnetic moment of N atoms in 1H-TcN2 are quenched and the ground state transfers to non-magnetic. In contrast, when a decreases to 3.18 Å, monolayer 1H-NbN2 undergoes N − N bonding-nonbonding transition at which the dN–N suddenly increases from 1.79 Å to 1.97 Å. The N–N bonding-nonbonding transition induces the magnetic moments to transfer from the d orbitals of Nb atom to the p orbitals of N atom, while ground state of monolayer 1H-NbN2 remains FM metal.

Electronic structure, mechanical and physical properties of Ag alloyed α-Nb5Si3: First-principles calculations

The effect of Ag additions on the structural, mechanical and thermodynamic properties of α-Nb5Si3 is systematically investigated using the first-principles calculations based on density functional theory. The calculated formation enthalpies show that when Nb atoms in α-Nb5Si3 are replaced by one and two Ag atoms, Ag atoms prefer Nb4c sites rather than Nb16l sites. With increasing concentration of Ag atoms, the elastic moduli and theoretical Vickers hardness of the α-(Nb1-xAgx)5Si3 compounds decrease, while they become more ductile. Ag additions slightly raise the mechanical anisotropies of α-(Nb1-xAgx)5Si3 compounds and increases the linear thermal expansion coefficients of the α-(Nb1-xAgx)5Si3 compounds along the c and a axes, with a decrease in the αc/αa ratios. Finally, the anisotropy of acoustic velocities and Debye temperatures for the α-(Nb1-xAgx)5Si3 compounds were also calculated.

High Ionic Conductivity in Oxygen-Deficient Ti-Substituted Sodium Niobates and the Key Role of Structural Features

NaNb1–xTixO3–0.5x (0 < x ≤ 0.15) compounds were prepared using a two-step synthesis process involving a hydrothermal route at T = 200 °C in an autoclave followed by heat treatments under air or reductive conditions. Rietveld structural refinements from X-ray diffraction data combined with 23Na and 93Nb nuclear magnetic resonance evidenced the formation of new complex oxides crystallizing in the P21ma space group. Ti substitution for Nb atoms contributes to stabilize the acentric polymorph rather than the well-known thermodynamically stabilized network (Pbma space group) of sodium niobate. Taking into account the competitive bond sequence Na1(2)–O1(2)–Nb–O3(4), the large variation of Na1–O1 and Na2–O2 bond lengths after Ti substitution leads to reduction of the Nb/Ti–O1 and Nb/Ti–O2 apical distortion (elongation in one direction) and consequently exaltation of the distortion in the equatorial plane. Then, transition metal crystal field splitting, as well as the second-order Jahn–Teller effect, increases, a...

Influence of Nb concentration on the structure, stability, and electronic and mechanical properties of D022 Al3Ti by first-principles calculations and experiments

The crystal structure, stability, and electronic and mechanical properties of D022 Al3Ti-Al3Nb pseudobinary solid solution were systematically investigated by a first-principles method and experiment. The calculated formation energies demonstrated that all the Al3(Ti1-xNbx) (0 ≤ x ≤ 1) solid solutions considered are thermodynamically stable and that the stability decreases monotonously with increasing Nb concentration. Lattice parameter a of Al3(Ti1-xNbx) exhibits strong negative deviation from a linear composition dependence, while the trend is reversed for lattice parameter c. Study of the mechanical properties (including the bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and Debye temperature) reveals that Nb atom substitution in Al3Ti enhances the mechanical stability and strength. The analysis of calculated electronic structures reveals that the metallic behavior of Al3(Ti1-xNbx) depends on the Nb concentration. Experimentally, Al3(Ti1-xNbx) was synthesized and was characterized by means of X-ray diffraction and scanning electron microscopy. Rietveld refinement results confirmed that the lattice parameters are in the range of our previous calculated values. The differences between the calculated and experimental lattice parameters are also discussed in detail.

