Nb-O Systems Research Articles

UV and X-ray induced photochromic material based on defect state exchanges

Inorganic photochromic (PC) materials are gaining increasing recognition as promising candidates for applications in optical storage and information display. However, conventional photochromic materials exhibit sluggish coding rates and a deficiency in intelligent response for writing and erasing under multiple excitation sources. This deficiency contributes to suboptimal readout/decoding efficiency and storage security. Here, we show a Ba2NaNb5O15:Er3+ photochromic material with photoluminescence reversible modulation that can fast respond to both ultraviolet light and 532 nm laser within about 1 s, which accompanied by thermal bleaching. The PC mechanism which is based on defect state exchanges between oxygen vacancy defects and color centers is attributed to the structural flexibility of Nb-O systems and oxygen vacancy increased under irradiation. More importantly, it shows differential PC decay behavior in response to X-ray and UV light due to the difference in the number of trapped carriers. Therefore, a simple strategy based on different self-recovery levels induced by double excitation is constructed to develop an encrypted optical memory model. This fast and differential excitation source response behavior hopefully stimulates the development of new encrypted optical memory.

Regulating anionic redox activity of lithium-rich layered oxides via LiNbO3 integrated modification

Lithium-rich layered oxide cathodes suffer from severe interfacial degradation, capacity attenuation and voltage fading which mainly result from poor reversibility of anionic redox. In this study, a LiNbO3 integrated strategy including LiNbO3 coating, spinel heterostructure and Nb5+ doping has been proposed for stabilizing lattice oxygen and improving structural stability. Among them, the LiNbO3 coating layer can protect highly active peroxo-like oxygen from the electrolyte and spinel heterostructure with three-dimensional (3D) lithium transport channels facilitates the diffusion kinetics. More importantly, Nb5+ dopants inserting into subsurface lattice regulate localized electron configuration and strengthen the reversibility of anionic redox. Theoretical calculation results including density of states and crystal orbital overlap populations unravel the active O-O dimer still coordinates with crystal framework under fully delithiated state after Nb doping exhibiting enhanced anionic redox reversibility and structural stability. Corresponding analysis suggest that doping Nb5+ cations possessing no valence electron (4d0) and low electronegativity could transform Mn-O system into electrochemical inactive Nb-O system with large charge transfer and low d orbital repulsion (Mott-Hubbard regime). Moreover, this inactive Nb-O system cannot provide any reactive electron from Nb5+ cation and oxygen holes have to be isolated on O 2p orbitals, leading to the enhanced reversibility of anionic redox. This integrated modification strategy provides inspiring insights for understanding and regulating the reversibility of anionic redox, showing great potential in designing high-performance lithium-rich Mn-based cathodes.

Multiple synergistic effects of Zr-alloying on the phase stability and photostability of black niobium oxide nanotubes as efficient photoelectrodes for solar hydrogen production

Niobium oxides exist in a plethora of metastable, stoichiometric, nonstoichiometric, and mixed phases, rendering the Nb-O systems very complicated and hard to study. These structures significantly differ in their catalytic activity, electrical conductivity, and photoresponse. Herein, we demonstrate the ability to selectively fabricate pure T-Nb2O5via the addition of small amount of Zr as a phase stabilizer. Moreover, we were able to tune the photoactivity of the material via hydrogen annealing. The photoactivity and stability of the fabricated black Zr-doped Nb2O5 nanotubes were correlated with the nature of induced defects upon hydrogen annealing and Zr doping. The H2-treated nanotubes showed extraordinary and remarkable stability and photoactivity upon their use for solar water splitting. This was accompanied by a noticeable reduction in the bandgap energy from 3.23 eV to 2.5 eV, which is mainly correlated with the introduced oxygen vacancies within the lattice with a remarkable conductivity. Most importantly, the black-defective nanotubes exhibited a photocatalytic activity that is ∼ 65 times that of the air-annealed counterparts. The optimized photoanodes attained a hydrogen production rate of ∼ 496 μmol h−1 cm−2 in 1 M KOH, revealing increased charge carriers transport and separation. The Mott-Schottky and valence band XPS analyses confirmed the increased charge carriers’ concentration and the appropriate band positions of the fabricated black nanotubes relative to the redox potentials of the water.

Structural Studies of Bulk to Nanosize Niobium Oxides with Correlation to Their Acidity.

