XAS investigation of tantalum and niobium in nanostructured TiO2 anatase

Sol-gel routes were used to prepare Ta 10 at% and Nb 5 at% and 10 at% doped titania nanosized powders. When fired between 410°C and 850°C the doped titania powders are in the anatase phase; further heating up to 1050°C is required to obtain the rutile phase. The presence of dopant atoms delays the rate of transformation as compared with pure titania powders. Doping also affects the rate of grain growth and increases the conductance response to gas. To better understand the role played by dopant atoms in inhibiting both phase transformation to rutile and grain growth, X-ray Absorption Spectroscopy measurements were performed at the L_III-L_I absorption edges of Ta and Nb K absorption edge. Analysis was restricted to the anatase phase because the transformation to rutile phase, obtained by firing at 1050°C, is accompanied by the formation of undesired Ta and Nb oxides (Ta₂O₅ and Nb₂TiO₇, respectively). Extended X-ray Absorption Fine Structure and X-ray Absorption Near-Edge Spectroscopy analysis results indicate that in nanostructured anatase both tantalum and niobium atoms substitute Ti cations with +5 valence state.     

Preparation,crystalline properties and X-ray absorption spectra of rare earth oxides ROx (R ≡ Ce,PrandTb; 1.5 ⩽ x ⩽ 2)

High purity rare earth (R ≡ Ce, Pr, Tb) sesquioxides, intermediate oxide and dioxide powders and thin flakes were prepared using different methods. Their crystalline properties were controlled by X-ray and electron diffraction. The X-ray absorption spectra (M_IV–V and L_III edges) have been used to determine the rare earth valence in the samples. From R₂O₃ to RO₂ the absorption edges show noticeable changes, indicating the evolution of the valence. The L_III and M_IV–V spectra respectively give information on the sd and 4f symmetry vacant states. The M_IV–V spectra of a given RO_x show that inhomogeneous valence can be exactly deconvoluted into suitably weighted R₂O₃ and RO₂ contributions according to the chemical formula. In the case of L_III edges, remarkable discrepancies in the deconvolution results are observed. This behaviour may be correlated with the change in the sd density of states due to oxygen vacancy ordering in the phase stability region of RO_x.     

Use of X-Ray Absorption Spectra as a “Fingerprint” of the Local Environment in Complex Chalcogenides

The local environment of tin, titanium, iron, and sulfur in spinel compounds Cu₂FeSn₃S₈ and Cu₂FeTi₃S₈ was studied by X-ray absorption spectroscopy (XAS) at the titanium, iron, sulfur K edges, and the tin L_I-edge. As detailed calculations of the electronic structure of these compounds are difficult to carry out due to the large number of atoms contained in the unit cell, the XAS spectra of the spinels are compared to those of relatively simple binary sulfides like SnS₂, TiS₂, and FeS. Indeed, the metal environments in these binary compounds are very similar to those in the spinels, and they can be considered good model compounds allowing the interpretation of electronic transitions observed in the spectra of quaternary phases. In the latter, the bottom of the conduction band is mainly formed by Sn 5s–S 3p, Sn 5p–S 3p antibonding states for the tin-based compounds and by Ti 3d_t2g–S 3p, Ti 3d_eg–S 3p antibonding states for the titanium-based compounds. It it shown that the local environment of iron atoms remains unchanged when substituting tin with titanium atoms, according to a topotactic substitution.     

Electron-doping through La-for-Sr substitution in (Sr1−xLax)2FeTaO6: Effects on the valences and ordering of the B-site cations, Fe and Ta

We have employed aliovalent A-site cation substitution, La^III-for-Sr^II, to dope the Sr(Fe_0.5Ta_0.5)O₃ perovskite oxide with electrons. Essentially single-phase samples of (Sr_1−xLa_x)(Fe_0.5Ta_0.5)O₃ were successfully synthesized up to x≈0.3 in a vacuum furnace at 1400 °C. The samples were found to crystallize (rather than with orthorhombic symmetry) in monoclinic space group P2₁/n that accounts for the partial ordering of the B-site cations, Fe and Ta. With increasing La-substitution level, x, the degree of Fe/Ta order was found to increase such that the La-richest compositions are best described by the B-site ordered double-perovskite formula, (Sr,La)₂FeTaO₆. From Fe L₃ and Ta L₃ XANES spectra it was revealed that upon electron doping the two B-site cations, Fe^III and Ta^V, are both prone to reduction. Magnetic susceptibility measurements showed spin-glass type behaviour for all the samples with a transition temperature slightly increasing with increasing x.     

