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.     

High resolution electron microscopy studies of Ba4Nb14O23

The new compound, Ba₄Nb₁₄O₂₃, has been prepared by heating mixtures of Ba₅Nb₄O₁₅, Nb₂O₅ and Nb at 1 450°C under Ar. Ba₄Nb₁₄O₂₃ has been studied by means of high resolution electron microscopy and X-ray powder diffraction techniques. It has a C-centered orthorhombic unit cell with a=20.782(4), b=12.448(3), c=4.148(1) Å and Z=2. The structure of Ba₄Nb₁₄O₂₃ can be considered as being an intergrowth between BaNbO₃ and NbO. Characteristic building units are triple chains of corner sharing Nb₆ octahedra which are connected via columns of the perovskite type structure to a three dimensional network.     

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?     

Structure and magnetic susceptibility of MnNb3O6

The mixed valence state niobium compound MnNb₃O₆ was found while studying phase formations in the system MnO-Nb₂O₅-NbO. It is isostructural with A_xNb₃O₆, x ≤ 1 and A = Na, Ca. Single crystals were obtained by heating MnC₂O₄ · 2H₂O and Nb₂O₅ in a flow of H₂ at 1300 °C. Monophasic samples were also prepared by heating stoichiometric mixtures of MnO, Nb₂O₅ and Nb in niobium ampoules under Ar(g) at 1100 °C. The crystal structure of MnNb₃O₆ (Immm, Z = 4, a = 7.1057(5), b = 10.1420(6), c = 6.5341(5) Å) was refined, using singlecrystal MoKα X-ray diffraction data, to a weighted R value of 0.018 for 329 unique reflections. The structure contains undulating layers of NbO₆ octahedra of the type _α²[NbO_6/2]in the ac-plane, with the octahedra sharing edges along [001]and corners along [100]. Between the layers there are columns along [001]of edge-sharing square MnO₈ prisms alternating with columns containing Nb₂O₈ clusters with an Nb-Nb distance of 2.6163(5) Å. The magnetic susceptibility shows a Curie-Weiss behaviour: χ_M = C/(T+θ) with θ ≈ −22 K and μ_eff = 6.0(1) μ_B for T ≥ ca. 35 K, with a small deviation from this dependence at lower temperatures, indicating Mn²⁺ ions with localised magnetic moments and antiferromagnetic interactions.     

Zur Kristallstruktur von Ni4Nb2O9

Crystal Structure of Ni₄Nb₂O₉ Single crystals of Ni₄Nb₂O₉ were prepared and examined by X-ray work (space group C–Fd2d; a = 10.101(13), b = 17.5126(51), c = 28.6364(87) Å; Z = 32). Ni₄Nb₂O₉ has 480 atoms per unit cell, thus forming a complicated threedimensional NiO₆-octahedra framework. NbO₆-double octahedra are deposited in this framework. The relations to other A₄B₂O₉ compounds are discussed.     

Structural evolution in a hydrothermal reaction between Nb2O5 and NaOH solution: from Nb2O5 grains to microporous Na2Nb2O6.2/3H2O fibers and NaNbO3 cubes

Niobium pentoxide reacts actively with concentrate NaOH solution under hydrothermal conditions at as low as 120 degrees C. The reaction ruptures the corner-sharing of NbO(7) decahedra and NbO(6) octahedra in the reactant Nb(2)O(5), yielding various niobates, and the structure and composition of the niobates depend on the reaction temperature and time. The morphological evolution of the solid products in the reaction at 180 degrees C is monitored via SEM: the fine Nb(2)O(5) powder aggregates first to irregular bars, and then niobate fibers with an aspect ratio of hundreds form. The fibers are microporous molecular sieve with a monoclinic lattice, Na(2)Nb(2)O(6).(2)/(3)H(2)O. The fibers are a metastable intermediate of this reaction, and they completely convert to the final product NaNbO(3) cubes in the prolonged reaction of 1 h. This study demonstrates that by carefully optimizing the reaction condition, we can selectively fabricate niobate structures of high purity, including the delicate microporous fibers, through a direct reaction between concentrated NaOH solution and Nb(2)O(5). This synthesis route is simple and suitable for the large-scale production of the fibers. The reaction first yields poorly crystallized niobates consisting of edge-sharing NbO(6) octahedra, and then the microporous fibers crystallize and grow by assembling NbO(6) octahedra or clusters of NbO(6) octahedra and NaO(6) units. Thus, the selection of the fibril or cubic product is achieved by control of reaction kinetics. Finally, niobates with different structures exhibit remarkable differences in light absorption and photoluminescence properties. Therefore, this study is of importance for developing new functional materials by the wet-chemistry process.     