Hydrated niobium oxides are used as strong solid acids with a wide variety of catalytic applications, yet the correlations between structure and acidity remain unclear. New insights into the structural features giving rise to Lewis and Brønsted acid sites are presently achieved. It appears that Lewis acid sites can arise from lower coordinate NbO5 and in some cases NbO4 sites, which are due to the formation of oxygen vacancies in thin and flexible NbO6 systems. Such structural flexibility of Nb-O systems is particularly pronounced in high surface area nanostructured materials, including few-layer to monolayer or mesoporous Nb2O5·nH2O synthesized in the presence of stabilizers. Bulk materials on the other hand only possess a few acid sites due to lower surface areas and structural rigidity: small numbers of Brønsted acid sites on HNb3O8 arise from a protonic structure due to the water content, whereas no acid sites are detected for anhydrous crystalline H-Nb2O5.

Thermodynamic Properties of the Phases in the V-O and Nb-O Systems at High Temperature

Thermodynamic Properties of the Phases in the V-O and Nb-O Systems at High Temperature

N Doping of Oxide Nanosheets

Photo-N doping for Nb-O system oxide nanosheets such as Nb(6)O(17), TINbO(5), and Ca(2)Nb(3)O(10) was succeeded in by treatment under mild conditions, where their H-restacked forms containing tetrabuthylammonium (TBA) ion were illuminated by UV light at room temperature. A relatively strong absorption in visible light region (400 approximately 600 nm) was observed for the N-doped samples because of formation of p-band or valence band hybridization of N in the Nb-O bandgap. The N doping proceeded only under TBA presence in water without O(2), and Pt loading promoted this doping reaction. It is proposed as a mechanism from the results of the doping for the Ti-O and Nb-O nanosheet mixed samples, that the Nb-O nanosheet acts as a photocatalyst for the N-doping reaction. This is the first report of N doping to oxide nanosheets.

Synthesis of the LnNb7O12 (Ln = La, Ce, Pr) Discrete-cluster compounds

The new compounds CeNb7O12 and PrNb7O12, isostructural with LaNb7O12, have been synthesized and characterized by x-ray diffraction. The structures of the LnNb7O12 (Ln = La, Ce, Pr) compounds have space group P21/c (Z = 4), with lattice parameters a = 10.764(2), 10.743(2), 10.738(4) A; b = 9.192(2), 9.146(2), 9.137(3) A; c = 10.313(2), 10.304(2), 10.303(3) A; and β = 104.25(2)°, 104.31(2)°, and 104.21(3)°, respectively. The reduction in the lattice parameters of LnNb7O12 in going from La to Pr correlates with the ionic radii of Ln: r(La3+) > r(Ce3+) > r(Pr3+). In the Nd(Sm)-Nb-O systems, phases of stoichiometry LnNb7O12 have not been obtained.

HREM studies of intergrowth between NbO and perovskite in the Ba-, Sr and K-Nb-O systems

Ordered and disordered intergrowth structures between perovskite and NbO type elements in the K-, Sr- and Ba-Nb-O systems have been studied by means of electron microscopy and X-ray powder diffraction techniques. The ordered structures KNb4O6 and A(II)2Nb5O9 (A(I)= K and A (II) = Sr, Ba) consist of lamellar intergrowths of A(I, II)NbO3 and NbO slabs, the latter being one unit cell thick. In the A (II)-Nb-O systems a two dimensional intergrowth between A(I, II)NbO3 and NbO blocks of varying sizes also exist, in a phasoid region called 03B1. Combined HREM studies and EDS-analysis of the Ba-analogue of the 03B1-phasoid gives the approximate composition ~ BaNb4O6. In the HREM images one can also find small domains of more ordered intergrowth. These are discussed in terms of micro-phases and homologous series. Some factors influencing the block sizes in the structure are also discussed.

ChemInform Abstract: REVIEW ON PHASE EQUILIBRIA AND DEFECT STRUCTURES IN THE NIOBIUM‐OXYGEN SYSTEM

Abstract The phase relations, defect structures and partial molar quantities of oxygen in the Nb-O system are reviewed. Phase diagrams or the niobium-oxygen solid solution and the nonstoichiometric NbO 2± x phase at high temperatures above 2000 K, and the phase relations between NbO 2+ x and Nb 2 O 5 below 1600 K were recently obtained. From the dependence of electrical conductivity and composition on the oxygen partial pressures, the defect structures of nonstoichiometric phases are discussed. The partial molar enthalpies and entropies of oxygen of the phases in the Nb-O system are summarized as a function of composition and discussed in relation to defect structure.

Review on phase equilibria and defect structures in the niobium-oxygen system

The phase relations, defect structures and partial molar quantities of oxygen in the Nb-O system are reviewed. Phase diagrams or the niobium-oxygen solid solution and the nonstoichiometric NbO 2± x phase at high temperatures above 2000 K, and the phase relations between NbO 2+ x and Nb 2O 5 below 1600 K were recently obtained. From the dependence of electrical conductivity and composition on the oxygen partial pressures, the defect structures of nonstoichiometric phases are discussed. The partial molar enthalpies and entropies of oxygen of the phases in the Nb-O system are summarized as a function of composition and discussed in relation to defect structure.

