Structural characterization and Curie temperature determination of a sodium strontium niobate ferroelectric nanostructured powder

The Curie temperature and its correlation with the magnitude of the displacement of the niobium atom from the center of [NbO₆] octahedra in NaSr₂Nb₅O₁₅ nanostructured powder were investigated. A single powder was prepared by high-energy ball milling. A powder with an average crystallite size of 37 nm was prepared by calcining the precursor at 1423 K. The refinement of the structural parameters was carried out by the Rietveld method. NaSr₂Nb₅O₁₅ exhibits tetragonal symmetry with the tungsten bronze structure (a=b=12.3495 (6) Å, c=3.8911 (2) Å, V=593.432 (5) Å³, and Z=2). The site occupancy of the Na⁺ and Sr²⁺ cations and the interatomic distances between the niobium and oxygen atoms were derived. The [NbO₆] octahedron undergoes both rotation and tilting depending on the crystallographic site. The Curie temperature of the powder was derived using both the impedance and infrared spectroscopy methods.     

Formation of rutile-type Nb0.94O2 by shock reduction of Nb2O5

Shock-recovery experiments on Nb₂O₅ powder specimens are made in pressure ranges up to 50 GPa using the gun method. “NbO₂” with the rutile structure is formed above about 40 GPa when an open recovery fixture is used. The tetragonal unit cell dimensions are measured to be a = 4.784(2) Å, c = 3.029(2) Å, and V = 69.34(6) ų. The metal-to-oxygen ratio is determined to be Nb_0.94O₂ by means of thermogravimetry. A comparative study is made on the shock reduction behavior of Nb₂O₅ and Ta₂O₅.     

Synthesis,Crystal Structure and Magnetism of the New Oxysulfide Ce3NbO4S3

The orange cerium‐niobium‐oxysulfide Ce₃NbO₄S₃ was synthesized by the solid state reaction of CeO₂, Ce‐metal, Nb₂O₅ and sulfur at 1100 °C. The crystal structure has orthorhombic symmetry (space group Pbam, a = 7.055(1), b = 14.571(3), c = 7.627(2) Å, Z = 4) and contains isolated [Nb₂S₄O₆]¹⁰⁻ ions consisting of two strongly distorted, edge sharing NbO₃SS_2/2 octahedra. Niobium is connected to three oxygen and three sulfur atoms. The cerium atoms are eightfold coordinated by oxygen and sulfur atoms. Certain oxygen and sulfur atoms are not connected to niobium, but exclusively surrounded by cerium. By connecting these cation polyhedra, one recognizes layers of polycations perpendicular to the c‐axis. The magnetic susceptibility shows Curie‐Weiss behavior with an effective magnetic moment μ_eff = 2.63(1) μ_B/Ce in agreement with Ce³⁺. A Weiss‐constant θ_p = –12(1) K indicates weak antiferromagnetic coupling. No magnetic ordering was detected above 2 K.     

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.     

Na3Al2Nb34O64 und Na (Si,Nb) Nb10O19: Clusterverbindungen mit isolierten Nb6-Oktaedern

Na₃Al₂Nb₃₄O₆₄ and Na (Si, Nb) Nb₁₀O₁₉. Cluster Compounds with Isolated Nb₆-Octahedra Hexagonal ormolu coloured plates of the new compounds Na₃Al₂Nb₃₄O₆₄ ( I ) and Na(Si, Nb)Nb₁₀O₁₉ ( II ) were prepared by heating pellets of NaF, Al₂O₃, NbO₂ and NbO (3:1:8:2) and NaF, NbO₂ and NbO (1:4:2), respectively, at approx. 850°C. I was contained in a sealed gold capsule, II in a silica tube. The Si incorporated in II originates from the container material. Both compounds crystallize in R 3 , I with a = 784.4(1), c = 7065(1) pm, Z = 3 and II with a = 784.1(1), c = 4221.8(5) pm, Z = 6. I and II represent new structure types. They contain the same characteristic structural units, namely discrete Nb₆O₁₂ clusters (d_Nb–Nb = 283 ± 4 pm) and Nb₂O₁₀ units with Nb–Nb dumbells (d_Nb–Nb ≈? 269 pm) in edgesharing coordination octahedra. In addition NbO₆ octahedra containing Nb in the oxidation state + 5 and NaO₁₂ cube-octahedra occur in both compounds besides AlO₄ and SiO₄ tetrahedra in I and II , respectively. The structures can be described in terms of a common closepacking of O and Na atoms together with Nb₆ octahedra.     

