Phase equilibria in the system NbO2Nb2O5: Phase relations at 1300 and 1400°C and related thermodynamic treatment

Phase equilibria in the system NbO₂Nb₂O₅ at 1300 and 1400°C were studied and the variations of composition of solid oxide phases with oxygen pressure in the atmosphere were determined by using $\text{CO}{}_2\text{H}{}_2$ gas mixture. Five discrete compunds, Nb₁₂O₂₉, Nb₂₂O₅₄, Nb₄₇O₁₁₆, Nb₂₅O₆₂, and Nb₅₃O₁₃₂ were observed to exist between NbO₂ and Nb₂O₅ at both temperatures. No appreciable tendency of increase of existence region was found for any of the studied compounds at higher temperatures. A mean for thermodynamic treatment of crystallographic shear planes including Wadsley intergrowth defects is proposed and, as a consequence, it was shown that Wadsley intergrowth defects in the studied compounds are probably of nonequilibrium nature.     

(Nb2W4O19), TMA2, Na4(OH2)14(SO4): a new layered structure with Lindqvist heteropolyanions, XAS characterization of the HPAs

The new (Nb₂W₄O₁₉),TMA₂, Na₄(OH₂)₁₄(SO₄) has been evidenced as a minor phase during the Nb₂W₄O₁₉TMA (tetramethylammonium) salt synthesis. Its crystal structure has been refined from single crystal X-ray diffraction data, system monoclinic, a=10.166(5) Å, b=17.93(1) Å, c=24.81(1) Å, β=93.057(7)°, space group (S.G.) C2/c, Z=4, R₁=3.96%, wR₁=4.50%. It shows the stacking of cationic and anionic bidimensional layers. The anionic layer of formula [(Nb₂W₄O₁₉), TMA₂ ]²⁻ is formed of isolated Lindqvist HPAs surrounded by TMA groups. The isolated layers adopt a trigonal symmetry that is lost in the crystal by the association of the cationic sheets. These later, of formula [Na₄(OH₂)₁₄(SO₄)]²⁺ form porous net-like sheets with nearly circular cavities of diameter 7.5 Å. groups host the available cavities in a disordered manner. The cohesion between the sheets is performed by both electrostatic interactions and a set of hydrogen bonds. In the cationic layers, the highly symmetrical surrounding of HPAs by TMA groups yields a homogeneous electrostatic field at their external surface leading to a statistic Nb/W disorder over the three available independent metallic positions. Then, XAS experiments at the L₁/L₃-W edge complementarily helped to highlight the preferential cis configuration of (Nb₂W₄O₁₉)⁴⁻ anions, help to the strong Nb vs W contrast in their contribution to the backscattering paths. Previously to these experiments, it was of course checked that both the two phases present in the prepared sample contain Nb₂W₄O₁₉ anions with nearly unchanged geometry.     

Mixed valence tungsten oxides with a tunnel structure,P4W4nO12n+8: A nonintegral member P4W10O38 (n = 2.5)

The mixed-valence oxide P₄W₁₀O₃₈, which can be considered as the nonintegral member n = 2.5 of the series P₄W_4nO_12n+8, crystallizes in the monoclinic system with unit-cell dimensions a = 6.5656(25), b = 5.2850(15), c = 20.573(15) Å, β = 96.18(4)°, and space group P2₁. The crystal structure was solved by conventional Patterson and Fourier techniques using 2339 counter-measured reflections that obeyed the condition I > 3σ(I) and refined to an R factor of 0.074 (R_w = 0.077). Basically, the framework of the structure built up from ReO₃-type slabs connected through PO₄ tetrahedra looks like that of P₄W₈O₃₂ previously described. Unlike P₄W₈O₃₂, two successive ReO₃-type slabs have a different width corresponding to two and three WO₆ octahedra so that the structure can be considered as an intergrowth of the integral members n = 2 and n = 3 of the series P₄W_4nO_12n+8.     

Electron microscopy studies of W18O49. 2. Defects and disorder introduced by partial oxidation

W₁₈O₄₉ was oxidized in air at about 500K for different intervals of time. Defects of various kinds, related to structures of higher oxides, were observed. These were a coherent intergrowth of W₁₂O₃₄, {102}, and {103} crystallographic shear, and WO₃-type structures. A new type of TTB structure was also observed as a defect. Its formation mechanism is proposed and discussed.     

Electrochemical Li insertion studies on WNb12O33—A shear ReO3 type structure

Electrochemical lithium insertion studies on WNb₁₂O₃₃ synthesized by solid state reaction (SSR) are carried out in the voltage range 1.0-3.2 V. During first discharge 15.6 Li are inserted with a specific capacity of 221 mAh/g. WNb₁₂O₃₃ is also synthesized by sol-gel (SG) technique with a view to enhance the rate capability and cycling properties. The SSR and SG samples are characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and galvanostatic cycling. Electrochemical cycling performance of SG samples is superior to that of the SSR sample at high ‘C’ rates. The sample synthesized by SG method exhibits high specific capacity of 142 mAh/g after 20 cycles at 20C rate.     

