Nb Substitution Research Articles

Growth and optical properties of Nb-doped WS2 monolayers

We report the chemical vapor deposition growth of Nb-doped WS2 monolayers and their characterization. Electron microscopy observations reveal that the Nb atom was substituted at the W site at a rate of approximately 0.5%. Unlike Mo doping, Nb-doped samples have photoluminescence (PL) peaks at 1.4–1.6 eV at room temperature. The peak energies are lower than the optical bandgap of 1.8 eV, and a saturation behavior of PL intensity is observed with the increase in excitation power. These results indicate that the observed PL peaks are assignable to the emission from impurity states generated by the substitution of Nb.

Effect of Nb substitution for Cu on glass formation and corrosion behavior of Zr[sbnd]Cu[sbnd]Ag[sbnd]Al[sbnd]Be bulk metallic glass

The influence of Nb substitution for Cu on glass forming ability and corrosion behavior in ZrCuAgAlBe alloy was systematically studied. It was found that although Nb addition deteriorated glass forming ability and thermal stability, corrosion resistance, which was evaluated by both potentiodynamic polarization and immersion tests, was improved in 0.5 M NaCl, 0.1 M HCl and 0.5 M H2SO4 solutions. X-ray photoelectron spectroscopy depth profiles on the polished specimens revealed that Nb addition promoted the oxidation of Zr and resulted in thicker oxide film with high valence. Meanwhile, composition analyses before and after immersion in 0.5 M NaCl demonstrated that Cu had a strong affinity for reacting with chloride ions, causing the pitting susceptibility, and the substitution of Nb for Cu not only decreased the pitting nucleation site but also restrained the dissolve of Cu significantly through the passive film. As the immersion time was further prolonged, oxygen content in surface film increased remarkably, and only the fraction of Nb among all metallic elements rose, suggesting that the stability of Nb2O5 in chloride-containing solutions is crucial to improving corrosion resistance.

Unusual lattice evolution and magnetochemistry of Nb doped CeO2

We report on the effect of Nb substitution on the lattice evolution and magnetic properties of the Ce1−xNbxO2. Based on the microstructure analysis the solubility limit of Nb was revised to x = 0.03. The lattice response to the Nb dopant shows unexpected non-monotonic dependence which cannot be described by an ideal solution of non-interacting point defects. By slow cooling, quenching, and sealed sample anneals, the ideal solution model, including oxygen vacancies, Nb dopant, and oxygen interstitials as key defects does not adequately describe the observed concentration of Ce3+. Magnetic measurements reveal that for ceramics prepared in air at ambient pressure the concentration of Ce3+ ions linearly increases with Nb content at approximately 1:2 ratio up to the Nb solubility limit. The effect of the crystal field on the energy splitting between the Ce3+ Γ8 and Γ7 multiplets shows strong dependence on the Nb concentration especially at low doping levels. This serves as an evidence of the possible point defect association in the title compound even at a very diluted Nb concentration.

Solid solubility in 1:13 phase of doping element for La(Fe,Si)13 alloys

The influences of Ni, Cr and Nb as substitution elements for Fe were investigated. The change in microstructure and the magnetic properties have been discussed in detail. Substitution elements Ni, Cr and Nb not only have limited solubility in NaZn13-type (1:13) phase, but also hinder the peritectoid reaction. Ni element mainly enters into La-rich phase while Cr element mainly concentrates in α-Fe phase, which both have detriment effect on the peritectoid reaction, leading to a large residual of impurity phases after annealing and a decrease of magnetic entropy change. Besides, Ni and Cr participated in peritectoid reaction by entering parent phases but slightly entering 1:13 phase, which would cause the disappearance of first order magnetic phase transition. A new phase (Fe,Si)2Nb was found when Nb element substitutes Fe in La(Fe,Si)13, suggesting that Nb does not participate in peritectoid reaction and only exists in (Fe,Si)2Nb phase after annealing. The alloy with Nb substitution maintains the first order magnetic phase transition character.