Detection of nanocrystallinity by X-ray absorption spectroscopy in thin film transition metal/rare-earth atom,elemental and complex oxides

Nanocrystallinity has been detected in the X-ray absorption spectra of transition metal and rare-earth oxides by (i) removal of d-state degeneracies in the (a) Ti and Sc L₃ spectra of TiO₂ and LaScO₃, respectively, and (b) O K₁ spectra of Zr(Hf)O₂, Y₂O₃, LaScO₃ and LaAlO₃, and by the (ii) detection of the O-atom vacancy in the O K₁ edge ZrO₂–Y₂O₃ alloys. Spectroscopic detection is more sensitive than X-ray diffraction with a limit of ∼2 nm as compared to >5 mm. Other example includes detection of ZrO₂ nanocrystallinity in phase-separated Zr(Hf) silicate alloys.     

The ionic conductivity and local environment of cations in Bi9ReO17

The influence of temperature on the structure of Bi₉ReO₁₇ has been investigated using differential thermal analysis, variable temperature X-ray diffraction and neutron powder diffraction. The material undergoes an order-disorder transition at ∼1000 K on heating, to form a fluorite-related phase. The local environments of the cations in fully ordered Bi₉ReO₁₇ have been investigated by Bi L_III- and Re L_III-edge extended X-ray absorption fine structure (EXAFS) measurements to complement the neutron powder diffraction information. Whereas rhenium displays regular tetrahedral coordination, all bismuth sites show coordination geometries which reflect the importance of a stereochemically active lone pair of electrons. Because of the wide range of Bi-O distances, EXAFS data are similar to those observed for disordered structures, and are dominated by the shorter Bi-O bonds. Ionic conductivity measurements indicate that ordered Bi₉ReO₁₇ exhibits reasonably high oxide ion conductivity, corresponding to 2.9×10⁻⁵ Ω⁻¹ cm^-1 at 673 K, whereas the disordered form shows higher oxide ion conductivity (9.1×10⁻⁴ Ω⁻¹ cm⁻¹ at 673 K).     

Electronic and optical properties of magnesium phthalocyanine (MgPc) solid films studied by soft X-ray excited optical luminescence and X-ray absorption spectroscopies

In a study to link the optical and structural properties of solid films of magnesium Phthalocyanine (MgPc), a range of synchrotron based spectroscopic methods have been used. These include X-ray excited optical luminescence (XEOL) together with X-ray absorption spectroscopy (XAS) measured both by total electron yield methods (TEY) and by using the optically detected photoluminescence yield method (PLY). XEOL spectra below K shell threshold show a broad emission peak at approximately 860 nm which can be attributed to the optical Q-band of these organic systems, which is then suppressed above the threshold. The shift to higher wavelength compared to optical emission spectra from MgPc in solution is consistent with intermolecular coupling of the excited states in the loosely intermolecular bonded phthalocyanine crystal structure. Zero order total PLY spectra at both C and N K edges are compared to TEY spectra where at the C K edge an inversion of intensity ratios between features is observed. Wavelength-specific PLY absorption spectra taken at 860 nm at the N K edge show a role for sigma* states participating in the luminescence process possibly through the sigma-like lone pair of bridging nitrogen atom, denoted the n --> pi* transition.     

Electronic structure of the spinel Li4Ti5O12 studied by ab initio calculations and X-ray absorption spectroscopy

The electronic structure of the spinel compound Li₄Ti₅O₁₂, used as anode material in Li-ion batteries, is studied both theoretically and experimentally. The partial densities of occupied and vacant s, p and d electronic states were calculated for Li, Ti and O, using the linear augmented plane wave formalism (LAPW) in order to obtain information on the chemical bond. X-ray absorption spectra were recorded at the Ti L₂₃, Ti K and O K absorption edges and calculated within the dipolar approximation by considering both the electronic ground state and the effect of core holes. The atomic origin of the observed main peaks is analysed from the comparison between the experimental and calculated spectra. We show that the two types of lithium in Li₄Ti₅O₁₂ which are found in tetrahedral and octahedral sites could be distinguished by X-ray absorption spectroscopy. We suggest to use such a technique for the investigation of Li-insertion mechanisms in lithium titanate spinels.     

