Synthesis of fine particles of niobium-oxide compounds from metal alkoxides

We synthesized ultrafine particles of niobium oxide and niobium-oxide compounds by the metal-alkoxide method: Nb₂O₅, LiNbO₃ and Sr_x- Ba_1-xNb₂O₆ (x = 0 ∼ 1). The structure was analyzed by X-ray diffraction and Raman scattering. All the Nb compounds have the same NbO₆- octahedral network, independent of the substituted metal ions.     

The investigation of room temperature ferromagnetism in (1 0 0) oriented BaNb2O6 PLD films on LaAlO3 (1 0 0) substrate

(1 0 0) oriented BaNb₂O₆ films have been successfully grown on LaAlO₃ (1 0 0) substrate at 750 °C or 450 °C in vacuum by pulsed laser deposition. The deposited BaNb₂O₆ PLD films exhibit room-temperature ferromagnetism. Ab initio calculations demonstrate that stoichiometric BaNb₂O₆ and that with barium vacancy are nonmagnetic, while oxygen and niobium vacancy can induce magnetism due to the spin-polarization of Nb s electrons and O p electrons respectively. Moreover, ferromagnetic coupling is energetically more favorable when two Nb/O vacancies are located third-nearest-neighbored. The observed room temperature ferromagnetism in BaNb₂O₆ films should be mainly induced by oxygen vacancies introduced during vacuum deposition, with certain contribution by Nb vacancies.     

Surface quasi-ordered nanostructures NbO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/Nb(110): investigation by surface analysis methods

The oxygen-induced surface structure on the Nb(110) face have been investigated by X-ray photoelectron spectroscopy and X-ray photoelectron diffraction. It is shown that the states (in terms of coordination and chemical bonding) of the niobium atoms belonging to surface oxide structures on Nb(110) are similar the metallic states in NbO. The thickness of the NbO _x layer was estimated to be 0.5 nm. Two nonequivalent chemical states of oxygen atoms on Nb(110) have been selected. It is suggested that the first state is the chemisorbed state of atomic oxygen on hexagonally packed surface areas of a Nb monolayer surface and the second state is the state of oxygen atoms belonging to NbO _x -like linear streaks formed on the Nb(110) face.     

Electronic structure of defective lithium cobaltites Li<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>2</Subscript>

X-ray absorption, resonant X-ray emission, and X-ray photoelectron spectroscopical methods have been applied for the study of the electronic structure of defective lithium cobaltites Li _x CoO₂ (0.6≤x≤1.0). Resonant O K α X-ray emission spectra of LiCoO₂ showed localized excitonic states due to a dd transition between occupied and unoccupied Co 3d states. On the base of measurements of Co 3s X-ray photoelectron, Co 2p, and O 1s X-ray absorption spectra, it was established that in defective cobaltites the electronic holes are localized mainly in O 2p states. An evidence of phase separation in Li _x CoO₂ has been found. It was shown that the semiconductor-to-metal transition in Li _x CoO₂ (x<0.76) at about 160 K is not accompanied by changes in the Co 3d electronic configuration which remains 3d ⁶.     

Interaction of Li<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>(Ni,Mn,Co)O<Subscript>2</Subscript> cathode materials with single and complex oxides at 900 °C

In order to find efficient barrier materials and inert dopants for the high temperature processing of Li-ion battery cathode materials, a chemical stability of Li_1+x (Ni,Mn)O₂ at 900 °C in air in contact with Al₂O₃, Nb₂O₅, SnO₂, TiO₂, and CeO₂ is studied. The interaction of Li_1+x (Ni,Mn)O₂ with Al₂O₃, Nb₂O₅, and SnO₂ results in the formation of the corresponding complex oxides—LiAlO₂, Li₃NbO₄, and Li₂SnO₃. A first stage of the chemical degradation of Li_1+x (Ni,Mn)O₂ is usually accompanied by the transformation of its hexagonal crystal structure into the cubic one. The reaction of Li_1+x (Ni,Mn)O₂ with titania is accompanied by the disappearance of TiO₂ and the formation of the Li_1+x (Ni,Mn)O₂-based solid solution. XRD analysis confirmed the absence of chemical interaction of Li_1+x (Ni,Mn)O₂ with CeO₂ while SEM data demonstrated the absence of eutectic melting at 900 °C. The similar absence of traces of the high temperature chemical interaction with Li_1+x (Ni,Mn)O₂ is found also for LiAlO₂, Li₃NbO₄, and Li₂SnO₃. Galvanostatic and cyclic voltammetry studies of Li_1+x (Ni,Mn,Co)O₂–CeO₂ composites demonstrated the increase in the initial discharge capacity of the composite cathodes compared to the native Li_1+x (Ni_,Mn_,Co)O₂.     