Raman spectra and ferromagnetism of nanocrystalline Fe-doped Li0.43Nb0.57O3+δ

Nanocrystalline undoped LiNbO3 and LiNbO3 doped with x% Fe (x=0.5, 1, 2, 3, 5) were synthesized via a combustion method. Fe-doped LiNbO3 with a 1mol% doping concentration exhibited a room-temperature ferromagnetism of 0.06emu/g. There was an abrupt change in properties when the doping concentration of Fe reached 2mol%, where the lattice contracted obviously and the saturation magnetization (Ms) increased an order of magnitude to 0.275emu/g; Ms slightly increased to its maximum value of 1.18emu/g when the doping concentration was further increased to 5mol%. Raman spectra showed that the substitution of Li by Fe occurred at small doping concentrations and the substitution of Nb at the Nb site occurred at higher doping concentrations. The results suggest that Fe3+ replaced NbLi4+ first and the weaker ferromagnetism is due to the minor fraction of NbLi4+ in LiNbO3. Then, Fe3+ substituted Li+, resulting in large lattice distortion and much stronger spin coupling of Fe–Nb. Finally, the excess Fe3+ started to replace Nb5+at the Nb sites, where the spin coupling of Fe–Nb is weaker than that at the Li site. An analysis of the experimental results suggests that the congruent Fe-doped LiNbO3 is a promising room-temperature single-phase multiferroic material.

Phase evolution and dielectric properties of fluorite-type Bi3(Nb0.9M0.1)O7+δ ceramics (M=Ti, Zr, Sn, W, δ=±0.05)

Phase structure and dielectric behavior of Bi3(Nb0.9M0.1)O7+δ ceramics (M = Ti, Zr, Sn, W, δ = ±0.05) were discussed. Cubic fluorite solid solutions Bi3(Nb0.9M0.1)O7+δ (M = Zr, Sn, W, δ = ±0.05) were obtained through a conventional solid state reaction method. An analysis of the microwave dielectric properties showed that substitution of Nb5+ by W6+ was favorable to improving the temperature coefficient of resonant frequency (TCF value). A temperature stable dielectric material Bi3(Nb0.9W0.1)O7.05 was introduced. The Bi3(Nb0.9W0.1)O7.05 ceramics sintered at 930 °C for 2 h exhibited a high permittivity of 92.4, a Qf value of 260 GHz and a TCF value of −34 ppm/°C.

Effect of Ge addition on structure and soft magnetic properties of Si-rich Fe-based nanocrystalline alloys

Soft magnetic Fe72.7Al0.8Si17.5B5Cu1Nb3-xGex (x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) amorphous alloys were produced by the single roller melt spinning method. The effects of Ge substitution for Nb on the microstructure and magnetic properties were investigated. Magnetic properties of all the alloys annealed at different temperatures were measured by the temperature dependence of initial permeability (μi-T curves) from the ambient temperature up to 650 °C. Structure and crystalline process have been analyzed by XRD patterns and DSC curves, respectively. Partial substitution of Nb by Ge enhanced the Curie temperatures of amorphous phase and crystalline phase, and therefore improved the soft magnetic properties at high temperature. The Fe72.7Al0.8Si17.5B5Cu1Nb2.6Ge0.4 alloy annealed at 570 °C for 0.5 h shows the optimum high-temperature μi, together with higher room-temperature μi. The origin of the improved magnetic softness both at room- and high-temperature was analyzed.

Thermoelectric Properties of Fe2VAl and Fe2V0.75M0.25Al (M = Mo, Nb, Ta) Alloys: First-Principles Calculations

Ab initio investigations of the structural, electronic, and thermoelectric properties of stoichiometric Fe2VAl full-Heusler alloy and Fe2V0.75M0.25Al (M = Mo, Nb, Ta) nonstoichiometric alloys have been performed using density functional theory on the basis of the full-potential linearized augmented plane wave method with the generalized gradient approximation. The thermoelectric properties are calculated using semiclassical Boltzmann transport theory within the constant-relaxation-time approximation. Fe2VAl, Fe2V0.75Nb0.25Al, and Fe2V0.75Ta0.25Al alloys are found to exhibit a semimetallic behavior, while Fe2V0.75Mo0.25Al acts as a metal. We found that Fe2VAl has a pseudogap of about −0.13 eV, whereas Fe2V0.75Nb0.25Al and Fe2V0.75Ta0.25Al are characterized by a zero energy gap around the Fermi level. Thermoelectric calculations showed that Fe2VAl has both p- and n-type thermoelectric properties, where the p-type thermopower values are found to be higher than those of n-type. The Seebeck coefficient S has maximum values from 20 μV K−1 to 125 μV K−1 and from 19 μV K−1 to 90 μV K−1 in the temperature range of 100 K to 800 K for p- and n-type, respectively. The maximum thermoelectric properties can be obtained at carrier concentration of the order of 1020 cm−3 for p- or n-type doping. Substitution of Nb and Ta atoms enhanced the thermoelectric properties to 150 μV K−1 at 800 K. The optimum concentrations for the three partially substituted alloys were found to be between 1020 cm−3 and 1021 cm−3.

