Electronic structure and Chemical bonding in Sr4Nb17O26

The electronic structure of oxoniobate Sr₄Nb₁₇O₂₆ is studied by the linear muffintin orbital (LMTO) method. It is shown that the highenergy conduction band consists of the Nb4d states and the hybridized valence band is formed by the Nb4d and O2p states. The band structure of this compound is characterized by superposition of the bands of the 2p states of perovskite oxygen atoms and the 4d states of monoxide niobium atoms. The degree of oxidation of the perovskite and monoxide niobium atoms is +5 and +2.56, respectively. Chemical bonding is analyzed using the electron localization function and model Hückel calculations. The niobiumoxygen bond is shown to be the strongest. The Fermi level is localized in the vicinity of the bottom of the niobium antibonding state band, which explains the existence of Sr_4−xNb₁₇O₂₆ in the homogeneity region corresponding to 0 < x < 0.3. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 5, pp. 771–780, September–October, 1998. This work was supported by RFFR grant No. 96-03-32015a.     

Ein weiteres Gemischtvalentes Oxoniobat: Sr5Nb34+Nb25+O16

About a Mixed Valence Oxoniobate: Sr₅Nb₃⁴⁺Nb₂⁵⁺O₁₆ The hitherto unknown compound Sr₅Nb₅O₁₆ was prepared and examined by X-ray single crystal work. It crystallizes with orthorhombic symmetry (space group D–Pmn2₁, a = 3.992(1), b = 32.476(10), c = 5.677(2) Å; Z = 2). Sr₅Nb₅O₁₆ consists of stacked perovskite-like blocks cut by a plane perpendicular to the cube face diagonal of the perovskite structure. The coordination relations of the intersections between those blocks and the distribution of Nb⁵⁺ and Nb⁴⁺ are discussed.     

An electron microscope study of perovskite-related oxides in the SrTiO system

In order to investigate the relationship between nonstoichiometry and the static dielectric constant, phases in the SrOTiO₂ pseudo-binary system have been studied by X-ray diffraction and electron microscopy. Three ternary oxides were found in the temperature interval 1373–1673°K, Sr₂TiO₄, Sr₃Ti₂O₇, and SrTiO₃. In all of these oxides, nonstoichiometry seems to be taken up by coherent intergrowth of lamellae of various homologous oxides Sr_n+1Ti_nO_3n+1 The results are discussed in terms of the crystal chemistry of the perovskite oxides and factors which influence the magnitude of the static dielectric constant of solids.     

原位制备钛酸锶钡/铌酸锶钡复相陶瓷的研究

0引言驰豫型铁电陶瓷是近年来广泛研究和迅速发展的一种新型功能陶瓷[1]。通过把具有不同相变温度的驰豫型铁电体层层叠加,可以获得具有稳定的介电常数鄄温度关系的复合材料[2]。钛酸锶钡Ba1-xSrxTiO3和铌酸锶钡SrxBa1-xNb2O6都是重要的驰豫型铁电体,并且其居里温度均随Ba/Sr比     

Investigation of s, p, d-element Niobates and Their Solid Solution Formation Processes by Thermal Analysis

The binary and ternary systems M'O(M'CO₃)-Nb₂O₅ and M'O(M'CO₃)-MO-Nb₂O₅, where M'=Ca,Sr and Ba and M=Cu, Ni, Cd, Zn and Pb, were investigated by means of thermal analysis in the temperature range 20–1500°C. The boundaries of stability of the solid solutions Sr_2−xMe_xNb₂O₇,Sr_2−xM_xNb₂O₇, Sr_4−xM_xNb₂O₉ and Sr_6−xM_xNb₂O₁₁ were determined by means of X-ray diffraction, and IR and Raman spectroscopy. The possibility of prognostication of the phase fields of stable solid solutions by calculation from the diagrams of the ‘comparative electronegativity of atoms vs. tolerance factor’ was demonstrated. The kinetic parameters of the interactions in theSrCO₃+MO+Nb₂O₅ powder mixtures were established. This revised version was published online in August 2006 with corrections to the Cover Date.     