Thermal expansion of phases formed in the system Nb2O5-MoO3

Studies on thermal expansion of phases formed in the system Nb₂O₅-MoO₃ (WO₃) have been carried out in the high-temperature X-ray diffraction attachment. In the case of Nb₁₄Mo₃O₄₄, Nb₁₂MoO₃₃ and Nb₁₂WO₃₃ the structure that consists of ReO₃ type blocks, the direction of minimal thermal expansion is consistent with direction in which the chains of corner-sharing polyhedra spread to infinity. On the contrary, for Nb₂Mo₃O₁₄, the structure of which resembles the structure of tetragonal tungsten bronzes, the maximal thermal expansion direction is consistent with above mentioned direction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Microstructures of Some Bi-W-Nb-O Phases

Microstructures of three Bi-W-Nb-O phases have been examined by using high-resolution transmission electron microscopy. Bi₁₇W₂Nb₃O₃₉ and Bi₁₇WNb₃O₃₆ have incommensurate superstructures derived from the defect fluorite-type δ-Bi₂O₃ and can be regarded as intermediate phases between the type II solid solutions in the Bi-Nb-O and Bi-W-O systems. Bi₈W₂Nb₂O₂₃ has a Bi₂WO₆-like subunit cell with a stepped superstructure. Formation mechanisms of various superstructures are discussed.     

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.     

Cluster intergrowth of perovskite and defect sodium chloride type structures in the K---Nb---O system: The structure of KNb4O6

A new reduced potassium niobate (KNb₄O₆) of intergrowth type structure containing condensed Nb₆O₁₂ clusters has been found. The structure has been determined from HREM images. The atomic positions have been refined with the Rietveld technique using X-ray powder diffraction data. The space group of KNb₄O₆ is P4/mmm; Z = 1, and its unit cell parameters are a = 4.1393(1) and c = 8.2537(2). KNb₄O₆ consists of alternating slabs of KNbO₃ (perovskite) and NbO (ordered deficient NaCl-type) both being a single unit thick. The structure is closely related to that of A₂Nb₅O₉ (A = Ba, Sr). Both phases can be considered as members (n = 1 and 2 respectively) of a homologous series A_nNb_3+nO_3+3n. Electron microscopy studies show the presence of defects, both as extra perovskite layers and missing NbO slabs, together with areas of more disordered intergrowth. The profile refinement and microanalysis of individual crystal fragments both indicate the structure to be niobium deficient according to the formula K_1+x/2Nb_4−xO₆.     

High n-value phases in the complex bismuth oxides with layered structure,Bi2CaNan−2NbnO3n+3

Complex bismuth oxides with layered structure are prepared with a series of compositions in the system Bi₂CaNb₂O₉-NaNbO₃. It is found by X-ray powder diffraction that each compound is composed of more than two phases, which are described by a formula Bi₂CaNa_n?2Nb_nO_3n+3, e.g., in the sample with the nominal composition Bi₂CaNb₂O₉ · 8NaNbO₃, the phases with n = 6 to 8 appear predominantly. These phases are closely intergrown to each other. Moreover, high-resolution electron microscopy reveals that microsyntactic intergrowth frequently occurs in the phases with n > 5. The occurrence of the latter intergrowth is explained in terms of the bond length obtained.     

Phase stability and non-stoichiometry in M-phase solid solutions in the system LiO0.5-NbO2.5-TiO2

Phase relations at 1050°C have been determined for M-phase solid solutions in the LiO_0.5-NbO_2.5-TiO₂ ternary phase system by the quench method. Rietveld analysis has been used to help determine phase boundaries and to study structure composition relations. The M-phases have trigonal structures based on intergrowth of corundum-like layers, [Ti₂O₃]²⁺, with slabs of (N−1) layers of LiNbO₃-type parallel to (0001). Ideal compositions are defined along the pseudobinary join LiNbO₃-Li₄Ti₅O₁₂ by the homologous series formula Li_NNb_N−4Ti₅O_3N, N?4. Homologues with N?10 lie to the low-lithia side of the LiNbO₃-Li₄Ti₅O₁₂ join and show extended single-phase solid solution ranges separated by two-phase regions. The composition variations along the solid solutions are controlled by a major substitution mechanism, Li⁺+3Nb⁵⁺↔4Ti⁴⁺, coupled with a minor substitution 4Li⁺↔Ti⁴⁺+3□, where □=vacancy. The latter substitution results in increasing deviations from the stoichiometric compositions A_2N+1O_3N with increasing Ti substitution. The non-stoichiometry can be reduced by re-equilibration at lower temperatures. Expressions have been developed to describe the compositional changes along the solid solutions.     