Core-Level Spectroscopy of 2-Thiouracil at the Sulfur L1- and L2,3-Edges Utilizing a SASE Free-Electron Laser

In this paper, we report X-ray absorption and core-level electron spectra of the nucleobase derivative 2-thiouracil at the sulfur L₁- and L_2,3-edges. We used soft X-rays from the free-electron laser FLASH2 for the excitation of isolated molecules and dispersed the outgoing electrons with a magnetic bottle spectrometer. We identified photoelectrons from the 2p core orbital, accompanied by an electron correlation satellite, as well as resonant and non-resonant Coster–Kronig and Auger–Meitner emission at the L₁- and L_2,3-edges, respectively. We used the electron yield to construct X-ray absorption spectra at the two edges. The experimental data obtained are put in the context of the literature currently available on sulfur core-level and 2-thiouracil spectroscopy.     

Biotransformations of Anticancer Ruthenium(III) Complexes: An X‐Ray Absorption Spectroscopic Study

An anti‐metastatic drug, NAMI‐A ((ImH)[Ru^IIICl₄(Im)(dmso)]; Im=imidazole, dmso=S‐bound dimethylsulfoxide), and a cytotoxic drug, KP1019 ((IndH)[Ru^IIICl₄(Ind)₂]; Ind=indazole), are two Ru‐based anticancer drugs in human clinical trials. Their reactivities under biologically relevant conditions, including aqueous buffers, protein solutions or gels (e.g, albumin, transferrin and collagen), undiluted blood serum, cell‐culture medium and human liver (HepG2) cancer cells, were studied by Ru K‐edge X‐ray absorption spectroscopy (XAS). These XAS data were fitted from linear combinations of spectra of well‐characterised Ru compounds. The absence of XAS data from the parent drugs in these fits points to profound changes in the coordination environments of Ru^III. The fits point to the presence of Ru^IV/III clusters and binding of Ru^III to S‐donor groups, amine/imine and carboxylato groups of proteins. Cellular uptake of KP1019 is approximately 20‐fold higher than that of NAMI‐A under the same conditions, but it diminishes drastically after the decomposition of KP1019 in cell‐culture media, which indicate that the parent complex is taken in by cells through passive diffusion.     

Valence fluctuation in some Yb intermetallics by X-ray photoemission and X-ray absorption

The valence state of Yb in some of its intermetallics, YbNi₂Ge₂, YbCu₂Si₂ and YbPd₂Si₂ has been investigated by L_III(Yb) absorption edges and X-ray pnotoelectron spectra in the 4f and 4d regions. These studies establish the presence of mixed valence in all three systems and illustrate the utility of 4f and 4d spectra in the study of mixed valence in Yb compounds.     

Ba3MIIITiMVO9 (MIII = Fe,Ga, Y,Lu; MV = Nb,Ta, Sb) perovskite oxides: Synthesis,structure and dielectric properties

We describe the synthesis, structures and dielectric properties of new perovskite oxides of the formula, Ba₃M^IIITiM^VO₉, for M^III = Fe, Ga, Y, Lu and M^V = Nb, Ta, Sb. While M^V = Nb and Ta oxides adopt disordered/partially ordered 3C perovskite structures where M^III/Ti/M^V metal-oxygen octahedra are corner-connected, the M^V = Sb oxides show a distinct preference for the 6H structure, where Sb^V/Ti^IV metal-oxygen octahedra share a common face forming (Sb,Ti)O₉ dimers that are corner-connected to the M^IIIO₆ octahedra. The preference of antimony oxides (Sb^V:4d¹⁰) for the 6H structure – which arises from a special Sb^V–O chemical bonding that tends to avoid linear Sb–O–Sb linkages unlike Nb^V/Ta^V:d⁰ atoms which prefer ～180° Nb/Ta–O–Nb/Ta linkages – is consistent with the crystal chemistry of M^V–O oxides in general. The dielectric properties reveal a significant difference among M^III members. All the oxides with the 3C structure excepting those with M^III = Fe show a normal low loss dielectric behaviour with ε = 20–60 in the temperature range 50–400 °C; the M^III = Fe members with this structure (M^V = Nb, Ta) display a relaxor-like ferroelectric behaviour with large ε values at frequencies ≤1 MHz (50–500 °C).     