Vacancy-interface-helium interaction in Zr-Nb multi-layer system: A first-principles study

Driven by our previous experimental findings, we have performed ab-initio simulations to investigate the mechanical properties of and vacancy-interface-helium interaction at Zr-Nb (HCP-BCC) multi-layer composite at Burgers orientation ((0 0 0 2)//(1 1 0) - 〈2 -1 -1 0〉 // 〈1 -1 1〉). HCP-BCC interfaces have not been previously modelled using density functional theory. We present many aspects of this interfacial system such as elastic constants, charge densities and densities of states which, although being crucial in understanding the properties of the materials with interfaces at the atomistic level, are usually ignored by similar works. A thorough analysis of the mechanical properties of Zr-Nb multi-layers has been performed and compared with the behaviour of their constituents in bulk form. The results are promising for applicational purposes as the multi-layers retain most of the mechanical properties of the bulk forms except for the anisotropy index. Furthermore, we present electronic structure analysis which reveals formation of bonding between opposing Zr and Nb atoms and transfer of some net charge at the interface. These results, together with the energetics of the systems, are used to understand the interaction among vacancy, He and the interface which gives insight for foreseeing the behaviour of the system in the presence of He atoms.

Niobium-Doped TiO2: Effect of an Interstitial Oxygen Atom on the Charge State of Niobium.

The structural and electronic properties of Nb-doped rutile TiO2 with several doping configurations were investigated by first-principles calculations based on the density functional theory. The calculations show that although the band gap in the NbTi, 2NbTi, and NbTi + Oi systems is small, the intragap states would be the electron-hole recombination center, leading to low photocatalytic efficiency. However, for the 2NbTi + Oi configuration, the impurity states are mainly located at the top of the valence band and the electron-hole recombination would be inhibited, indicating relatively higher photocatalytic efficiency. On the basis of the charge-compensated theory, two electrons on the transition-metal Nb atoms compensate for the same amount of holes on the acceptor level of a nonmetal interstitial O atom, in the model of 2NbTi + Oi. Such donor-acceptor codoping may not only suppress the electron-hole recombination but also maintain a reduced band gap, suggesting that the doping models would exhibit higher photocatalytic activity than pure TiO2. The calculation results show that the interstitial O atoms play an essential role in manipulating the valence state of impurity Nb. The role of the interstitial O atom in Nb-doped rutile TiO2 suggests that it can give rise to beneficial charge-compensation effects.

Influence of 1%Nb Solute Addition on the Thermal Stability of In Situ Consolidated Nanocrystalline Cu

Nanocrystalline (nc) Cu and Cu–1% Nb bulk materials are synthesized using a combination of cryogenic and room temperature ball milling. The grain size values of these in situ consolidated Cu and Cu–1% Nb, determined using transmission electron microscopy, are found to be 22 nm and 18 nm, respectively. In this investigation, isochronal heat treatments are performed for 1 h to establish grain size and microstructural changes as a function of temperature. The annealing of nc Cu–1% Nb at a temperature of 1073 K reveals a slight increase in the average grain size from 18 to 45 nm. The grain size of nc Cu, however, increases from 22 nm to about 3 μm after annealing at the same conditions. The present results indicate that solute entrapment plays a major role in thermal stability of the high purity contaminant‐free Cu with the addition of only 1 at% Nb after annealing for 1 h up to a homologous temperature of 0.8. Kinetic stabilization via clustering of Nb atoms on the grain boundaries and the triple junctions is also observed after annealing at high temperature for longer times.

The effect of isothermal aging on microstructure and mechanical behavior of modified 2.5Al alumina-forming austenitic steel

Microstructural evolution of modified 2.5Al alumina-forming austenitic steel after isothermal aging at 973 K was investigated by scanning electron microscope (SEM), and X-ray diffraction (XRD). Tensile and vicker's hardness tests were also carried out to study mechanical properties during isothermal aging. The average particle diameter of precipitates was statistically measured and the kinetic model was employed to describe the coarsening of precipitates. The results show that the precipitation sequence of Laves and NiAl phases was related to aging time, and precipitated after 20 h and 100 h in this work, respectively. The average particle diameter of precipitates continuously increased with aging time. And the coarsening rate of the precipitates decreased at 1000 h due to the exhaustion of Nb atoms in matrix. The precipitation of NiAl and Laves phases plays an important role in mechanical properties. The hardness and the work hardening rate continuously increased. In contrast, the elongation gradually decreased. The above results indicated that the effect of aging strengthening. The room-temperature yield strength firstly sluggish increased and then gradually decreased, finally increased significantly due to the increase in σgb, the decrease in σdis and σss, and the change in σppt.