Phase relations in the system V/Nb/O. V. Investigation of mixed crystals V1-xNbxO2

Mixed crystals V_1-xNb_xO₂ exist over the whole area of the quasibinary line VO₂-NbO₂. The existence of Nb_{5+ beside V3+ and V4+ on the V-rich side and V3+ beside Nb5+ and Nb4+ on the Nb-rich side of the mixed crystals is demonstrated by XANES-measurements. The compound VNbO}4_(V0.5_Nb0.5_O2_{) is described as a double oxide with vanadium only as V3+ and niobium only as Nb5+. At this point the electric resistivity of the solid solution shows a maximum.} Received: 11 May 1998 / Revised: 4 August 1998 / Accepted: 10 August 1998     

Zu den Phasenverhältnissen im System V/Nb/O. I. Koexistenzbeziehungen im Bereich V2O5/Nb2O5/VO2/NbO2

The Phase Relations in the System V/Nb/O. I. Coexistence Relations in the Section V₂O₅/Nb₂O₅/VO₂/NbO₂ . Phase relations in the section Nb₂O₅/V₂O₅/VO₂/NbO₂ of the ternary system V/Nb/O have been studied by X-ray diffraction. The investigated samples were prepared by high temperature synthesis at 900°C–1000°C. The section Nb₂O₅/V₂O₅/VO₂/NbO₂ is charakterized by fife three phase regions: The limits of solubility of the pseudobinary system were ascertained by determination of lattic parameters of powder samples:      

Thermochemische Untersuchungen im System V/Nb/O. II Zum chemischen Transportverhalten im Bereich V2O5/Nb2O5/VO2/NbO2

Thermochemical Investigations in the System V/Nb/O. II. Chemical Transport in the Region V₂O₅/Nb₂O₅/VO₂/NbO₂ Transport experiments were used to support the phase relationships of the V₂O₅/Nb₂O₅/VO₂/NbO₂ system, which were established by annealing experiments of powder mixtures. The phase relations were studied in the NbO₂-rich region of the system by means of X-ray and ESMA methods. The NbO₂-rich section is characterized by the following two phase and three phase regions: Two phase region: V₃Nb₉O₂₉/rutile mixed crystal V_1?xNb_xO₂ Two phase region: BI-mixed crystal/V_xNb_1?xO₂ Three phase region: V₃Nb₉O₂₉/solubility limit LG1 (V_1?xNb_xO₂)/BI-mixed crystal Three phase region: solubility limit LG1 (V_1?xNb_xO₂)/BI-mixed crystal/solubility limit LG2 (V_xNb_1?xO₂). The composition of the solubility limits LG1 and LG2 was ascertained by means of ESMA-investigation: LG1: 57.5 ± 5 mol% NbO₂/43.5 ± 5 mol% VO₂ LG2: 22.5 ± 5 mol% NbO₂/78.5 ± 5 mol% VO_2?     

Na(V3−xNbx)Nb6O14 – ein neues Oxoniobat mit [Nb6O12]- und [M2O9]-Clustern

Na(V_3?xNb_x)Nb₆O₁₄ — A Novel Oxoniobate with [Nb₆O₁₂] and [M₂O₉] Clusters Goldcolored single crystals and black powders of Na(V_3?xNb_x)Nb₆O₁₄ have been prepared by heating a pellet containing a mixture of NaNbO₃, NbO₂, NbO, VO₂ and NaF or Na₂B₄O₇ (as mineralizers) at 900°C in a sealed gold capsule. The analytically determined Nb : V ratio is 5 : 1 and means that x is about 1.5. The compound crystallizes in P6₃/m with a = 603.4(1), c = 1807.9(5) pm and Z = 3. The crystal structure can be described in terms of common close packing of sheets of O and Na atoms together with Nb₆ octahedra. Characteristic building groups of the new structure type are [Nb₆O₁₂] clusters, [M₂O₉] clusters and NbO₅ bipyramids. V atoms are distributed only on the positions of the Nb atoms within the trigonal bipyramids or the [M₂O₉] clusters. The [Nb₆O₁₂] clusters show characteristicaly short distances d_Nb-Nb = 279.4 and 281.3 pm, respectively. In the [M₂O₉] units, which are built from two MO₆ octahedra that share a common face, V or Nb atoms form M–M dumbbells with d_M–M = 255.9 pm. The electronic structure is discussed using Extended Hückel calculations.     