Structure and properties of (1−x)(0.6Pb(Zn1/3Nb2/3)O3-0.4Pb (Mg1/3Nb2/3)O3)-xPbTiO3 thin films with perovskite phase promoted by polyethylene glycol

The preferential formation of a pyrochlore structure is a knotty problem in the preparation of Pb(Zn_1/3Nb_2/3)O₃ (PZN)-based thin film materials and its presence is significantly detrimental to the dielectric and piezoelectric properties. In this study, 40 mol% of PZN was replaced with Pb(Mg_1/3Nb_2/3)O₃ (PMN) for obtaining a perovskite composition around a morphotropic phase boundary (MPB), (1−x)(0.6PZN-0.4PMN)-xPT ((1−x)PZMN-xPT, PT: PbTiO₃) where x = 0.23. The thin films with this composition were prepared with a polyethylene glycol (PEG) modi-fied sol-gel method on LaAlO₃ substrates. The microstructural evolution of the films on heat treatment was examined with X-ray diffraction. With the aid of PEG, the formation of the pyrochlore phase was suppressed and the perovskite phase formed directly from the amorphous gel film. The multilayer films with a thickness around 0.25 μm showed a single perovskite phase without any detectable pyrochlore structure. Microscopic images showed uniform grain size of a few tens of nanometers. The role of the polymer dramatically promoting the perovskite phase was investigated with the aid of X-ray photoelectron spectroscopy and thermal analysis. The dielectric constant of the obtained film was 4160 at 1 kHz. The film demonstrated typical ferroelectric hysteresis loops and exhibited excellent piezoelectric performance.     

Quantitative WDXS Microanalysis of Bismuth-Based BaBi4Ti4O15 Perovskites Doped with Nb and Fe

 Quantitative electron-probe microanalysis was used to determine the chemical composition of an Fe- and Nb-doped bismuth-based BaBi₄Ti₄O₁₅ perovskite compound. Elemental concentrations of Fe, Nb, Bi, Ba and Ti were accurately measured using wavelength-dispersive X-ray spectroscopy that was optimised for the analysis of a complex oxide matrix containing minor concentrations of dopants. Measurements were performed with a JEOL JXA 840A electron probe microanalyser at 20 and 26 kV, 50 nA beam current, 100 s maximum counting time and 0.3% preset counting deviation (σ_c) using both PET and LiF crystals. K-ratios were quantified by the ZAF and the φ(ρz) PAP matrix-correction procedures. The results showed that dopants incorporate into the BaBi₄Ti₄O₁₅ at Ti^4 + sites according to the Ba_1−4XBi_4 + 4XTi_4−4XFe_4XO₁₅ and Ba_1 + 4XBi_4−4XTi_4−4XNb_4XO₁₅ solid-solution formulae. The majority of the excess charge introduced by the substitution of Ti^4 + with Fe^3 + or Nb^5 + is compensated for the change in the Ba^2 +/Bi^3 + ratio.     

Preparation and Properties of Highly Oriented Sr0.3Ba0.7Nb2O6 Thin Films by a Sol-Gel Process

We succeeded in the preparation of epitaxial or highly oriented strontium-barium niobate (Sr_0.3Ba_0.7Nb₂O₆) thin film by a sol-gel process. A homogeneous coating solution was prepared with Sr and Ba acetates and Nb(OEt)₅ as raw materials, and acetic acid and diethylene glycol monomethyl ether as solvents. Sr_0.3Ba_0.7Nb₂O₆ film sintered at 900°C on MgO(1 0 0) was oriented with c-axis perpendicular to the substrate surface. Sr_0.3Ba_0.7Nb₂O₆ film sintered at 700°C on SrTiO₃(1 0 0) was an epitaxial and oriented with c-axis in parallel to the substrate surface. Transmittance of Sr_0.3Ba_0.7Nb₂O₆ film (film thickness: 144 nm) was more than 60% at the range from 400 to 800 nm. Refractive index was 2.33 at 633 nm. Dielectric constant and dielectric loss of the Sr_0.3Ba_0.7Nb₂O₆ thin films prepared on polycrystal Pt substrates were 600 and 0.06 at room temperature and 1 kHz, respectively. The curie temperature (T_c) of polycrystalline Sr_0.3Ba_0.7Nb₂O₆ thin films was about 200°C. At room temperature and 50 kHz, remanent polarization (P_r) and coercive field (E_c) of the polycrystalline thin films were 1.79 C/cm² and 2.69 kV/cm, respectively.     

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.     