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.     

Ein neues Gemischtvalentes Oxoniobat: Sr7Nb24+Nb45+O21 \documentclass{article}\pagestyle{empty}\begin{document}$ \widehat = $\end{document} Sr1,167 NbO3,5

A New Mixed Valence Strontium Niobium Oxide Sr₇Nb₂⁴⁺Nb₄⁵⁺O₂₁ \documentclass{article}\pagestyle{empty}\begin{document}$ \widehat = $\end{document} Sr_1.167NbO_3.5 The unknown compound Sr₇Nb₆O₂₁ kristallisiert nach Einkristall-Röntgenbeugungsdaten rhomboedrisch (Raumgruppe C? R3 ; a = 16,450(5) Å, α = 19,85(1)° trigonale Aufstellung: a = 5,670(1), c = 48,364(13) Å). The compound is built up by perovskite blocks with a width of 6 octahedra. The crystal chemistry especially of the interspace between those blocks is discussed in respect to related compounds.     

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.     

A niobium phosphate with a tunnel structure: Ca0.5+xCs2Nb6P3O24

A new niobium phosphate, Ca_0.5+xCs₂Nb₆P₃O₂₄ has been isolated. It crystallizes in the R32 space group, with the following parameters of the hexagonal cell: a_H = 13.379 Å, c_H = 10.371 Å. The determination of the structure by a single crystal X-ray diffraction study shows that its host lattice [Nb₆P₃O₂₄]_∞ can be described as the assemblage of mixed chains [Nb₂PO₁₃]_∞ running along c_H in which one PO₄ tetrahedron alternates with two NbO₆ octahedra. This framework delimits huge tunnels where the cesium cations are located and cages formed by [Nb₆P₃O₃₆] units occupied by calcium. The most striking feature of this framework deals with its similarity with the hexagonal tungsten bronze of Magnéli (HTB). The latter is discussed here by considering the stacking along c of [Nb₂PO₈]_∞ layers whose geometry is closely related to that of the HTBs. The possibility of nonstoichiometry leading to a mixed valency of niobium is considered.     

Structure and luminescence of the α-LnNb3O9-type rare earth niobates

α-LnNb₃O₉ (Ln = La, Pr, Nd) compounds have been prepared hydrothermally from acidic solutions. In comparison to the previously reported orthorhombic β modifications, α-LnNb₃O₉ compounds are monoclinic. The structure of α-PrNb₃O₉ was determined with a = 5.3784(6), b = 7.602(2), c = 16.344(2) Å, and β = 92.21(1)°, space group $\text{P2}{}_1\text{c}$. It is built of double and single chains of corner-shared NbO₆ octahedra extended along the b axis. Praseodymium atoms reside in tunnels along the b axis and are in eight-coordination with oxygen. All α-LnNb₃O₉ compounds can be irreversibly converted to the β modification by heating in air to 1200°C. The X-ray excited luminescence of Sm-, Eu-, Tb-, and Dy-doped α-LaNb₃O₉ is also reported.     