High temperature vaporization studies on the niobium-oxygen system by mass spectrometric method

The vapor pressures over Nb 2O 5(s,1) and two phase NbO 2+x(s) - Liquid mixture (2.063≦0/Nb≦2.386) were measured by the mass spectrometric method in the temperature range 1726–2271 K. The melting temperature of Nb 2O 5(s) and the liquidus line of two phase NbO 2+x(s) - Liquid mixture were determined from the vapor pressure measurements. The enthalpy and entropy of fusion of Nb 2O 5 were also obtained. Based on the present results together with our previous results, the phase diagram and the compositional dependences of the pressures of NbO 2(g), NbO(g), O(g) and O 2(g) for the Nb-O system were summarized.

Thermotransport of oxygen and nitrogen in niobium and tantalum

Previous studies of thermotransport in the Nb-O, Nb-N and Ta-O systems were extended to a wider concentration range of oxygen and nitrogen. It turns out that the strong concentration dependence of the heat of transport Q + found by Lohnert and Fromm in the Ta-O system appeared in the Nb-O system, too, and could be described by the following empirical formula Q + = − 2,4 c kcal at.%/mol . As the concentration c goes to zero Q + reaches a constant value of about —16 kcal/mol. The same behaviour was measured in the Ta-O system with a limit of —19 kcal/mol for c → 0. In the Nb-N system no thermotransport was found. Correlations between thermotransport and electrotransport which are discussed from a phenomenological point of view, indicate that electron drag forces play a dominant role in the mechanisms of interstitial thermotransport. This is confirmed in the present study by an argument, which is based upon the two band model of H.B. Huntington and upon the similar concentration dependence of the absolute thermoelectric power and the heat of transport in the Ta-O system.

Étude du système ternaire Niobium-Oxygène-Soufre et estimation des propriétés thermodynamiques des sulfures de niobium

The ternary phase diagram Nb-O-S has been studied in the region bounded by the compounds NbO, Nb 1− x S, NbS 2 and Nb 2O 5. Only the known binary phases present in the Nb-S and Nb-O systems have been observed. In its most stable form, Nb 1− x S is in equilibrium either with NbO or NbO 2 depending upon its composition. The NbO 2-Nb 2O 5 transition takes place at the sulphur-rich limit of 2s Nb 1+ x S 2. The 3s Nb 1+ x S 2, 3s NbS 2 and Nb 2O 5 phases are in equilibrium at about 750°C under p s 2 = 1 atm. From these results and Larson and Elliott's measurements, the free energy of formation of Nb 1,12S 2 has been calculated and those of the other sulphides estimated at 1000°C.

Superconductivity in the TiO and NbO systems

This paper presents electrical resistivity and superconductivity studies for the B1 (NaCl) structure oxides of titanium, vanadium, and niobium. In samples of nominal composition TiO x , VO x , and NbO x ,x was varied from 0.8 to 1.2. It was found that all three of these oxides exhibit room-temperature electrical resistivities characteristic of metallic behavior. With decreasing temperature, the resistivity drops steeply in the case of NbO x , but remains steady or rises somewhat in the case of TiO x and VO x , depending on the exact value ofx. It is suggested that in TiO x and VO x there is a large resistivity component due to scattering of carriers by disordered vacancies. Superconductivity was observed in NbO x (T c=1.38 K,x=1.0) and TiO x (T c=1.0 K,x=1.07). In the latter case the material showed a well-defined maximum ofT c as a function of composition, withT c 1.20. Several VO x samples remained normal to 0.07 K.

The solid-solubility of oxygen in Nb and Nb-rich Nb-Hf, Nb-Mo and Nb-W alloys: Part I: the Nb-O system

The interstitial solid-solubility of oxygen in niobium has been studied by means of X-ray diffraction, micrographic and thermal techniques using both a dynamic leak method and a Sieverts apparatus. The lattice parameter of degassed, zone-refined niobium was found to be 3.2986 Å, which is much lower than hitherto published values. At pressures above 10 −11 torr O 2, Nb is in thermodynamic equilibrium with oxide vapor and can take up to 6 at.% O 2 into solid solution at 1775°C. Above this temperature oxidation is “catastrophic” and the volatile oxides Nb 2O, NbO and Nb 2O 5 form on the surface. The “degassing” of Nb at 2200°C and above at a pressure of 10 −6 torr O 2 is, in effect, brought about by the volatilization of the oxide and not by the de-adsorption of gaseous oxygen.