X-ray spectra and electronic structure of Li x NbO2 superconductor and other niobium oxide compounds

X-ray emission Nb and OK spectra and O1s total photoelectron yield spectra for superconductor Li_0.71NbO₂ and other niobium oxide compounds NbO, BaNb₄O₆, Ba₂Nb₅O₉, BaNb₅O₈. LiNbO₂, NbO₂, LiNbO₃ were investigated. It was found that in stoichiometric LiNbO₂, niobium oxidation degree is 3+. Under the lithium deintercalation down to Li_0.71NbO₂ composition, the niobium ions oxidation degree increases up to 3.3+. As more electrons are supplied by Nb to fill the O2p band, the degree of O2p-Nb4d hybridization is increased. Using the full-potential linear muffin-tin orbital method, the self-consistent band structure calculations of Li _x NbO₂ (x=1.0; 0.7; 0.5) compounds were carried out. Agreement between partial Nb4d and O2p densities of states and corresponding X-ray spectra was observed. For thex=0.7; 0.5 the Fermi level is placed on the slope of sharp density of states peak formed mostly by the Nb4d states. With the decrease ofx thep-d hybridization becomes stronger, although remains well below the values typical for the high-temperature superconductors.     

Precipitation of lamellar phases in H: LiNbO3 layers

Structural defects in a surface layer of the proton-exchanged waveguide layer formed on a substrate prepared from an X-cut lithium niobate single crystal are revealed and investigated using scanning electron microscopy and optical interference profilometry in combination with selective etching. It is established that these structural defects, which are oriented along the [0 15 1] crystallographic direction and matched to the matrix (the β phase of the H_xLi_{1 ? x} NbO₃ solid solution) through a network of misfit dislocations, consist of one of the H_xLi_{1 ? x} NbO₃ phases precipitated in the form of lamellar regions ～100 nm thick. Precipitation of lamellar phases precedes the destruction of the surface of the proton-exchanged layer. The presence of residual internal stresses in the LiNbO₃ initial crystal favors the formation of the lamellar phases.     

Formation of ferroelectric phases in KNbO<Subscript>3</Subscript> and other niobates with perovskite structure

The Curie-Weiss temperatures T _CW of NaNbO₃ and AgNbO₃ are determined for the first time by extrapolating the temperature dependence of the reciprocal permittivity from the cubic phase. It is established that the values of T _CW of ANbO₃ perovskites (A = Na, Ag, K) are practically equal, i.e., that the cubic phases of all the compounds are potentially unstable to an almost equal extent with respect to polarization. From an analysis of the EXAFS spectra of the NaNbO₃ and KNbO₃ cubic phases, it follows that the potential-energy surfaces of the Nb cation in both compounds are similar and exhibit eight minima shifted by 0.16 and 0.19 Å, respectively, from the oxygen-octahedron center along the [111]-type directions. Based on these facts, it is inferred that the ANbO₃ instability with respect to the occurrence of polarization is due to the tendency of the Nb cations to be ordered over the eight minima and that the phase transitions induced by this instability are of the order-disorder type. A statistical model capable of describing the sequence of ferroelectric phase transitions occurring in KNbO₃ and in (Ba, Sr)TiO₃ is proposed and studied.     

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.     

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.     

Characterization of Fe–Nb sputtered thin films

Structure, composition and chemical behavior of co-sputtered Fe–Nb thin films are analyzed by different techniques, as conversion electron Mössbauer spectroscopy, x-ray photoelectron spectroscopy and x-ray diffraction. It is shown that oxygen is determinative in hindering the Fe–Nb alloy formation and, as a result, Fe_1−xO and Nb₂O₅ occur in significant amounts, even in vacuum. In spite of the oxygen role, a Fe–Nb alloy is formed in little amounts, which increase as the Nb content is increased. The increase of the Nb content is also related with the increase of Fe_1−xO and with a decreasing of the metallic Fe present. Mössbauer data indicate the Fe–Nb phase present is the Fe₂Nb Laves phase.     

Transport properties of Pr0.7Ca0.3MnO3/Nb:SrTiO3 heterojunctions

Current–voltage (J–V) characteristics of epitaxial hetero-junctions composed of Pr_0.7Ca_0.3MnO₃ and Nb:SrTiO₃ were studied under forward and reversed bias conditions. Detailed analysis showed that the J–V characteristics of these heterojunctions can be well-fitted by the thermally-assisted tunnelling model. While the dielectric constant of Nb:SrTiO₃ extracted under the forward bias was about one order of magnitude smaller than that of bulk SrTiO₃, the value obtained under reverse bias was very close to that of the bulk SrTiO₃. The result can be explained by the existence of an interface layer on the Nb:SrTiO₃ substrate with a smaller effective dielectric constant. The current finding suggested that the properties of interface layer should be taken into account in order to accurately simulate the J–V characteristics of such heterojunctions.     