Synthesis and electrical property study of La3Ni2MO9 (M=Nb and TA)

Synthesis of La3Ni2NbO9 and La3Ni2TaO9 double perovskite materials was successfully carried out. Both isostructural niobates and tantalates exhibited a monoclinic structure. Sample preparation via solid state reaction yielded a single phase of La3Ni2NbO9, while samples with impurities were obtained when using the sol–gel process. Nb and Ta substitution in La3Ni2MO9 had effects on the sintering behavior. The electrical conductivity of La3Ni2NbO9 was found to significantly increase with increasing temperature, but the electrical conductivity of La3Ni2TaO9 decreased with temperature when the temperature was lower than 300°C then slightly increased again at higher temperature.

Structural and magnetic phase transitions in EuTi1-xNbxO3: A resonant ultrasound spectroscopy study

We have investigated the structural and magnetic phase transitions in EuTi1-xNbxO3 (0 ≤ x ≤ 0.3) with synchrotron powder X-ray diffraction (XRD), resonant ultrasound spectroscopy (RUS), and magnetization measurements. The Pm-3m ↔ I4/mcm structural transition in pure and doped compounds is marked by a pronounced step-like softening of the elastic moduli near TS, which resembles that of SrTiO3 and can be adequately modeled using the Landau free energy model employing the same coupling between strain and octahedral tilting order parameter as previously used to model SrTiO3. Upon Nb doping, the cubic-to-tetragonal structural transition shifts to higher temperatures and the room temperature lattice parameter increases while the magnitude of the octahedral tilting decreases. In addition, Nb substitution for Ti destabilizes the antiferromagnetic ground state of the parent compound and long range ferromagnetic order is observed in the samples with x ≥ 0.1.

Phase, microstructure and microwave dielectric properties of Nb and Ga doped Ca0.6La0.267TiO3 ceramics

Nb and Nb0.5Ga0.5 doped A-site deficient Ca0.6La0.267TiO3 perovskites were prepared via a mixed oxide solid state sintering route. Analysis of X-ray diffraction patterns of Nb-doped Ca0.6La0.267TiO3 samples sintered at 1300–1500 °C revealed the formation of single phase ceramics with orthorhombic symmetry. Ca0.6La0.267Ti1−x (Nb0.5Ga0.5) x O3 compositions crystallized into single phase ceramics at x = 0.3; however, the formation of a secondary phase (Ca2Nb2O7) was also observed at x > 0.3. The microstructure of Nb-doped samples comprised randomly oriented sharp-edged rectangular and/or square plate-like grains along with some voids while the microstructure of Nb/Ga-doped samples comprised relatively larger densely packed irregular-shaped grains with deformed edges. The substitution of Nb increased the relative permittivity (e r) from 114 to 136 but at the cost of both the quality factor (Q × f o) and temperature coefficient of resonant frequency (τ f). On the other hand, Nb and Ga co-doping was effective in minimizing τ f accompanied by an increase in Q × f o. The optimum microwave dielectric properties (i.e. e r = 41, Q × f o = 16,052 GHz and τ f = −13 ppm/ °C) were achieved for Ca0.6La0.267Ti0.5(Nb0.5Ga0.5)0.5O3.

Investigation of Dielectric Relaxation Processes in Ba2NdFeNb4-xTaxO15 Ceramics

Dielectric response of Ba2NdFeNb4-xTaxO15 (x = 0.3, 0.6 and 2) solid solutions with Tetragonal Tungsten Bronze structure has been investigated by means of broadband dielectric spectroscopy in the frequency range from 20 Hz to 37 GHz. Pure Ba2NdFeNb4O15 compound displays ferroelectric behavior. Substitution of Nb5+ by Ta5+ considerably alters dielectric response of this system. Ferroelectric state was observed only for x = 0.3 compound with slightly lowered ferroelectric phase transition temperature. Further increase of x causes formation of relaxor state in these solid solutions and considerably lowers corresponding dielectric permittivity dispersion temperature region.