Das erste Diniobat mit ‚isolierten’︁ Anionen: KLi4[NbO5]=K2Li8[Nb2O10]

The First Diniobate with ‘Isolated’ Anions: KLi₄[NbO₅]=K₂Li₈[Nb₂O₁₀] [1] . By heating of well ground mixtures of the binary oxides [K₂O, Li₂O, Nb₂O₅, K:Li:Nb=1.1:4.4:1, Pt-tube, 1100°C, 3d] colourless, triclinic single crystals of KLi₄NbO₅ have been prepared for the first time: space group P1 (Nr. 2) with a=816.9(2) pm, b=592.2(2) pm, c=589.7(2) pm, α=121.00(2)º, β=91.78(2)°, γ=99.23(2)°, Z=2. The crystal structure was solved by four-cycle diffractometer data [Mo-Kα , 1386 from 1386 I_o(hkl), R=3.4%, R_w=2.6%], parameters see text. Characteristic for this structure are “isolated” groups of [Nb₂O₁₀] and the tetrahedral coordination of Li(1), Li(2), and Li(3). Li(4) has a tetragonal-pyramidal coordination. The structural relations are deduced by Schlegel Diagrams. The Madelung Part of Lattice Energy, MAPLE, the Effective Coordination Numbers, ECoN and the charge distribution have been calculated and discussed.     

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:      

NaCa4Nb5O17: a layered perovskite AnBnO3n+2 compound

Sodium tetracalcium pentaniobium heptadecaoxide, NaCa₄­Nb₅O₁₇, corresponds to the n = 5 term of the homologous A_nB_nO_3n+2 family of structures composed of ABX₃ perovskite layers. The structure consists of perovskite‐type blocks of n = 5 layers of NbO₆ octahedra, separated by an interblock region. Successive blocks along the b axis are displaced by c with respect to each other. The deformation of the NbO₆ octahedra is a minimum at the middle of each block, and increases along the direction of the b axis to a maximum at each end of the block. Ca and Na share the same intrablock sites, coordinated by 12 O atoms, whereas only Ca atoms are found in the interblock cavities, at sites with different coordination geometries.     

Photoelectron spectroscopy and diffraction of surface nanoscale NbO/Nb(110) structures

X-ray photoelectron spectroscopy and diffraction (XPS and XPD) are applied to analyze oxygen-induced surface structures on the Nb(110) face formed due to oxygen segregation from the crystal bulk on thermal annealing to 2000 K in vacuum and/or oxygen adsorption in situ at temperatures above 1100 K. The Nb3d, O1s electronic states and valence band spectra of the NbO _x /Nb(110) surface are studied by XPS, and the results are compared with data for NbO, NbO₂, and Nb₂O₅ oxides. It is shown that niobium atoms entering the composition of surface oxide structures on Nb(110), from the standpoint of the nearest environment and chemical bond, are similar to metal states in NbO. The NbO _x layer thickness is estimated to be 0.5 nm. Two chemically inequivalent oxygen states are distinguished on Nb(110), which are, presumably, atomic chemisorbed oxygen on the parts of the clean surface of the Nb monolayer with hexagonal packing and oxygen in the composition of NbO _x -like linear clusters on Nb(110). A model of the NbO _x /Nb(110) surface takes into account a distortion of the structure of NbO _x clusters: a periodic vertical shift of metal atoms in Nb-chains and changes in Nb-O bond angles. Original Russian Text Copyright ? 2009 by M. V. Kuznetsov, A. S. Razinkin, and E. V. Shalaeva __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 50, No. 3, pp. 536–543, May–June, 2009.     

The investigation of NbO2 and Nb2O5 electronic structure by XPS,UPS and first principles methods

In this paper, the electronic structures of NbO₂ and Nb₂O₅ are theoretically and experimentally analyzed. The oxides in the samples are mainly consisted of NbO₂ and NbO, whereas the outmost layer of the samples is NbO₂. After exposure to air, the outermost layer on all niobium samples is Nb₂O₅. The photoelectrons from the first 2–4 Å contribute to the spectra, so the valence band structure of NbO₂ and Nb₂O₅ can be confirmed from ultraviolet photoelectron spectroscopy (UPS). By comparing the UPS with density of state results, the electronic structure of NbO₂ and Nb₂O₅ can be distinguished from each other, and then the electronic structure was deconvoluted into several electronic states. The agreement between experimental result and theory is, in the best case, satisfactory. Copyright © 2013 John Wiley & Sons, Ltd.     