Characterization of stoichiometric LiNbO3 grown from melts containing K2O

The growth of LiNbO₃ single crystals from a melt with the Li/Nb ratio of 0.946, to which 6 wt.% K₂O has been added, leads to stoichiometric specimens, essentially free of potassium, with (50±0.15) mol% Li₂O in the crystal. This is established by studying the composition dependence of the following properties: linewidths of the electron paramagnetic resonance (EPR) of Fe³⁺, energy of the fundamental absorption edge, Raman linewidths of phonon modes, and dispersion of the optical birefringence. Comparison of the results with relevant calibration scales leads to the above composition. In all cases the Li₂O content was found to be closer to 50% than that of a LiNbO₃ crystal vapor-phase equilibrated to 49.9mol% Li₂O. The photorefractive effect at light intensities I10⁷ W/m² is suppressed in this stoichiometric material. The features of the ternary system K₂O-Li₂O-Nb₂O₅, which are possibly responsible for the unexpected growth of stoichiometric LiNbO₃ from the indicated melts, are discussed.     

Local electronic structure of Cu2O,CuO and YBa2Cu3O7−δ

Cu 3d and O 2p electronic states of Cu₂O, CuO, and the highT _c compound YBa₂Cu₃O_7– have been probed by means of high resolution x-ray emission spectroscopy (XES). The CuL and OK XES bands are compared in detail with recently reported x-ray photoelectron and ultraviolet photoelectron spectroscopy (XPS and UPS) measurements and densities of states obtained by local density functional (LDF) theory. The XES data show that the hybridization between Cu 3d and O 2p states is completely modified in CuO and YBa₂Cu₃O_7–, whered-d correlation energy is large, as compared to LDF predictions. Such is not the case for Cu₂O where agreement between theory and experiment is good.The Cu 3d states are found to be highly localized in YBa₂Cu₃O_7– (though less so than in CuO). The O 2p states lie at lower binding energies than in the simpler oxides and are mainly situated above the Cu 3d states. The respective positions of the centre of gravity of the OK emission bands on an x-ray energy scale indicate that the oxygen sites are less well screened by the O 2p states in the highT _c compound. This provides indirect evidence for the presence ofd-like states at the oxygen sites.     

Local atomic structure of niobates and titanates from X-ray absorption spectroscopic data

The local atomic structure of PbTiO₃, BaTiO₃, and KNbO₃ perovskite-type crystals and K _x Na_{1 ? x} NbO₃ solid solutions in different phases is investigated using the angular dependence of the pre-edge structure of the Ti and Nb K X-ray absorption spectra and the EXAFS data. In noncubic phases, a considerable deviation of the local structure from the structure determined from diffraction data is observed only for the tetragonal phase of the BaTiO₃ crystal. It is revealed that, in the cubic phase of niobates, the niobium atoms are characterized by significant displacements from the centrosymmetric positions along the threefold axes, so that they are close in the magnitude and the direction to the displacements in the low-temperatures rhombohedral phases.     

Optoelectronic properties of LixAxNbO3 (A=Na, K, Rb, Cs, Fr) crystals

The structural and optoelectronic properties of Li_xA_xNbO₃ (A=Na, K, Rb, Cs, Fr and x=0, 0.5) compounds have been investigated by the generalized gradient approximation within density functional theory. The calculated fundamental direct band gap of pure LiNbO₃ is 3.32 eV. It is found that the substitution of alkali elements drastically change the optoelectronic nature of the compound from direct to indirect bandgap semiconductor and the fundamental gap also decreases. The nature of the compound is ionic with strong bonds between alkali ions and O, while there are partial covalent bonds between Nb and O. The calculated static refractive index of pure LiNbO₃ is 2.43 for the perpendicular plane to the c-axis, while 2.37 for the parallel plane to the c-axis. So these values are intensively dependent on the substitution of alkali metals. The calculated electron energy loss spectra are in good agreement with the experimental results. It also predicts some extra interesting peaks, which have not been observed in experiments.     

Study on the formation during the production of lead-free piezoceramics at the morphotropic phase boundary

Because of the environmental concerns, the manufacture of ceramics based on lead titanate zirconate [Pb(Zr_1−xTi_x)O₃−PZT] has been condemned because of the lead toxicity. In this context, the electromechanical properties of sodium, potassium and lithium niobate [(Na_0.5−x/2K_0.5−x/2Li_x)NbO₃−NKLN] at the morphotropic phase boundary granted these materials the position of most suitable candidate to replace PZT. However, the production of these ceramics is rather critical mainly because of a natural tendency of forming secondary phases. To help with the studies of the synthesis of this lead-free piezoceramic, this work presents an evaluation of the crystallization of the (Na_0.47K_0.47Li_0.06)NbO₃ phase by solid-state reactions. TG-DTA, XRD, dilatometric and ferroelectric hysteresis analyses indicated that a secondary phase (K₃Li₂Nb₅O₁₅) crystallizes at temperatures above 850 C and also during the sintering of the powders compacts at 1080 C. To prevent the formation of this phase, the addition of Na₂Nb₂O₆.nH₂O microfibers obtained through a microwave hydrothermal synthesis was performed in the sintering process. After to this addition, the suppression of the K₃Li₂Nb₅O₁₅ phase occurred and an increase of the NKLN electrical properties was then obtained.     