Reactivity of T-Nb2O5 or H-Nb2O5 towards V2O5. Synthesis in the solid state and properties of V4Nb18O55

The IR spectrum of V₄Nb₁₈O₅₅ has been compared with the IR spectra of selected niobates of known structures to show structural relations between these compounds. This comparison shows that V₄Nb₁₈O₅₅ has crystal structure related to T-Nb₂O₅, W16Nb18O94 and Ba₂NaNb₅O₁₅. On the other hand, reaction between V₂O₅ and H-Nb₂O₅ yields a solid solution of V₂O₅ in VNb₉O₂₅. It has been proposed two models of synthesized solid solution with formulas V_1+xNb_9-xO₂₅ or V_1+xNb₉O_25+5x/2.Independently of Nb₂O₅ polymorph, used for synthesis, the metastable compound VNbO5 cannot be synthesized in the solid state below 650°C      

Crystallisation of some mixed Na/V and K/V fluorides by solvothermal methods

Single crystals of several phases in the Na–V–(O)F and K–V–(O)F systems have been grown using a mild solvothermal route in water/ethylene glycol. At a temperature of 100 °C the V⁴⁺-containing oxyfluoride phases Na₄V₂O₂F₈ and K₂VOF₄ are prepared, exhibiting dimeric and chain-like vanadium oxyfluoride units, respectively. On raising the reaction temperature to 220 °C reduction to V³⁺ occurs, and three different two-dimensional sheet structures are crystallised, NaVF₄, KVF₄ and K₅V₃F₁₄. Precise crystal structures are reported for the latter two, which represent members of the Dion–Jacobson and Chiolite families, respectively.     

V2.38Nb10.7O32.7: A V2O5-Nb2O5 mixed oxide tunnel structure related to the tetragonal tungsten bronzes

A crystal structural model for the orthorhombic compound V_2.38Nb_10.7O_32.7, which is known as “V₂Nb₉O_27.5”, was developed by means of selected area electron diffraction (SAED), Rietveld refinement and high resolution electron microscopy (HREM). The metastable compound is obtained by thermal decomposition of freeze-dried precursors as chain-like agglomerated nanoparticles or by reaction of V₂O₅ with fresh-precipitated Nb₂O₅ as more compact micro-scaled crystals. With the latter, it was possible to identify its structure for the first time (space group Cmmm). The tetragonal tungsten bronze (TTB)-type structure shows high potential for ionic intercalation, since easily reducible [V⁵⁺₂O²⁻] units are implemented in the tunnels of a rigid niobium oxide framework.     

Semiconductor-metal transitions in NbO2 and Nb1−xVxO2

NbO₂, Nb_0.98V_0.02O₂, and Nb_0.95V_0.05O₂ transform from semiconducting to metallic state at temperatures higher than the DTA phase-transition temperatures. Vibrational mode softening and c-axis NbNb pairing appear to be important factors in the mechanism of these transitions. In the solid solutions, $\text{c}\text{a}$ ratio is maximum at x = 0.5 since the metal-metal interaction along the c axis becomes minimum; conductivity is minimum at this composition since there are no mobile electrons.     

Zur Bildung und Umwandlung von Oxidphasen im quasibinären System V2O5?Nb2O5

Formation and Transformation of Oxide Phases in the Quasibinary System V₂O₅? Nb₂O₅ In the quasibinary system V₂O₅? Nb₂O₅ three phases exist in addition to the boundary phases: VNbO₅, V₂Nb₉O_27.5, and VNb₉O₂₅. Only the latter phase is a thermodynamically stable one. The metastable phases VNbO₅ and V₂Nb₉O_27.5 are formed by thermal decomposition of freeze-dryed products of alkoxide hydrolysis. VNb₉O₂₅ can be formed by thermal treatment of a metastable solid solution with TT-Nb₂O₅ structure or, beside V₂O₅, by thermal decomposition of the other metastable phases. A reaction scheme of formation and decomposition of phases in the quasi- binary system is given and discussed.     