An x-pes study of the semiconductor-metal transition in NbO2

Marked changes in the Nb4d band in the X-ray photo-electron spectrum accompany the semiconductor-metal transition in NbO₂. The Nb4d band progressively shifts towards the O2p band with increase in temperature.     

Nb 3d and O 1s core levels and chemical bonding in niobates

A set of available experimental data on binding energies of Nb 3d_5/2 and O 1s core levels in niobates has been observed with using energy difference (O 1sNb 3d_5/2) as a robust parameter for compound characterization. An empirical relationship between (O 1sNb 3d_5/2) values measured with XPS for Nb⁵⁺-niobates and mean chemical bond length L(NbO) has been discussed. A range of (O 1sNb 3d_5/2) values possible in Nb⁵⁺-niobates has been defined. An energy gap ∼1.4–1.8 eV is found between (O 1sNb 3d_5/2) values reasonable for Nb⁵⁺ and Nb⁴⁺ states in niobates.     

Dependences of the refractive indices on the proton concentration in H:LiNbO3 waveguides

Proton-exchanged optical waveguides in lithium niobate crystals exhibit a rich variety of structures and phases. It is established that seven H_xLi_1−x NbO₃ crystalline phases with a lithium niobate structure may form under various conditions of proton exchange and post-exchange annealing. A method is proposed to determine the proton concentration in the various phases identified. Relationships are established between the structural parameters, the proton concentration, and the ordinary and extraordinary refractive indices of various H_xLi_1−x NbO₃ crystalline phases. The results can explain various optical phenomena observed in proton-exchanged waveguides and permit prediction of the characteristics of light-guide structures. Zh. Tekh. Fiz. 69, 47–57 (March 1999)     

Structural and electrochemical characterization of mesoporous thin films of Nb2xV2 − 2xO5 upon lithium intercalation

Nb_2xV_2 ? 2xO₅ (0 ≤ x ≤ 1) powders were prepared by a synthetic route based on the inorganic polymerization of alkoxy-choride precursors and characterized by a combination of X-ray diffraction, ⁵¹V and ⁹³Nb NMR and Raman spectroscopy. Amorphous mesoporous thin films of similar compositions were successfully prepared by a modified Evaporation Induced Self Assembly method using polystyrene-b-polyethyleneoxide diblock copolymer as structuring agent. The electrochemical properties of the mesoporous films upon lithium insertion–deinsertion are investigated by cyclic voltammetry. This study highlights the advantages of such nanoarchitecture in terms of increased capacity to insert lithium.     

KTN晶体及其熔体结构的高温拉曼光谱研究

测量并研究了不同温度(室温—1573 K)范围内KTN晶体的拉曼光谱及其熔体的高温拉曼光谱,分析了KTN晶体结构随温度变化的规律及其熔体的结构特征.随着温度的升高,KTN晶体的拉曼光谱谱峰都不同程度地向低波数方向移动,同时存在不同程度的展宽,并伴随强度的减弱.观察并解释了温度353 K附近KTN晶体样品的四方—立方转相现象.研究了KTN晶体拉曼光谱中538cm^-1,585cm^-1,835cm^-1和877cm^-1谱峰及其 关键词： 高温拉曼光谱 熔体 KTN晶体     

Synthesis and physicochemical properties of Li2 Me x Zr1 − x O3 − δ (Me = Nb,Ti; x = 0.05, 0.1) solid solutions

Complex lithium metallates Li₂ Me _x Zr_{1 ? x} O_{3 ? δ} (Me = Nb, Ti, x = 0.05, 0.1) with iso-and heterovalent substitutions for Zr⁴⁺ ions in lithium zirconate are synthesized for the first time using a citrate technique. The inclusion of Ti⁴⁺ and Nb⁵⁺ ions in the crystal structure of Li₂ZrO₃ is confirmed by means of X-ray diffraction and NMR. It is shown that in the temperature range of 750–820 K, Li₂Ti_0.1Zr_0.9O₃ solid solution has higher conductivity than phases of undoped lithium zirconate.     

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.     

The ionic conductivity of Li6BaLa2M2O12 with coexisting Nb and Ta on the M sites

In a view to balancing cost and lithium ion conductivity, Li₆BaLa₂Nb _x Ta_2???x O₁₂ (x?=?0–2) was prepared by solid-state reaction, and its corresponding AC impedances were tested at temperatures ranging from 20 to 250 °C in air. Li₆BaLa₂Ta₂O₁₂ exhibits the highest conductivity, 8.77?×?10^?6?S/cm, and the second highest is Li₆BaLa₂Nb₂O₁₂ with 6.69?×?10^?6?S/cm. Partial replacement of Ta with Nb cannot bestow the advantages of cost saving or the enhancement of lithium ion conductivity. X-ray diffraction patterns revealed a gradual change as an increasing amount of Nb replaces Ta in Li₆BaLa₂Nb _x Ta_2???x O₁₂ (x?=?0–2), and it is thought that the trending of Nb and Ta to rest on the crystallographic planes is different.     