Crystal chemistry of lithium in octahedrally coordinated structures. I. Synthesis of Ba8(Me6Li2)O24 (Me = Nb or Ta) and Ba10(W6Li4)O30. II. The tetragonal bronze phase in the system BaONb2O5Li2O

The preparation, single crystal growth, and crystallographic properties of a close-packed, eight-layer, hexagonal (a = 5.803 Å, c = 19.076 Å) modification having the stoichiometry Ba₈Nb₆Li₂O₂₄ and of a close-packed, ten-layer, hexagonal (a = 5.760 Å, c = 23.742 Å) phase with Ba₁₀W₆Li₄O₃₀ stoichiometry are discussed. The isostructural Ba₈Ta₆Li₄O₂₄ form of the eight-layer phase was also prepared (a = 5.802 Å, c = 19.085 Å). Proposed crystal structures involve the pairing of lithium and metal (Nb, Ta, or W) octahedra to yield face-sharing units. The relationship of this phenomenon to other known close-packed phases containing Li is demonstrated. An investigation of the Ba₈Nb₆Li₂O₂₄Ba₁₀W₆Li₄O₃₀ system is reported.A tetragonal bronze phase homogeneity region was delimited at 1200°C in the BaONb₂O₅Li₂O system. A new orthorhombic phase (a = 10.197 Å, b = 14.882 Å, c = 7.942 Å) was prepared with the stoichiometry Ba₄Li₂Nb₁₀O₃₀.     

The structure of a barium niobium silicon oxide with the probable composition Ba6+xNb14Si4O47 (x ⋍ 0.23)

Reaction of BaO, Nb₂O₅, and Nb in mole ratios of 2.4:1.6:1 in an evacuated silica capsule at 1250°C produces a mixture of at least two products, one of which has the probable composition $\text{Ba}{}_{\mathrm{6+x}}\text{Nb}{}_{14}\text{Si}{}_4\text{O}{}_{47}\text{(x ? 0.23)}$. This compound has an hexagonal unit cell of dimensions a = 9,034 ± 0.004 Å, c = 27.81 ± 0.02 Å, probable space group $\text{P6}{}_3\text{mcm}\text{, Z = 2}$. Its structure has been determined from 942 independent reflections collected by a counter technique and refined by least squares methods to a conventional R value of 0.062. The basic structure consists of strings of four NbO₆ octahedra sharing opposite corners, each string joined to the next by edge sharing of the end octahedra, so that the c axis corresponds to the length of a strand of seven corner-linked octahedra. Chains of three such strands are formed by corner sharing between the strands. The chains in turn are joined by NbO₆ octahedra and Si₂O₇ groups in which the SiOSi linkage is linear. Barium atoms are in sites between the chains coordinated by 13 oxygen atoms. A second site, 15 coordinated, probably has a small amount of barium as well; the fractional occupancy for barium in this site is 0.076.     

Pentaguanidinium monohydrogendiphosphopentamolybdate(VI) 2.5‐hydrate, (CH6N3)5[HP2Mo5O23]·2.5H2O: hydrogen bonding and π‐stacking in guanidinium cations

The asymmetric unit of the title compound consists of two crystallographically independent, but structurally identical, [HP₂Mo₅O₂₃]⁵⁻ anions, ten guanidinium cations and five water molecules. Each singly protonated diphosphopentamolybdate(VI) anion retains the typical geometry of a ring of five edge‐sharing MoO₆ octahedra [Mo...Mo = 3.3265 (8)–3.4029 (10) Å], except for one corner‐sharing link [Mo...Mo = 3.6642 (7) and 3.6826 (8) Å]. Two capping PO₄ tetrahedra share corners with the five octahedra. Despite being surrounded by an extensive network of hydrogen bonds, predominantly from the guanidinium cations, short P—O—H...O=P contacts [O...O = 2.519 (7) and 2.457 (7) Å] associate the anions into infinite columns generated by the c‐glide. In addition to their heavy involvement in hydrogen bonding, with all N—H donors being utilized, the guanidinium cations assemble into extensive π‐stacked columns with an average interplanar spacing of 3.53 Å.     