Magnetische Eigenschaften von Ti3−xMxO5-Phasen (M = V3+, Cr3+, Nb4+)

Magnetic Properties of Ti_3?xM_xO₅ Phases (M = V³⁺, Cr³⁺, Nb⁴⁺) The magnetic properties of Ti_3?xV_xO₅, Ti_3?xCr_xO₅, and Ti_3?xNb_xO₅ phases are reported. In the case of V³⁺ and Cr³⁺ the magnetic leaping-temperature decreases, however Nb⁴⁺ shift the phase-transition towards higher temperatures. All samples show a “memory-effect” in magnetic properties, i. e. the results of heating- and cooling-cycles are higher susceptibilities of the α-phase of Ti₃O₅. Endowed Ti₃O₅ phases show for the α- and β-Ti_3?xM_xO₅ til the leap Curie-Weiss characteristic in 1/X vs. temperature measurements. Exception is β-Ti_3?xNb_xO₅, its susceptibility is independend of the temperature up to x ? 0.3.     

Phase composition and magnetic properties of niobium-iron codoped TiO2 nanoparticles synthesized in Ar/O2 radio-frequency thermal plasma

Nanoparticles of Nb⁵⁺-Fe³⁺ codoped TiO₂ with various Nb⁵⁺ concentrations (Nb/(Ti+Fe+Nb)=0-10.0 at%) and Fe³⁺ (Fe/(Ti+Fe+Nb)=0-2.0 at%) were synthesized using Ar/O₂ thermal plasma. Dopant content, chemical valence, phase identification, morphology and magnetic properties were determined using several characterization techniques, including inductively coupled plasma-optical emission spectrometer, X-ray photoelectron spectroscopy, X-ray diffraction, UV-vis diffuse reflectance spectrometer, field-emission scanning electron microscopy, transmission electron microscopy and SQUID commercial instrument. The XRD revealed that all the plasma-synthesized powders were exclusively composed of anatase as major phase and rutile. The rutile weight fraction was increased by the substitution of Fe³⁺ for Ti⁴⁺ whereas it was reduced by the Nb⁵⁺ doping. The plasma-synthesized Nb⁵⁺-Fe³⁺ codoped TiO₂ powders had intrinsic magnetic properties of strongly paramagnetic and feebly ferromagnetic at room temperature. The ferromagnetic properties gradually deteriorated as the Fe³⁺ concentration was decreased, suggesting that the ferromagnetism was predominated by the phase composition as a carrier-mediated exchange.     

Crystal growth and structure of V2VSe2IVO9; comparison with V2Te2O9

High quality single crystals of V₂Se₂O₉ have been obtained as a side compound during a study of the CuCl₂-V₂O₅-SeO₂ system. V₂Se₂O₉ crystallises in the monoclinic system, space group P2₁/n with a = 8.0560(2), b=10.3650(2), c = 8.4500(2) Å, β = 102.893(2) °, Z = 4 and ρ_x =3.90 g/cm³. Full matrix least-squares refinement gave an R = 0.0347 and R_w = 0.0802 using 2232 independent reflections. The structure is built up by very distorted octahedra occupied by V⁵⁺ sharing one edge and one apex to form [V₄O₁₈]¹⁶⁻ entities interconnected by SeO₃E tetrahedra (E designs the 4s² lone pair of the Se (IV) cation). V₂Se₂O₉ is not isostructural with V₂Te₂O₉, in which the V⁵⁺O₆ octahedra share edges to form along [011] infinite chains linked together via Te₂O₅E₂ groups.     