Reduction of the titanium niobium oxides. I. TiNb2O7 and Ti2Nb10O29

Reduction of the titanium-niobium oxides follows a common pattern. TiO₂ is eliminated, to form a new phase richer in titanium than the original compound, and Nb(iv) replaces Ti(iv) in the original block structure, which is thereby enriched in niobium. With TiNb₂O₇, the second phase is a TiO₂NbO₂ solid solution, with the rutile structure, initially with a high titanium content, in equilibrium with a solid solution of composition Me₃O₇, isostructural with TiNb₂O₇. At log p_O2 (atm) about ?9.0 this reaches the limiting composition Ti_0.72Nb_2.28O₇, in equilibrium with Ti_0.56Nb_0.44O₂. The Me₃O₇ block structure then transforms into the Me₁₂O₂₉ block structure (Ti₂Nb₁₀O₂₉Nb₁₂O₂₉ solid solution), which rapidly increases in niobium content as reduction continues. Reduction of Ti₂Nb₁₀O₂₉ at oxygen fugacities above log p_O2 (atm) = ?9.0 forms the Me₃O₇ phase as the titanium-rich phase. At log p_O2 = ?9.0, and a composition about Ti_1.6Nb_10.4O₂₉, the rutile solid solution takes over as second phase. The niobium/titanium ratio in both phases rises as reduction proceeds, and the last vestiges of the Me₁₂O₂₉ phase, in equilibrium with the final product, Ti_0.17Nb_0.67O₂, are almost denuded of titanium.     

Über hexagonale Perowskite mit Kationenfehlstellen. XXVIII. Die Struktur der rhomboedrischen 9 L-Stapelvarianten Ba3WNb□O9−x/2□x/2

On Hexagonal Perovskites with Cationic Vacancies. XXVIII. Structure of Rhombohedral 9 L Stacking Polytypes Ba₃W Nb □O_9?x/2□_x/2 According to the intensity calculations for Ba₃W_4/3Nb_2/3□O_26/3□_1/3 and Ba₃Nb₂□O₈□(II) these rhombohedral 9 L compounds crystallize in the space group R3m, sequence (hhc)₃. The refined, intensity related R′ values are 6.9% (Ba₃W_4/3Nb_2/3□O_26/3□_1/3) and 7.2% (Ba₃Nb₂□O₈□(II)). The relations between the rhombohedral 9 L structure (A₃M₂□O₉) and the palmierite type (A₃M₂□O₈□) are discussed.     

Phase relations in the K2W2O7-K2WO4-KPO3-Bi2O3 system and structure of K6.5Bi2.5W4P6O34

The phase relations in the cross-section of the K₂W₂O₇-K₂WO₄-KPO₃ containing 15 mol% Bi₂O₃ were undertaken using flux method. Crystallization fields of K_6.5Bi_2.5W₄P₆O₃₄, K₂Bi(PO₄)(WO₄), Bi₂WO₆, KBi(WO₄)₂ and their cocrystallization areas were identified. Novel phase K_6.5Bi_2.5W₄P₆O₃₄ was characterized by single-crystal X-ray diffraction: sp. gr. P−1, a=9.4170(5), b=9.7166(4), c=17.6050(7) Å, α=90.052(5)°, β=103.880(5)° and γ=90.125(5)°. It has a layered structure, which contains {K₇Bi₅W₈P₁₂O₆₈}_∞ layers stacked parallel to ab plane and sheets composed by potassium atoms separating these layers. Sandwich-like {K₇Bi₅W₈P₁₂O₆₈}_∞ layers are assembled from [W₂P₂O₁₃]_∞ and [BiPO₄]_∞ building units, and are penetrated by tunnels with K/Bi atoms inside. FTIR-spectra of K₂Bi(PO₄)(WO₄) and K_6.5Bi_2.5W₄P₆O₃₄ were discussed on the basis of factor group theory.     

Structure Study of Bi2.5Na0.5Ta2O9 and Bi2.5Nam−1.5NbmO3m+3 (m=2-4) by Neutron Powder Diffraction and Electron Microscopy