Structural evolution of the KTa0.6Nb0.4O3 alkoxide-based solutions: probing the transition metals local environment by X-ray absorption spectroscopy

The KTa_0.6Nb_0.4O₃ sols for chemical solution deposition of thin films were prepared from potassium acetate and transition metal ethoxides by the 2-methoxyethanol based route. The local environment of both transition metals after reflux times 1, 4, 24, and 48 h, whereby the crystallization behavior of the films was strongly affected, was monitored by extended X-ray absorption fine structure spectroscopy, at Ta L₃ and Nb K edges. The Ta species existed in the sols as monomers, remaining stable even with prolonged reflux time. The Ta–O–K correlations were confirmed in all cases. In contrast, the Nb-alkoxide formed dimers, with a gradual formation of oligomeric species with prolonged refluxing. The Nb–O–K correlations were present after all reflux times. The number of K neighbours around Nb increased upon refluxing, saturating at 24 h.     

Unoccupied electronic states in graphite oxides

Unoccupied electronic states of graphite oxides with different oxidation times have been investigated using X-ray absorption spectroscopy (XAS) in total electron yield (TEY) and total fluorescence yield (TFY) modes. The graphite oxides exhibit graphite-like sp² states with additional electronic states originating from functional groups. The intensity of the π^* state decreases as the atomic ratio O:C increases. The intensity ratio of carboxyl to π^* states in XAS TEY mode corresponds to the area ratio of carboxyl to sp² carbon in X-ray photoelectron spectroscopy. In addition, the two modes in XAS can identify electronic states of functional groups originating from surface or bulk regions.     

Zr/Ti摩尔比对锶锆钛复合氧化物在可见光下光催化性能的影响

通过分步沉积法制备了不同Zr/Ti 摩尔比的锶锆钛(SZT)复合氧化物催化剂, 以X射线衍射(XRD)、扫描电镜(SEM)、透射电镜(TEM)、紫外-可见(UV-Vis)漫反射光谱等表征手段考察不同Zr/Ti 摩尔比下SZT催化剂的结构形态, 以可见光下光催化降解亚甲基蓝为模型反应考察样品的光催化活性. 结果表明: Zr/Ti 摩尔比<1 时SZT催化剂发生Zr⁴⁺与Ti⁴⁺同质替换, 引起晶格缺陷, 光催化活性小幅提高; Zr/Ti 摩尔比≥1 时SZT催化剂产生SrZrO₃/Ti_nO_2n-1 (n=4, 9)的新晶相, Ti_nO_2n-1 (n=4, 9)的存在有利于光生电子-空穴的传导与分离, 可大幅提高催化剂光催化活性. 其中, SZT-5/5 表现出最高的光催化活性, 其一级反应速率常数达到0.2133 min^-1, 是同等光照条件下纯SrTiO₃样品(0.0158 min^-1)的13.5倍.     

Quantitative surface analysis by total electron yield

When the surface of a solid sample is irradiated under vacuum by x-rays an electron emission, owing to photoabsorption, can be measured. As the electrons are detected under neglection of their kinetic energies the total electron yield (TEY) is determined. With a tuneable x-ray monochromator the TEY is measured below and above of one of the absorption edges of a given element. A jumplike increase of the TEY signal, due to the additional photoabsorptions in the corresponding atomic level, can be observed – qualitative analysis. The height of this jump can be correlateted to the concentration – quantitative analysis. It can be shown by a fundamental parameter approach for primary and secondary excitations how to use TEY for a quantitative analysis. The information depth of this new method is approximately 2–400 nm depending on the chemical elements and on the original kinetic energies of Auger and photoelectrons. Thus, TEY is located between photoelectron spectrometry and x-ray fluorescence analysis.     