Zur Präparation und Struktur gemischtvalenter Niob-Wolframoxide der Zusammensetzung (Nb,W)17O47

Preparation and Structure of Niobium Tungsten Oxides (Nb,W)₁₇O₄₇ with Mixed Valency The formal substitution of 2Nb⁵⁺ by Nb⁴⁺ or W⁴⁺, respectively, and W⁶⁺ leads to tungsten niobium oxides (Nb,W)₁₇O₄₇ with mixed valency. The phases Nb_8-nW_9+nO₄₇ with n = 1 to 5 could be obtained by heating (1 250°) mixtures of NbO₂ or WO₂, respectively, with Nb₂O₅ and WO₃. The products crystallize with the structure of Nb₈W₉O₄₇. This is proved by X-ray powder diffraction and transmission electron microscopy. A further decrease of the Nb-content results in two-phase products.     

Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XX. Metastabile Oxydation von Mischkristallreihen — ein Zugang zu W-reichen Blockstrukturen im System Nb2O5/WO3

Contributions to the Investigation of Inorganic Non-stoichiometric Compounds. XX. Metastable Oxidation of a Series of Solid Solutions — an Access to W-rich Block Structures in the System Nb₂O₅/WO₃ The characteristical region of existence of block structures with building elements that are limited in size in two directions ends in the system Nb₂O₅/WO₃, as was shown by previous investigations, under conditions of equilibrium at a maximum value of 2.654 O/ΣM. For the occurring phases with the ratios Nb₂O₅: WO₃ = 6:1, = 7:3, = 8:5 and = 9:8 as well we now were successful in substituting W for Nb. The original block structure and the corresponding ratio O/ΣM were preserved. The “9:8”-phase W_4/4[Nb₁₈W₇O₆₉], for example, forms solid solutions W_4/4[Nb₁₁W₁₄O₆₉] leaving the size of the building elements ([5 times; 5] blocks) unchanged. Hereby the ratio W/Nb is drastically enhanced from 0.444 to 1.364. By metastable oxidation of these solid solutions at temperatures of about 500°C, for instance in air, one comes back to the system Nb₂O₅/WO₃. In this way the region of existence of block structures could be expanded far beyond the limit at 2.654 O/ΣM to higher W/Nb values.     

Electrochemistry of vanadium-doped tetragonal and monoclinic ZrO2 attached to graphite/polyester composite electrodes

The electrochemistry of monoclinic and tetragonal vanadium-doped zirconias (VZrO₂), prepared from gel precursors with vanadium loadings ranging from 0.5 to 15 mol%, has been studied using abrasive-conditioned graphite/polyester composite electrodes immersed in aqueous HCl and HClO₄ solutions. Isolated vanadium centers form a solid solution in the zirconia lattice with a solubility limit close to 5 mol%. Above 5 mol%, finely dispersed V₂O₅ is formed. Vanadium centers located at the boundary sites of the zirconia lattice display successive one-electron transfer processes near to +0.25 and +0.10 V vs. SCE, whereas finely dispersed V₂O₅ yields three successive reduction processes at +0.46, +0.30, and +0.16 V vs. SCE. Electrochemical data indicate the presence of both V⁵⁺ and V⁴⁺ centers in the lattice of monoclinic and tetragonal zirconias, the V⁵⁺/V⁴⁺ ratio decreasing as the vanadium loading increases. Electronic Publication     

Spin dimer analysis of the magnetic structures of A2V3O9 (A = Ba,Sr) and η-Na9V14O35: Importance of the V4+–O⋯O–V4+ super-superexchange interactions mediated by the O–V5+–O bridges

Spin dimer analysis was carried out for the magnetic oxides of V⁴⁺ (d¹) ions, A₂V₃O₉ (A = Ba, Sr) and η-Na₉V₁₄O₃₅, to account for their magnetic structures. After identifying the V(O_eq)₄ square planes containing the magnetic orbitals of the V⁴⁺ (d¹) ions, we analyzed the relative strengths of the various V–O_eq–V and V–O_eq⋯O_eq–V spin exchange interactions. The V–O_eq⋯O_eq–V spin exchange interactions mediated by O_eq–V⁵⁺–O_eq bridges are found to be crucial in determining the magnetic structures of A₂V₃O₉ (A = Ba, Sr) and η-Na₉V₁₄O₃₅. Our analysis suggests that η-Na₉V₁₄O₃₅ should have two different spin gaps.     

Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XIII. Oxydationsprodukte von monoklinem Nb12O29 Elektronenoptische Untersuchung

Contributions to the Investigation of Inorganic Non-stoichiometric Compounds. XIII. Oxidation Products of Monoclinic Nb₁₂O₂₉, Electron Optical Investigation The electron optical investigation shows that the starting material Nb₁₂O₂₉(mon.) is well ordered and that the oxidation products Nb₂O₅(Ox1BI) and Nb₂O₅(Ox2BI) have different structures. Nb₂O₅(Ox1BI) has a similar structure as Nb₁₂O₂₉(mon.), however differs from the latter by characteristic point defects, which in the electron microscope easily disappear by reduction. Nb₂O₅(Ox2BI) has not a well ordered structure; characteristic are rows of [2×n]-blocks which on the average are separated by five [3×n]-blocks. The average block length is n = 4 octahedra. The observed composition O/Nb = 2.500 can be explained by a structure model with unoccupied tetrahedral sites of Nb.     

Exafs and electron-microscopy studies on V/SiO2 catalysts

Extended x-ray-absorption fine structure and scanning electron microscopy have been applied to the structure of the vanadium oxide layers on impregnated and grafted vanadium aerosils. When aerosil is impregnated with NH₄VO₃ solution, V₂O₅ crystals are formed; when vanadium is grafted by reacting the oxychloride with carrier OH groups, there are no visible crystals. On the other hand, the EXAFS spectra for the grafted specimens show all the oscillations found for crystalline V₂O₅. It is concluded that the vanadium oxide layers in these grafted materials have a long-range order similar to that in V₂O₅ and contain microcrystals having sizes up to 5 nm.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 5, pp. 652–655, September–October, 1987.     

The Critical Effect of Niobium Doping on the Formation of Mesostructured TiO2: Single‐Crystalline Ordered Mesoporous Nb‐TiO2 and Plate‐like Nb‐TiO2 with Ordered Mesoscale Dimples

Highly ordered mesoporous niobium‐doped TiO₂ with a single‐crystalline framework was prepared by using silica colloidal crystals with ca. 30 nm in diameter as templates. The preparation of colloidal crystals composed of uniform silica nanoparticles is a key to obtain highly ordered mesoporous Nb‐doped TiO₂. The XPS measurements of Nb‐doped TiO₂ showed the presence of Nb⁵⁺ and correspondingly Ti³⁺. With the increase in the amount of doped Nb, the crystalline phase of the product was converted from rutile into anatase, and the lattice spacings of both rutile and anatase phases increased. Surprisingly, the increase in the amount of Nb led to the formation of plate‐like TiO₂ with dimpled surfaces on one side, which was directly replicated from the surfaces of the colloidal silica crystals.     

Nb-Ta, Nb-Mo and Nb-V oxides prepared from hybrid organic-inorganic precursors

New hybrid organic-inorganic materials based on group 5 elements and a well-defined polymeric matrix have been prepared and used as precursors for Nb-Ta and Nb-Mo mixed oxides. In this non-conventional but easily accessible route to multimetallic oxides, a copolymer of N,N-diallyl-N-hexylamine and maleic acid was synthesised and used as matrix to stabilise inorganic species generated in solution from (NH₄)₆Mo₇O₂₄·4H₂O, NH₄VO₃, (gu)₃[Nb(O₂)₄] and (gu)₃[Ta(O₂)₄]. Solid-state studies indicate that the homogeneity of the blends can be kept up to about 0.5 mol Nb^V and Ta^V and 0.25 mol V^V per mol of repeat units of the copolymer. The calcination conditions of these homogeneous hybrid precursors were optimised to produce Nb-Mo, Nb-Ta and Nb-V oxides. While the thermal treatment of the Nb-V hybrid blends led only to a mixture of different phases, the characterisation of the final phases by X-ray diffraction (XRD) proved the formation of pure Nb₂Mo₃O₁₄ and showed that Nb-Ta oxides could be synthesised as single phases corresponding to a continuous series of solid solutions.