Zur kenntnis ‚Kationen-reicher’︁ Tantalate und Niobate Über Na5TaO5 und Na5NbO5

On Tantalates and Niobates ‘rich in Cations’. On Na₅TaO₅ and Na₅NbO₅ Colourless, transparent single crystals of Na₅TaO₅ [annealed mixtures of Na₂O, Li₂O, and Ta₂O₅, Na : Li : Ta = 6.6 : 1.1 : 1, Ni-cylinder, 1000°C, 75 d] as well as Na₅NbO₅ [annealed mixtures of Na₂O, Li₂O, and Nb₂O₅, Na : Li : Nb = 6.6 : 1.1 : 1, Ni-cylinder, 1000°C, 75 d] have been prepared. Single crystal data show that both isotypic oxides represent a deformed variant of the NaCl-type of structure [Na₅TaO₅: 1154 from 1250 I₀ (hkl), four-cycle diffractometer Philips PW 1100, ω2-θ scan, Ag? Kα , R = 4.88%, space group c2/c with a = 629.3(1) pm, b = 1025.4(2) pm, c = 1004.6(2) pm, b? 106.80(2)°, z = 4 and Na₅NbO₅: 998 from 1247 I₀(hkl), four-cycle diffractometer Philips PW 1100, ω-2θ scan, Ag? Kα , R = 8.58% and R_w = 7.67%, space group C2/2 with a = 629.1(1) pm, b = 1024.4(2) pm, c = 1004.2(2) pm, b? = 106.80(2)°, Z = 4]. The Madelung Part of Lattice Energy, MAPLE, and Effective Coordination Numbers, ECoN, the latter derived from Mean Effective Fictive Ionic Radii, MEFIR, as well as Charge Distribution, CHARDI, are calculated.     

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.     

Neubestimmung der Kristallstruktur von NbOCl3

NbOCl₃ was obtained from a reaction of NbCl₅ and Nb₂O₅ at 260?C. Contrary to the literature data, NbOCl₃ crystallizes in the non‐centrosymmetric space group P&4macr;2₁m as determined by single‐crystal and powder X‐ray diffraction data (crystal: a = b = 1089.59(6) pm, c = 394.79(2) pm, Z = 4, R₁ = 0.0229, wR₂ = 0.0459, powder: a = b = 1086.36(6) pm, c = 393.65(2) pm). The niobium atoms are surrounded by distorted octahedra built of four chlorine atoms and two oxygen atoms in trans positions. Two such octahedra are edge‐bridged through shared chlorine atoms, forming dimers. These units are linked to each other by apical oxygen atoms forming one‐dimensional Nb₂Cl₆O₂ chains parallel [001]. Contrary to the literature data two different Nb‐O distances are obtained.     

Kristallchemische Untersuchungen an den Phasen: (I) Zn(V0,12Nb0,88)2O6, (II) Zn(V0,28Ta0,72)2O6 und (III) Zn(Nb0,58Ta0,42)2O6

Crystal Chemistry Examinations of the Phases: (I) Zn(V_0.12Nb_0.88)₂O₆, (II) Zn(V_0.28Ta_0.72)₂O₆, and (III) Zn(Nb_0.58Ta_0.42)₂O₆ Single crystals of the quaternary phases ( I ), ( II ), and ( III ) were prepared by high temperature technique and investigated by X-ray single crystal methods. Phase ( I ) crystallizes in the columbite Structure. Phase ( II ) deviates from the ternary compounds (ZnV₂O₆, ZnTa₂O₆) and appears in trirutile type. In phase ( III ) the octahedra arrangement of the tri-α-PbO₂ Structure is realized.     

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.     

Darstellung,Kristallstruktur und elektronenmikroskopische Untersuchung von UNb6O16 – einem neuen niobreichen Oxid im System U/Nb/O

Preparation, Crystal Structure and Electron Microscopic Investigation of UNb₆O₁₆ – a New Niobium-rich Phase in the System U/Nb/O Powdery UNb₆O₁₆ was produced by heating (1 000°C or 1 100°C; evacuated silica tube) mixed powders of UO₂, NbO₂ and Nb₂O₅ (1:2:2). Single-crystals of UNb₆O₁₆ were obtained by chemical transport in a small temperature gradient (1 000°C → 990°C; transport agent NH₄Cl). The lattice constants are a = 22 339(4) Å; b = 3.7750(6) Å; c = 7.249(3) Å; β = 97.61(3)° and Z = 2. The structure determination (space group C2) let to R = 0.026 (R_w = 0.026). Eight oxygen atoms surround U⁴⁺ like a trans-bis-capped octahedron, Nb⁴⁺ and Nb⁵⁺ are coordinated distorted octahedraly. The structure was checked and the occupation of the positions O8 and O9 was clarified with the program MAPLE4 [3]. A through focus series of high resolution transmission electron microscopic images was obtained which is in acceptable agreement with images calculated on the basis of the multi-slice method.     