Dopant substitution and oxygen migration in the complex perovskite oxide Ba3CaNb2O9: A computational study

Atomistic simulation methods have been used to study the defect chemistry of the complex perovskite oxide Ba₃CaNb₂O₉. Calculations were carried out for the hexagonal (P-3m1) phase and the cubic (Fm-3m) phase. The hexagonal structure is predicted to be energetically more stable at room temperature. In both structures the most favourable dopant for Nb⁵⁺ is found to be Ca²⁺ rather than Mg²⁺, in contrast to the generally accepted rule that size similarities govern such processes. The diffusion of oxygen vacancies in the hexagonal and cubic phases occurs within different networks of corner-sharing NbO₆ and CaO₆ octahedra. Irrespective of the arrangement of octahedra, however, migration of oxygen vacancies around NbO₆ octahedra takes place with lower activation energies than around the CaO₆ octahedra.     

Zur Darstellung und Struktur von LaNb5O14

Preparation and Structure of LaNb₅O₁₄ Single crystals of LaNb₅O₁₄ could be prepared by chemical transport reactions (T₂ → T₁; T₂ = 1050°C; T₁ = 950°C) using chlorine as transport agent. LaNb₅O₁₄ crystallizes in the orthorhombic space group Pbem with cell dimensions a = 3.8749(2) Å; b = 12.4407(6) Å and c = 20.2051(9) Å; Z = 4; R = 6.28%, R_w = 3.74%. The structure consists of two types of Nb? O-polyhedra. Especially remarkable are chains of edge-sharing pentagonal NbO₇-bipyramids, which are interconnected by corner-sharing NbO₆-octahedra. Tunnels running in a-direction are created by this framework of NbO₆- and NbO₇-polyhedra. Lanthanum atoms are located in these tunnels at levels inbetween the niobium atoms. The relationship to O? LaTa₃O₉ and M? CeTa₃O₉ type structures will be discussed.     

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.     

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.     

Syntheses and Structures of Two New Cu/Nb/pyrazine Complexes: Three dimensional CuNb(pyz)2OF5 · (pyz)(H2O) and two dimensional [Cu(pyz)2.5]+ [NbF6]− · (pyz)

Crystals of CuNb(pyz)₂OF₅ · (pyz)(H₂O) ( 1 ) and [Cu(pyz)_2.5]⁺ [NbF₆]^? · (pyz) ( 2 ) were grown (150°C and autogeneous pressures) from CuO, 1/2(Nb₂O₅), (HF)_x · pyridine, and H₂O in excess pyrazine. Light blue single crystals of ( 1 ) are orthorhombic, crystallizing in space group Cccm (No. 66), with a = 14.547(1) Å, b = 16.135(2) Å, c = 13.803(2) Å, and Z = 8. The structure of ( 1 ) contains corner shared [Cu(pyz)_4/2F_2/2]⁺, [Cu(pyz)_4/2O_2/2], and [NbF₄O_1/2F_1/2]^?0.5 octahedra. Orange crystals of ( 2 ) are monoclinic, crystallizing in space group C2/c (No. 15), with a = 11.792(8) Å, b = 17.123(3) Å, c = 17.051(5) Å, β = 90.04(4)°, and Z = 8. The structure of ( 2 ) contains puckered rings of corner shared [Cu(pyz)(pyz)_3/2]⁺ tetrahedra and isolated [NbF₆]^? anions within the rings.     

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.     

Crystal structure of BiNbTe2O8

BiNbTe₂O₈ crystallises with orthorhombic symmetry (space group Pbca) and unit cell parameters: a=5.6109(7) Å, b=8.1277 (8) Å, c=31.205 (3) Å, Z=8. Its crystal structure has been solved from single crystal X-ray diffraction data and refined to a final reliability factor R1=0.0512. It can be described as a regular succession along [001] of ReO₃-like sheets of NbO₆ octahedra, and fluorite-like sheets of BiO₈ distorted cubes and Te(1)O₄ disphenoı̈ds. These sheets are connected by Te(2)O₃ pyramids via Nb–O(4)–Te(2)–O(8)–Bi bridges. The lone pairs of Te(1) and Te(2) atoms are stereochemically active and point toward the empty cuboctahedral sites of the ReO₃-like sheets.