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1.
Shigeyuki Kimura 《Journal of solid state chemistry》1973,6(3):438-449
Phase equilibria in the system NbO2Nb2O5 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 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. 相似文献
2.
Marcia Cristina Kaezer Frana Jean-Guillaume Eon Michel Fournier Edmond Payen Olivier Mentr 《Solid State Sciences》2005,7(12):1533
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. 相似文献
3.
The mixed-valence oxide P4W10O38, which can be considered as the nonintegral member n = 2.5 of the series P4W4nO12n+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 (Rw = 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 P4W4nO12n+8. 相似文献
4.
Wubeshet Sahle 《Journal of solid state chemistry》1982,45(3):334-342
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. 相似文献
5.
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. 相似文献
6.
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. 相似文献
7.
Wuzong Zhou 《Journal of solid state chemistry》2002,163(2):479-483
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. 相似文献
8.
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 Ba8Nb6Li2O24Ba10W6Li4O30 system is reported.A tetragonal bronze phase homogeneity region was delimited at 1200°C in the BaONb2O5Li2O system. A new orthorhombic phase (a = 10.197 Å, b = 14.882 Å, c = 7.942 Å) was prepared with the stoichiometry Ba4Li2Nb10O30. 相似文献
9.
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 AnNb3+nO3+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 K1+x/2Nb4−xO6. 相似文献
10.
Kunitaka Muramatsu Masaji Shimazu Junzo Tanaka Shigeo Horiuchi 《Journal of solid state chemistry》1981,36(2):179-182
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 Bi2CaNan?2NbnO3n+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. 相似文献
11.
Phase relations at 1050°C have been determined for M-phase solid solutions in the LiO0.5-NbO2.5-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 LiNNbN−4Ti5O3N, 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 A2N+1O3N 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. 相似文献
12.
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 TiO2NbO2 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 pO2 (atm) about ?9.0 this reaches the limiting composition Ti0.72Nb2.28O7, in equilibrium with Ti0.56Nb0.44O2. The Me3O7 block structure then transforms into the Me12O29 block structure (Ti2Nb10O29Nb12O29 solid solution), which rapidly increases in niobium content as reduction continues. Reduction of Ti2Nb10O29 at oxygen fugacities above log pO2 (atm) = ?9.0 forms the Me3O7 phase as the titanium-rich phase. At log pO2 = ?9.0, and a composition about Ti1.6Nb10.4O29, 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, Ti0.17Nb0.67O2, are almost denuded of titanium. 相似文献
13.
On Hexagonal Perovskites with Cationic Vacancies. XXVIII. Structure of Rhombohedral 9 L Stacking Polytypes Ba3W Nb □O9?x/2□x/2 According to the intensity calculations for Ba3W4/3Nb2/3□O26/3□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% (Ba3W4/3Nb2/3□O26/3□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. 相似文献
14.
K.V. Terebilenko V.N. Baumer N.S. Slobodyanik 《Journal of solid state chemistry》2008,181(9):2393-2400
The phase relations in the cross-section of the K2W2O7-K2WO4-KPO3 containing 15 mol% Bi2O3 were undertaken using flux method. Crystallization fields of K6.5Bi2.5W4P6O34, K2Bi(PO4)(WO4), Bi2WO6, KBi(WO4)2 and their cocrystallization areas were identified. Novel phase K6.5Bi2.5W4P6O34 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 K6.5Bi2.5W4P6O34 were discussed on the basis of factor group theory. 相似文献
15.
The crystal structures of Bi2.5Na0.5Ta2O9 and Bi2.5Nam-1.5NbmO3m+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 (Am-1BmO3m+1)2- (m=2-4) pseudo-perovskite slabs. Bi2.5Na0.5Ta2O9 (m=2) and Bi2.5Na2.5Nb4O15 (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. Bi2.5Na1.5Nb3O12 (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 Bi2.5Na0.5Ta2O9 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 Bi2.5Nam-1.5NbmO3m+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. 相似文献
16.
H.J. Rossell 《Journal of solid state chemistry》1976,19(2):103-111
Powder X-ray diffraction techniques have been used to determine the crystal structures of Ti3Sc4O12, Nb1.5Sc5.5O12, 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 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. 相似文献
17.
18.
Nik Reeves-McLaren Matthew C. Ferrarelli Yuan-Wei Tung Derek C. Sinclair Anthony R. West 《Journal of solid state chemistry》2011,184(7):1813-1819
Subsolidus phase relations in the CuOx-TiO2-Nb2O5 system were determined at 935 °C. The phase diagram contains one new phase, Cu3.21Ti1.16Nb2.63O12 (CTNO) and one rutile-structured solid solution series, Ti1−3xCuxNb2xO2: 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. 相似文献
19.
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. 相似文献
20.
The crystal structure of KxP4W14O50 (x = 1.4) has been solved by three-dimensional single crystal X-ray analysis. The refinement in the cell of symmetry , 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 Rw = 0.039 for 2436 reflections with . This structure belongs to the structural family KxP4O8(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. 相似文献