Oxygen ionic transport in Bi2O3-based oxides: The solid solutions Bi2O3–Nb2O5

The minimum concentration of niobium to stabilize the fluorite-type f.c.c. phase in the Bi₂O₃–Nb₂O₅ oxide system at temperatures below 996 K was ascertained to be about 10 mol%. Thermal expansion, electrical conductivity and crystal lattice parameters of the Bi(Nb)O_1.5+δ solid solutions decrease with increasing niobium content. Thermal expansion coefficients were calculated from the dilatometric data to be (10.314.5)×10⁻⁶ K⁻¹ at temperatures in the range 300–700 K and (17.526.0)×10⁻⁶ K⁻¹ at 700–1100 K. The conductivity of the Bi_{1− x} Nb _x O_1.5+δ ceramics is predominantly ionic. The p-type electronic transference numbers of the Bi(Nb)O_1.5+δ solid solutions in air were determined to be less than 0.1. Annealing at temperatures below 900 K results in a sharp decrease in conductivity of the Bi_{1− x} Nb _x O_1.5+δ ceramics. Received: 18 August 1997 / Accepted: 20 October 1997     

Bi5Nb3O15 as a photocatalyst: photocatalytic and photoelectrochemical studies

Bi₅Nb₃O₁₅ was prepared from a stoichiometric mixture of Bi₂O₃ and Nb₂O₅ at 300–500 °C. The prepared photocatalyst was characterized by diffuse reflection spectrum (DRS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and particle size analysis. The band gap, crystal structure and average grain size were determined from the above methods to be 3.25 eV, distorted pyrochlore and 4–5 μm respectively. The photoelectrochemical behavior of hydrogen-reduced Bi₅Nb₃O₁₅ was investigated in 0.1 M Na₂SO₄ and using the Fe(CN)₆ ^3−/4− redox couple for measuring the current-voltage characteristics. The cyclic voltammetric studies revealed that the onset potential for photocurrent generation existed at −0.45 V, which is more negative to water reduction level at pH 7.0, and that of the photocurrent at 1.0 V was observed as 0.58 mA/cm². Photocatalytic hydrogen production has been achieved by using Bi₅Nb₃O₁₅ as a photocatalyst in presence of methyl viologen. The quantum yield for hydrogen production for this system was found to be 0.83. All the studies clearly indicated that Bi₅Nb₃O₁₅ has potential in solar energy conversion. Received: 22 May 1997 / Accepted: 18 September 1997     

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.     

Phase equilibria,charge transport,and mass transport in Sr<Subscript>4</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript>-“M<Subscript>4</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript>” (M = Cd,Cu, Ni,and Zn) systems

Homogeneous fields of Sr_{4 − x} M _x Nb₂O₉ (M = Cd, Cu, Ni, or Zn) solid solutions were determined using powder X-ray diffraction. Phase fields were plotted proceeding from the tolerance factor t and electronegativity ratio $ \bar k_A /\bar k_B $ \bar k_A /\bar k_B with a satisfactory fit of experimental results. Thermogravimetry was used to establish the major kinetic laws of solid-phase synthesis (conversion, rate-controlling stage, and effective activation energy) in (4 − x)SrCO₃ + xMO + Nb₂O₅ powdery mixtures. Direct radiometry was used to determine ⁹⁰Sr, ⁶³Ni, and ⁶⁵Zn self-diffusion coefficients in solid solutions based on the Sr₄Nb₂O₉ phase. Electrical conductivity was measured as a function of temperature for all Sr₄Nb₂O₉-“M₄Nb₂O₉” samples. The conductivity of Sr_{4 − x} M _x Nb₂O₉ (M = Cd, Cu, Ni, or Zn) solid solutions has a mixed ion-electron character.     

How synthesis parameters influence the quality of lead magnesioniobate PbMg1/3Nb2/3O3

Lead magnesioniobate PbMg_1/3Nb_2/3O₃ (PMN) has been prepared using lead oxide and magnesium niobate. Factors that influence the perovskite/pyrochlore ratio in the PMN structure have been studied. In order for the maximal amount of the perovskite phase to be obtained, synthesis should be carried out at 850?C900°C; an excess of MgO (1?C5 wt %) does not exert a positive effect. An excess of PbO (5?C7 wt %) allows one to obtain a phase that contains 85% PMN, with the perovskite structure. The perovskite lattice is found to be stabilized as a result of cationic substitutions (Yb, Lu) in the Mg-Nb sublattice due to the formation of solid solution Pb(Mg_1/3Nb_2/3)_{1 ? x} Yb(Lu) _x O₃, where 0 ?? x ?? 0.9.     

Etude des systemes CaHPO4−MHPO4−H2O (M=Sr,Ba) A 50°C

The systems CaHPO₄−MHPO₄−H₂O (M=Sr, Ba) were studied at 50°C. ForM=Sr, the series of single phases, Ca_1−xSr_xHPO₄ for 0.95<X<0.75 and Ca_xSr_1−xHPO₄ for 0.4<X<1 have been prepared. These solid solution were caracterized by their infrared spectra and their crystallographic unit cell parameters. ForM=Ba a new phase Ca₂Ba(HPO₄)₃ has been determined. It was characterized by DRX, IR, ATD and chemical analyses.