Crystal Structure of BaNb6.3(1)Ti3.6(1)O16 containing Nb6O12, fused Ti3O13 Clusters and Ti3O10 Units

A new phase, BaNb_6.3(1)Ti_3.6(1)O₁₆, has been synthesised. Electron diffraction studies indicate an hexagonal substructure with unit cell parameters a ≈ 8.9 Å and c ≈ 9.5 Å. In some of the ED patterns superstructure reflections are present, indicating a supercell with a = √3 · a_sub and c = c_sub. However, X‐ray single‐crystal diffraction studies of a crystallite yielding reflections corresponding to the supercell revealed it to be monoclinic, with the unit cell parameters a = 26.811(2) Å, b = 15.4798(2) Å, c = 9.414(2) Å, β = γ = 90° and α = 90.0(3)°. The average crystal structure was refined, using the subcell with a = 8.937(2) Å, b = 15.479(2) Å, c = 9.414(2) Å, β = γ = 90° and α = 90.0(3)°, space group Cm11, and Z = 4, to R_I = 3.24% and R_wI = 3.44%. The structure can be described as an hexagonal close packing layers of Nb₆ octahedra, Ba, and O atoms (A1, A2) and layers of O atoms (B1, B2), appearing in the packing sequence: A1B1A2B2. The Nb₆ octahedra are found in isolated Nb₆O₁₂O₆ clusters, and the Ti atoms in Ti₃O₁₃ and Ti₃O₁₀ units in octahedral and tetrahedral voids formed by O atoms, respectively. The Ti positions were found to be only partly occupied. Microanalysis indicates that some Nb atoms are located in the Ti₃ triangles. A model is presented that interprets these not fully occupied Ti₃ triangles as a result of a superimposing of three different structures. Two of these consist of two fused Ti₃O₁₃ units, forming an Ti₆O₁₉ unit, and a Ti₃O₁₀ unit, while the third consists of alternating Ti₃O₁₃ units.     

Ein neues Praseodymniobat Pr2Nb11O30

A New Praseodymiumniobate Pr₂Nb₁₁O₃₀ By chemical vapor transport (T₂ → T₁, T₂ = 950 °C, T₁ = 900 °C, 3 d) of a mixture of PrOCl and Nb₂O₅ (1 : 1) using 5 mg NH₄Cl as transport agent we obtained the new compound Pr₂Nb₁₁O₃₀. The crystal structure determination [a = 6.2325(5) Å, c = 32.3677(36) Å, Z = 2, 1631 independent I₀, 69 parameters, R1 = 2.07%] shows CN = 8 (twofold capped octahedrally) for Pr, CN = 7 (pentagonal bipyramidally) for Nb(1,2) and CN = 6 (octahedrally) for Nb(3). The structure is closely related to that of Cu₅Ta₁₁O₃₀.     

Thiocyanate Compounds of [Nb6Cl12]2+: The structure of A4[Nb6Cl12(NCS)6](H2O)4 (A = K,Rb, NH4)

New compounds of the general formula A₄[Nb₆Cl₁₂(NCS)₆](H₂O)₄ (A = K, Rb, NH₄) were synthesized from Nb₆Cl₁₄ and ASCN in aqueous solutions. X-ray structure refinements were performed on single-crystal data of the three compounds. They are isotypic and crystallize with the space group P1 (Z = 1) and the lattice parameters: a = 877.9(3) pm, b = 1176.6(3) pm, c = 1187.0(3) pm, α = 114.29(1)°, β = 98.96(2)°, γ = 100.91(2)° for K₄[Nb₆Cl₁₂(NCS)₆](H₂O)₄ ( 1 ); a = 887.6(3) pm, b = 1184.0(4) pm, c = 1195.4(4) pm, α = 114.95(2)°, β = 98.84(2)°, γ = 101.31(2)° for Rb₄[Nb₆Cl₁₂(NCS)₆](H₂O)₄ ( 2 ) and a = 886.0(4) pm, b = 1181.1(6) pm, c = 1183.9(6) pm, α = 114.49(2)°, β = 99.48(3)°, γ = 101.53(1)° for (NH₄)₄[Nb₆Cl₁₂(NCS)₆](H₂O)₄ ( 3 ). Each centrosymmetric [Nb₆Cl₁₂(NCS)₆]^4? ion of the isotypic compounds contains six terminal thiocyanate groups being bound to the corners of the octahedral niobium cluster through the nitrogen atoms (d_{Nb? N} = 221.5(6)–224.3(6) pm, bond angles Nb? N? C 168.6(5)–176.4(6)°). The [Nb₆Cl₁₂(NCS)₆]^4? ions are linked via A? S and A? Cl interactions with the A cations. Half of the cations occur to be disordered along two crystallographic sites.