Calculation of Chemical Shifts of X-Ray-Emission Spectra of Niobium in Niobium(V) Oxides Relative to Metal

Chemical shifts of the K_α1 and K_β1 lines of X-ray-emission spectra of niobium in oxides (Nb₂O₅)_n, n = 1–4, relative to metal Nb have been calculated. Stoichiometric clusters (Nb2O5)n the electronic structure of which was calculated using accurate relativistic pseudopotentials and two-component version of the density functional theory are considered as prototypes for modeling different crystal forms of niobium(V) oxide. The chemical shifts were calculated using the method based on using the property of approximate proportionality of valence spinors in the core region of the heavy atom [11]. Corrections to values of chemical shifts have been determined with allowance for deviations from the abovementioned proportionality. Rapid convergence of results with respect to the size of the niobium oxide cluster has been demonstrated.     

Ab initio density functional study on changes in local structure in perovskite compound,LixLa1/3NbO3

Present paper describes the changes in local structure of perovskite type electrode material, Li_xLa_1/3NbO₃, by using ab initio density functional theory calculations. Although the lithium insertion leads to the elongation of Nb–O bonds due to reduction of Nb, no marked change in cell volume is indicated in the calculated results as well as experimental observation previously reported. (M. Nakayama et al. J. Phys. Chem. B 109 (2005) 14648) The distribution of bond angles of Nb–O–Nb deviated from 180° by lithium insertion. Such behaviors was ascribed to the tilt of the NbO₆ octahedron induced by lithium insertion.     

X-ray photoelectron spectroscopy of tin oxide nanolayers

X-ray photoelectron spectra of 30- and 100-nm nanolayers, recorded in the energy range 0–35 eV, show a strong dependence of both the distribution of the density of Sn 5s, p+ O₂ p valence states and the change in the intensity ratio for the Sn 4d and O 2s subvalence states on the annealing temperature and nanolayer thickness. In the nanolayers fabricated at an annealing temperature of 450°C, an unusually strong band of O 2s states of unbound oxygen is observed, which is retained for nanolayers doped with palladium and disappears for nanolayers doped with gold and silver.     

Doping effect on the phase transition temperature in ferroelectric SrBi2−xNdxNb2O9 layer‐structured ceramics: a micro‐Raman scattering study

The temperature dependence of Raman spectra for SrBi_2−xNd_xNb₂O₉ ceramics (x from 0 to 0.2) has been studied in a wide temperature range from 80 to 873 K. It is found that the peak position of the A_1g[Nb] phonon mode at 207 cm^–1, which is directly associated with the distortion of NbO₆ octahedron, decreases with increasing Nd composition, while the A_1g[O] phonon mode at 835 cm^–1 increases. Moreover, both the peak position and intensity of the A_1g[Nb] phonon mode reveal strong anomalies around the ferroelectric to paraelectric phase transition temperature. It indicates that the phase transition temperature decreases from about 710 to 550 K with increasing Nd composition, which is due to the fact that the introduction of Nd ions in the Bi₂O₂ layers reduces the distortion extent of NbO₆ octahedron. Copyright © 2011 John Wiley & Sons, Ltd.     

氮化铌表面氧化物的XPS研究

本文用X射线光电子能谱(XPS)对室温下自然氧化和射频氧化的氮化铌薄膜表面进行了成分分析。指出:与在类似条件下纯铌膜的氧化不同,NbN膜的表面氧化物中不存在NbO和NbO₂,而是以Nb₂O₃作为从NbN到Nb₂O_5-y的过渡相。氮化铌在氧化过程中表现出一种抑制Nb₂O,NbO等低价传导性氧化物生成的能力,这对于用氧化物作势垒的超导隧道结的研制具有实际参考价值。 关键词：     

X-ray excited photoelectron spectra of LiNbO3: A quantitative analysis

X-ray excited photoelectron spectra (XPS) using monochromatized Al — K radiation ( =1,486.7 eV) have been analysed quantitatively. The intensities of the various core lines (Li(1s), O(1s), O(2s), Nb(3s), Nb(3p), Nb(3d), Nb(4s), Nb(4p)) normalized to the intensity of the Nb (3d _5/2)-line, show good agreement with those calculated with theoretical values for the photoabsorption cross section of the free atoms and the theoretical estimates of the dependance of the electron escape depth on the electron kinetic energy. From the energy loss spectra observed at the high binding energy side of the XPS core lines the energy loss function Im {–1/()} is calculated and compared to published optical data. This comparison yields directly the total electron escape depth as function of the electron energy over a large energy range.     