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Zusammenfassung Bei 50°C wurde das System CaHPO₄−MHPO₄−H₂O (mitM=Sr, Ba) untersucht. FürM=Sr wurden Serien von Einzelphasen erhalten: Ca_1−xSr_xHPO₄ für 0.95<X< 0.75 und Ca_xSr_1−xHPO₄ für 0.4<X<1. Diese Mischkristalle wurden anhand ihrer Infrarotspektren und ihrer kristallographischen Elementarzellenparameter charakterisiert. FürM=Ba wurde die neue Phase Ca₂Ba(HPO₄)₃ ermittelt. Sie wurde mittels DRX, IR, ATD und chemischer Analyse charakterisiert.

     

Metastabile Oxometallate der Lanthanoidmetalle:.Zur Kenntnis von Sr3Pr4O9 und Sr3La2Sm2O9 mit einem Beitrag über SrPr2O4

Metastable Compounds of Rare Earth Oxides. About Sr₃Pr₄O₉ and Sr₃La₂Sm₂O₉ with a Remark about SrPr₂O₄ Sr₃Pr₄O₉ (A) and Sr₃La₂Sm₂O₉ (B) are for the first time prepared and investigated by X-ray single crystal work. Both compounds crystallize with monoclinic symmetry (space group C_s⁴? Cc, Z = 4) with (A) a = 11.468; b = 7.262; c = 13.218 Å; β = 115.61°, (B) a = 11.518; b = 7.263; c = 13.290 Å; β = 115.61°. (A) and (B) are metastable with high disorder in the metal positions. All of the metal positions are occupied with a statistical distribution of Sr²⁺ and Ln³⁺. (A) decomposes in the hitherto unknown compound SrPr₂O₄. It belongs to the calciumferrite type compounds.     

Transparent thin films of Bi2WO6, Bi4Ti3O12 and Sr0.75Ba0.25Nb2O6

Transparent thin films were prepared by a sol–gel method starting from precursor formation in solution, subsequent spin coating followed by a heating ramp up to a maximum of 700 °C. Starting from a Bi₂MoO₆ synthesis route, the phase formation and thin film processing of the bismuth containing materials Bi₂WO₆, Bi₃Ti₄O₁₂ and additionally of the tungsten–bronze structure Sr_0.75Ba_0.25Nb₂O₆ were studied. Spin coating was used to adjust the film thickness in a wide range from 6 to 200 nm. All films were obtained as multicrystalline pure phases according to X-ray diffraction analyses. Scanning electron micrographs revealed homogeneous coatings composed of nanoparticles with a crystallite size varying between 20 and 100 nm, furthermore the UV–VIS spectra demonstrated a high transparency of the films, 80–90% at 600 nm.     

Reaction mechanism in the formation of perovskite Pb(Fe1/2Nb1/2)O3 by calcining of mixed oxides (CMO)

A calcining at 300°C and sintering process were proposed to obtain a pure perovskite phase Pb(Fe_1/2 Nb_1/2)O₃ from a 4PbO/Nb₂O₅/Fe₂O₃ mixture, which is calcined at 300°C for several days and sintered at various temperatures for 2h; the resultant powder was air quenched. The X-ray powder diffraction pattern of the sintered sample is carefully analyzed to identify intermediate phases. The effects of calcining at 300°C and sintering on obtaining PFN are based on the deformation of Pb₅Fe₄Nb₄O₂₁. A reaction mechanism for the calcining cycle of Pb(Fe_1/2 Nb_1/2)O₃ is proposed.     

Electrochemical characterisation of a Zn/(PEO)4ZnCl2/Nb2O5 solid-state cell

The (PEO)₄ZnCl₂ electrolyte (PEO, polyethylene oxide) was studied in view of its potential application in a solid-state rechargeable zinc cell. The electrochemical stability window was established, and decomposition voltage values between 3.19 (20 °C) and 1.44 V (150 °C) were estimated. Cyclic voltammetry studies using a Pt/(PEO)₄ZnCl₂/Pt cell indicated reversibility of the Zn²⁺/Zn couple at the electrode/electrolyte interface. Laboratory cells Zn(−)/(PEO)₄ZnCl₂/Nb₂O₅(+) were assembled and studied at 55 °C, under various discharge current densities. Results of cell discharge profiles, capacity values, charge–discharge cycles and cell stabilities are reported.