The crystal structures of Bi_2.5Na_0.5Ta₂O₉ and Bi_2.5Na_m-1.5Nb_mO_3m+3 (m=3,4) have been investigated by the Rietveld analysis of their neutron powder diffraction patterns (λ=1.470 Å). These compounds belong to the Aurivillius phase family and are built up by (Bi₂O₂)²⁺ fluorite layers and (A_m-1B_mO_3m+1)^2- (m=2-4) pseudo-perovskite slabs. Bi_2.5Na_0.5Ta₂O₉ (m=2) and Bi_2.5Na_2.5Nb₄O₁₅ (m=4) crystallize in the orthorhombic space group A2₁am, Z=4, with lattice constants of a=5.4763(4), b=5.4478(4), c=24.9710 (15) and a=5.5095(5), b=5.4783(5), c=40.553(3) Å, respectively. Bi_2.5Na_1.5Nb₃O₁₂ (m=3) has been refined in the orthorhombic space group B2cb, Z=4, with the unit-cell parameters a=5.5024(7), b=5.4622(7), and c=32.735(4) Å. In comparison with its isostructural Nb analogue, the structure of Bi_2.5Na_0.5Ta₂O₉ is less distorted and bond valence sum calculations indicate that the Ta-O bonds are somewhat stronger than the Nb-O bonds. The cell parameters a and b increase with increasing m for the compounds Bi_2.5Na_m-1.5Nb_mO_3m+3 (m=2-4), causing a greater strain in the structure. Electron microscopy studies verify that the intergrowth of mixed perovskite layers, caused by stacking faults, also increases with increasing m.     

Crystal structures of some fluorite-related M7O12 compounds

Powder X-ray diffraction techniques have been used to determine the crystal structures of Ti₃Sc₄O₁₂, Nb_1.5Sc_5.5O₁₂, NbSc₆O₁₁F, NbSc₅HfO₁₂, Zr₃Er₄O₁₂, and Hf₃Sc₄O₁₂ and to reexamine the structures of Zr₃Sc₄O₁₂ and Zr₃Yb₄O₁₂. All the compounds have the same fluorite-related superstructure as reported previously for Pr₇O₁₂, UY₆O₁₂, Zr₃Sc₄O₁₂, and Zr₃Yb₄O₁₂. Most of the compounds exhibit cation ordering, a situation observed previously only in UY₆O₁₂. One-seventh of the cations occupy a set of special sites of symmetry $\text{3}$ and are octahedrally coordinated by oxygen. These cations are of the type that has the smallest available ionic radius. The remaining cations occupy a set of general sites randomly with respect to atom type and are sevenfold coordinated by oxygen. In Hf₃Sc₄O₁₂, only partial ordering of this nature occurs, while it is confirmed that no cation ordering occurs in Zr₃Sc₄O₁₂. Some aspects of the cation order-disorder situation are discussed.     

Synthesis, structure and electrical properties of Cu3.21Ti1.16Nb2.63O12 and the CuOx-TiO2-Nb2O5 pseudoternary phase diagram

Subsolidus phase relations in the CuO_x-TiO₂-Nb₂O₅ system were determined at 935 °C. The phase diagram contains one new phase, Cu_3.21Ti_1.16Nb_2.63O₁₂ (CTNO) and one rutile-structured solid solution series, Ti_1−3xCu_xNb_2xO₂: 0<x<0.2335 (35). The crystal structure of CTNO is similar to that of CaCu₃Ti₄O₁₂ (CCTO) with square planar Cu²⁺ but with A site vacancies and a disordered mixture of Cu⁺, Ti⁴⁺ and Nb⁵⁺ on the octahedral sites. It is a modest semiconductor with relative permittivity ∼63 and displays non-Arrhenius conductivity behavior that is essentially temperature-independent at the lowest temperatures.     

High resolution electron microscopy of TiO2·7Nb2O5

The defect structure of TiO₂·7Nb₂O₅ has been examined at about 0.3 nm resolution in an electron microscope. Under suitable conditions of crystal orientation and objective lens defocus, the contrast in images from very thin fragments can be interpreted directly in terms of structure. Proposed structures for Wadsley intergrowth defects and displacements are confirmed, and new observations of complex fault bands and grain boundaries are described. Microdomains of TiNb₁₄O₃₇, with the structure predicted by Wadsley, have also been found in this material.     

K1.4P4W14O50: An odd-m member (m = 7) of the monophosphate tungsten bronze series KxP4O8(WO3)2m

The crystal structure of K_xP₄W₁₄O₅₀ (x = 1.4) has been solved by three-dimensional single crystal X-ray analysis. The refinement in the cell of symmetry $\text{A2}\text{m}$, with a = 6.660(2) Å, b = 5.3483(3) Å, c = 27.06(5) Å, and β = 97.20(2)°, Z = 1, has led to R = 0.036 and R_w = 0.039 for 2436 reflections with $\text{σ(I)}\text{I}\text{≤ 0.333}$. This structure belongs to the structural family K_xP₄O₈(WO₃)_2m, called monophosphate tungsten bronzes (MPTB), which is characterized by ReO₃-type slabs of various widths connected through PO₄ single tetrahedra. This bronze corresponds to the member m = 7 of the series and its framework is built up alternately of strands of three and four WO₆ octahedra. The structural relationships with the P₄O₈(WO₃)_2m series, called M′PTB, are described and the possibility of intergrowth between these two structures is discussed.