Soft X-ray absorption spectroscopy study of (Ba0.5Sr0.5) (Co0.8Fe0.2)1−хNbхO3−δ with different content of Nb (5%–20%)

The mixed electronic ionic conducting materials (Ba0.5Sr0.5) (Co0.8Fe0.2)1−хNbхO3−δ with partial Nb substitution (x: 0.05, 0.10, 0.15, 0.20) for B cations (Co/Fe), synthesized using a solid state reaction method, have been studied by near edge X-ray absorption fine structure (NEXAFS). Co L2,3- absorption spectra of (Ba0.5Sr0.5) (Co0.8Fe0.2)1−хNbхO3−δ (BSCFN) powders were analyzed with the purpose to understand the influence of the Nb substitution on the atomic electronic structure of BSCFN. The joint analysis of the Co L2,3- absorption spectra reveals the presence of mixed oxidation states Co2+/Co3+ in all the studied BSCFN structures. It was established that the proportion of oxidation states Co2+/Co3+ and the corresponding coordinations of Co atoms depend on the content of Nb. In the 10% Nb substituted BSCF sample Co atoms mostly occur in the Co2+ oxidation state and are preferentially characterized by an octahedral coordination site. In all other structures Co atoms are rather characterized by Co2+/Co3+ oxidation states and occupy both octahedrally and tetrahedrally coordinated sites.

Structural and magnetic phase transitions inEuTi1−xNbxO3

We investigate the structural and magnetic phase transitions in EuTi1-xNbxO3 with synchrotron powder X-ray diffraction (XRD), resonant ultrasound spectroscopy (RUS), and magnetization measurements. Upon Nb-doping, the Pm-3m to I4/mcm structural transition shifts to higher temperatures and the room temperature lattice parameter increases while the magnitude of the octahedral tilting decreases. In addition, Nb substitution for Ti destabilizes the antiferromagnetic ground state of the parent compound and long range ferromagnetic order is observed in the samples containing more than 10% Nb. The structural transition in pure and doped compounds is marked by a step-like softening of the elastic moduli in a narrow temperature interval near TS, which resembles that of SrTiO3 and can be adequately modeled using the Landau free energy model employing the same coupling between strain and octahedral tilting order parameter as previously used to model SrTiO3.

Evolution of magnetic properties of CaMn1−xNbxO3 with Nb-doping

Magnetic and structural properties of Nb-doped CaMnO3 have been studied and the effect of doping with 0.02 ⩽ x ⩽ 0.1 has been investigated. Substitution of Nb5+ ion for the Mn4+ site of the parent matrix causes one-electron doping with the chemical formula , accompanied by a monotonous increase of the lattice parameters, unit-cell volume, average Mn–O bond distance and a decrease in Mn–O–Mn bond angle, with increasing x. Low temperature magnetic ground state of CaMn1−xNbxO3 has been found to be dependent on niobium doping level. The ground magnetic state evolves from mostly antiferromagnetic, with a weak ferromagnetic component for x = 0.02–0.08, to charge ordered C-type antiferromagnetic state at x = 0.1. Spontaneous magnetization increases sharply with increasing doping level, approaches a maximal value of 4.1 emu g−1 at T = 10 K for x = 0.08, and then decreases rapidly to reach a very small value of 0.2 emu g−1 for x = 0.1. Anomalous negative magnetization behavior below the magnetic transition temperature has been observed for the compound with x = 0.04 in the field cooled magnetization and remanent dc magnetization measurements. Vertical and horizontal shifts of the hysteresis loop of the field cooled sample have been observed for CaMn0.9Nb0.1O3 as possible signatures of the exchange bias effect. The effect of hydrostatic pressure on dc magnetization for the sample with x > 0.02 revealed a significant increase of the ferromagnetic phase volume under pressure, linked to progressive suppression of a negative magnetization in x = 0.04 sample.

Improvement in dehydrogenation performance of Mg(BH4)2·2NH3 doped with transition metal: First-principles investigation

The electronic structures, dehydrogenation properties and diffusion path of an H atom in pure and transition metal substituted Mg(BH4)2·2NH3 were investigated using the first-principles calculations. The occupation energies of dopants indicate that substituting Ti for an Mg1 atom is the easiest, while Ni and Nb substitutions are somewhat difficult. The electronic density of states and the topological analysis of electron density reveal the covalent characteristics of the N–H and B–H bonds and the weak ionic interaction between M (the metal atoms) and the NH3 and BH4 groups. The substitutions of Ti, Ni and Nb increase the interaction between metal and N atoms, which stabilize the NH3 groups and inhibit the release of ammonia during dehydrogenation. The hydrogen removal energies indicate that Ti, Ni or Nb doping weakens the interactions between B and H atoms, thereby facilitating the dissociation of B–H bonds in Mg(BH4)2·2NH3. The minimum energy paths of H diffusion demonstrate that the substitutions can reduce the energy barrier and thus favor H diffusion in the bulk phase. Thus, substitution is an effective technique for improving the hydrogenation/dehydrogenation performance of Mg(BH4)2·2NH3 hydrogen storage material.