Electronic states near fermi level in superconductors La2 − x Sr x CuO4 by means of x-ray absorption spectra near Cu-K and La-L edges

The electronic states of La_{2? x} Sr _x CuO₄ with 0.00 ≤ x ≤ 0.20 are studied by means of X-ray absorption spectroscopy (XANES, EXAFS) near the K-edge of Cu²⁺ ion and the L-edges of La³⁺ ion. It is found that characteristic white lines occurring near L _II and L _III edges of La³⁺ ion show a slight energy shift depending on substituted Sr²⁺ ions, x and temperature. The white lines suggest that unoccupied high-density 5dπ and 5dδ bands of La³⁺ ion just above a Fermi level transform to a hybridized single band of 5dπδ at 78?K in the superconductors with x = 0.10, 0.16 and 0.20. On the other hand, the XANES spectra near the Cu-K edge including a pre-edge region do not depend on x and temperature in the region of 0.00 ≤ x ≤ 0.20. It is considered that there is no reconstruction of electronic states at the Fermi level in a Mott–Hubbard band gap between an O 2p valence band and a Cu 3d conduction band. The electronic states at the Fermi level are probably consisted of the unoccupied 5dπδ band and an empty charge-transfer 3d?⁹ L band at low temperature, bands of which occur in a band gap between a filled O 2p valence band and an unoccupied O 2p conduction band. The insulator–superconductor–metal transitions in La_{2? x} Sr _x CuO₄ are related to the 5dπδ and 3d?⁹ L bands and holes, which site at a top region of the O 2p valence band near the Fermi level produced by a substitution of La³⁺ with Sr²⁺ ions.     

Enhanced luminescence from spontaneously ordered Gd2O3:Eu based nanostructures

Nanostructured Gd₂O₃:Eu³⁺ and Li⁺ doped Gd₂O₃:Eu³⁺ thin films were prepared by pulsed laser ablation technique. The effects of annealing and Li⁺ doping on the structural, morphological, optical and luminescent properties are discussed. X-ray diffraction and Micro-Raman investigations indicate a phase transformation from amorphous to nanocrystalline phase and an early crystallization was observed in Li⁺ doped Gd₂O₃:Eu³⁺ thin films on annealing. AFM images of Li⁺ doped Gd₂O₃:Eu³⁺ films annealed at different temperatures especially at 973 K show a spontaneous ordering of the nanocrystals distributed uniformly all over the surface, with a hillocks (or tips) like self-assembly of nanoparticles driven by thermodynamic and kinetic considerations. Enhanced photoemission from locations corresponding to the tips suggest their use in high resolution display devices. An investigation on the photoluminescence of Gd_2−xEu_xO₃ (x=0.10) and Gd_2−x−yEu_xLi_yO₃ (x=0.10, y=0.08) thin films annealed at 973 K reveals that the enhancement in luminescence intensity of about 3.04 times on Li⁺ doping is solely due to the increase in oxygen vacancies and the flux effect of Li⁺ ions. The observed decrease in the values of asymmetric ratio from the luminescence spectra of Li⁺ doped Gd₂O₃:Eu³⁺ films at high temperature region is discussed in terms of increased EuO bond length as a result of Li⁺ doping.     

X-ray spectra and specific features of the structure of lithium-sodium cobaltite Li<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Na<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>CoO<Subscript>2</Subscript>

The electronic structure and specific features of the structure of nonstoichiometric cobaltite Li _x Na _y CoO₂ (x = 0.42, y = 0.36) have been studied comprehensively. The calculated multiplet for the lowspin state of the Co³⁺ ion agrees with the experimental spectra. It has been established using X-ray absorption spectra measured in the total photoelectric effect yield and total fluorescence yield modes that the Li _x Na _y CoO₂ cobaltite is stoichiometric with respect to the alkali metal near its surface and is defective inside. It has been demonstrated that the charge compensation in the case of an alkali metal deficit in LixNayCoO₂ is due to holes in O 2p states.     

Physicochemical and technological peculiarities of multicomponent oxide crystal growth from melt by modified Stepanov technique

The peculiarities of the modified Stepanov technique are considered. The advantages of the method for growth of oxide crystals from melts of complex compositions are shown. Crystallization conditions are given to obtain optically homogeneous Sr _x Ba_{1 − x} Nb₂O₆, BaWO₄, SrMoO₄, LiNbO₃, and Ca₃(Nb, Ga)₂Ga₃O₁₂ single crystals using dies of capillary type.