Metastable phases in proton-exchanged waveguides on an X cut of lithium niobate

The formation and decomposition of high-temperature phases in proton-exchanged waveguide layers of H_xLi_{1 ? x} NbO₃ on an X cut of lithium niobate are studied using IR spectroscopy and x-ray diffraction. It is shown that the phase transitions that occur in the proton-exchanged layers between the high-temperature phases fixed by quenching from T = 200°C and the phase that is equilibrium at room temperature are accompanied by changes in the frequencies and integrated intensities of the spectral components of the absorption bands characterizing OH groups and hydrogen bonds in the crystal. These changes imply two paths of proton redistribution during these completely reversible phase transitions: (i) transfer of some protons from substitutional sites to interstitial sites and (ii) proton transfer between substitutional sites having different ionic environments. The phase transition from the phase that is equilibrium at room temperature into a high-temperature phase is found to be accompanied by a more than twofold increase in strain in the H_xLi_{1 ? x} NbO₃ layer lattice (x ≈ 0.50). The experimental data obtained confirm complete reversibility and the diffusionless character of these phase transitions.     

Determination of the local structure of NbO6 octahedra in the orthorhombic phase of a KNbO3 crystal using EXAFS

A new method proposed by us to determine the displacement direction of B atoms from centrosymmetric positions in ABO₃ crystals is used to study the local atomic structure of KNbO₃ in the orthorhombic phase. It is shown that the conventional treatment of the EXAFS yields serious errors in determinations of the local distortions of NbO₆ octahedrons. To eliminate these errors, it is suggested that diffraction data on the average atomic displacements should be combined with the results of direct calculations of the Nb K-EXAFS of KNbO₃ in the orthorhombic phase. This approach was used to establish that the preferential direction of displacement of Nb in the orthorhombic phase of KNbO₃ is in the direction of the polar twofold axis. Fiz. Tverd. Tela (St. Petersburg) 40, 1097–1101 (June 1998)     

A study of threshold switching of NbO2 using atom probe tomography and transmission electron microscopy

Threshold switching is a phenomenon where the resistivity of an insulating material changes and the insulator exhibits metallic behavior. This could be explained by phase transformation in oxide materials; however, this behavior is also seen in amorphous insulators. In this study, through an ex-situ experiment using transmission electron microscopy (TEM), we proved that threshold switching of amorphous NbO₂ accompanies local crystallization. The change in I–V characteristics after electroforming was examined by evaluating the concentration profile. Atom probe tomography (APT) combined with in-situ TEM probing technique was performed to understand the threshold switching in amorphous NbO₂. The local crystallization in amorphous NbO₂ was validated by the observed difference in time-of-flight (ToF) between amorphous and crystalline NbO₂. We concluded that the slower ToF of amorphous NbO₂ (a-NbO₂) compared with crystalline NbO₂ (c-NbO₂) is due to the resistivity difference and trap-assisted recombination.     

Physical investigation of the mechanisms of lithium insertion in sulfides as anode materials for lithium-ion batteries

Electrochemical and chemical lithium insertion (vian-butyllithium) were carried out on a family of spinel phases of the Cu₂S-In₂S₃-SnS₂ system. Electrochemical measurements have revealed great reversible capacities and the possible use of these materials as negative electrodes in lithium-ion batteries.¹¹⁹Sn Mössbauer spectroscopy,⁷Li NMR and S K edge EXAFS spectroscopy were carried out to characterize the mechanisms of lithium insertion. This study has shown the formation of lithium-metal alloys finely dispersed in an amorphous lithium sulfide matrix at low potentials. A model is proposed to explain the great capacities observed in these materials.     

Lithium in In2S3

The electrochemical insertion of lithium into β-In₂S₃ at different concentrations has been investigated by means of γγ-Perturbed Angular Correlations measurements using implanted radioactive ¹¹¹In nuclei. The study of the hyperfine interactions of the ¹¹¹Cd nuclei during the decay ¹¹¹In(EC)¹¹¹Cd allowed to determine the evolution of the three observed electric field gradients at different temperatures and lithium contents. This study reveals a great difference of behavior between the three indium sites of the structure, and shows the mobility of indium ions and their partial reduction after lithium insertion. In₂S₃ is good model material to study the properties of more complex materials based on this structure, which may be used as electrodes for lithium-ion batteries. This revised version was published online in September 2006 with corrections to the Cover Date.