Effect of Nb doping on electrochemical properties of LiNi1/3Co1/3Mn1/3O2 at high cutoff voltage for lithium-ion battery

Nb doped cathode materials with the formula LiNi1/3Co1/3Mn1/3−xNbxO2 (x=0, 0.01, 0.02, 0.03) have been prepared successfully by sol–gel method. The effect of Nb substitution on the crystal structure and electrochemical properties of LiNi1/3Co1/3Mn1/3O2 were studied systematically by X-ray diffraction (XRD) and various electrochemical measurements. The results showed Nb substitution played an important role in the good cycling performance of LiNi1/3Co1/3Mn1/3O2. Charge/discharge tests revealed that LiNi1/3Co1/3Mn1/3–0.02Nb0.02O2 showed a capacity retention of 94.1% at 1C after 50 cycles in a high cutoff voltage range (3.0–4.6V), while discharge capacity of LiNi1/3Co1/3Mn1/3O2 remains only 89.4% of that at 1C. Ex-situ XRD analysis and EIS analysis indicated that the improved electrochemical properties of Nb-doped sample result from the more stable structure and lower resistance during the electrochemical cycling.

Selecting Substituent Elements for Li-Rich Mn-Based Cathode Materials by Density Functional Theory (DFT) Calculations

Li2MnO3 is known to stabilize the structure of the Li-rich Mn-based cathode materials xLi2MnO3·(1 – x)LiMO2 (M = Ni, Co, Mn, etc.). However, its presence makes these materials suffer from drawbacks including oxygen release, irreversible structural transition, and discharge potential decay. In order to effectively address these issues by atomic substitution, density function theory (DFT) calculations were performed to select dopants from a series of transition metals including Ti, V, Cr, Fe, Co, Ni, Zr, and Nb. Based on the calculations, Nb is chosen as an dopant, because Nb substitution is predicted to be able to increase the electronic conductivity, donate extra electrons for charge compensation and postpone the oxygen release reaction during delithiation. Moreover, the Nb atoms bind O more strongly and promote Li diffusion as well. Electrochemical evaluation on the Nb-doped Li2MnO3 show that Nb doping can indeed improve the performances of Li2MnO3 by increasing its electrochemical activity and hindering...

A novel cobalt-free double-perovskite NdBaFe1.9Nb0.1O5+δ cathode material for proton-conducting IT-SOFC

A novel cobalt-free double-perovskite oxide NdBaFe1.9Nb0.1O5+δ (NBFNb10) was prepared by a solid-state reaction method and investigated as a suitable cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). It was discovered that partial substitution of Nb for Fe-site further enhanced the electrical conductivity of NdBaFe2O5+δ (NBF). The NBFNb10 exhibited the highest conductivity of 109Scm−1 at 300–800°C in air, and showed good chemical compatibility with the BaZr0.1Ce0.7Y0.2O3−α (BZCY) electrolyte. A single cell with the NBFNb10–BZCY composite cathode and NiO–BZCY composite anode-supported BZCY electrolyte membrane (ca. 30μm thickness) was fabricated and tested at 500–700°C using H2 as fuel and air as oxidant. The peak power density and the interfacial polarization resistance (Rp) were 392mWcm−2 and 0.14Ωcm2 at 700°C, respectively.

First-principles molecular dynamics study of water dissociation on the γ-U(1 0 0) surface

Based on first-principles molecular dynamics simulations at finite temperatures, we systematically study the adsorption and dissociation of water molecules on the γ-U(1 0 0) surface. We predict that water molecules spontaneously dissociate upon approaching the native γ-U(1 0 0) surface. The dissociation results from electronic interactions between surface uranium 6d states and 1b2, 3a1, and 1b1 molecular orbitals of water. With segregated Nb atoms existing on the surface, adsorbing water molecules also dissociate spontaneously because Nb 3d electronic states can also interact with the molecular orbitals similarly. After dissociation, the isolated hydrogen atoms are found to diffuse fast on both the γ-U surface and that with a surface substitutional Nb atom, which is very similar to the ‘Hot-Atom’ dissociation of oxygen molecules on the Al(1 1 1) surface. From a series of consecutive molecular dynamics simulations, we further reveal that on both the γ-U surface and that with a surface substitutional Nb atom, one surface U atom will be pulled out to form the U–O–U structure after dissociative adsorption of 0.44 ML water molecules. This result indicates that oxide nucleus can form at low coverage of water adsorption on the two surfaces.