Reactions of 2‐Substituted Pyridines with Titanocenes and Zirconocenes: Coupling versus Dearomatisation

Reactions of group 4 metallocene sources with 2‐substituted pyridines were investigated to evaluate their coordination type between innocent and reductive dearomatisation as well as to probe the possibility for couplings. A dependence on the cyclopentadienyl ligands (Cp, Cp*), the metals (Ti, Zr), and the substrates (2‐phenyl‐, 2‐acetyl‐, and 2‐iminopyridine) was observed. While 2‐phenylpyridine is barely reactive, 2‐acetylpyridine reacts vigorously with the Cp‐substituted complexes and selectively with their Cp* analogues. With 2‐iminopyridine, in all cases selective reactions were observed. In the isolated [Cp₂Ti], [Cp₂Zr], and [Cp*₂Zr] compounds the substrate coordinates by its pyridyl ring and the unsaturated side‐chain. Subsequently, the pyridine was dearomatised, which is most pronounced in the [Cp*₂Zr] compounds. Using [Cp*₂Ti] leads to the unexpected paramagnetic complexes [Cp*₂Ti^III(N,O‐acpy)] and [Cp*₂Ti^III(N,N′‐impy)]. This highlights the non‐innocent character of the pyridyl substrates.     

BATHOPRODUCTS and CONFORMERS OF ALL-trans-AND 13-ds BACTERIORHODOPSIN AT 90 K*

Abstract– At 90 K the photoproduct of the primary light reaction of (rani-bacteriorhodopsin, the bathoproduct K¹ consists of a mixture of at least three spectrally different species, K¹_I, K¹_II, and K¹_III having maxima in the difference absorption spectra at 645, 635 and 625 nm, respectively. The bathoproducts differ in their long wavelength absorption bands and in their rate constants for photo-conversion to trans-bacteriorhodopsin under far red light irradiation (λ > 720 nm). The bathoproducts are formed from different precursors–conformers of trans-bacteriorhodopsin, which are stable at 90 K, but are in equilibrium with each other at 213 K. We suggest that the bathoproducts may initiate parallel conversion cycles of trans-bacteriorhodopsin at low temperatures. The primary photoreaction of 13-cis-bacteriorhodopsin also yields three bathoproducts, K^c_I, K^c_II and K^c_III having maxima in the difference absorption spectra at 615, 605 and 595 nm, respectively.     

Investigation of the amorphous to crystalline phase transition of chemical solution deposited Pb(Zr0.3Ti0.7)O3 thin films by soft X-ray absorption and soft X-ray emission spectroscopy

Chemical solution deposited (CSD) complex oxide thin films attract considerable interest in various emerging fields as for example, fuel cells, ferroelectric random access memories or coated conductors. In the present paper the results of soft-X-ray spectroscopy between 280 and 560 eV on the amorphous to crystalline phase transition of ferroelectric Pb(Zr_0.3Ti_0.7)O₃ (PZT) thin films are presented. Five CSD samples derived from the same wafer coated with a PZT film pyrolyzed at 350 °C were heat treated at different temperatures between 400 and 700 °C. At first the samples were morphologically and electrically characterized. Subsequently the soft-X-ray absorption and emission experiments were performed at the undulator beamline 8.0 of the Advanced Light Source of the Lawrence Berkeley National Laboratory. Soft-X-ray absorption spectra were acquired for the Ti L _2,3-, O K-, and C K-edge thresholds by using simultaneously the total electron yield (TEY) and total fluorescence yield (TFY) detection methods. For two samples, annealed at 400 and 700 °C, respectively, the resonant inelastic soft-X-ray spectroscopy (RIXS) was applied for various excitation energies near the Ti L-, O K-edges. We observed clear evidence of a rutile phase at untypically low temperatures. This rutile phase transforms into the perovskite phase upon increasing annealing temperature. These results are discussed in the framework of current microscopic models of the PZT (111) texture selection.     

Effect of Sn additions on the microstructure,mechanical properties,corrosion and bioactivity behaviour of biomedical Ti–Ta shape memory alloys

Ti–Ta and Ti–Ta–xSn shape memory alloys (SMAs) were produced successfully by microwave sintering. Tin element was added to Ti–Ta SMA with three different atomic percentages (0.37, 0.745 and 2.26 at.%). The influences of Sn addition on microstructure, transformation temperatures, mechanical properties, shape memory behaviours, corrosion resistance and bioactivity were investigated. It was found that the morphologies of the modified and unmodified Ti–Ta alloys contain a plate-like and needle-like structure, where the former structure is related to the β phase and the latter related to the α phase. The transformation temperatures of M _s to M _f with and without Sn additions were observed to be in the range of 389.65 to 355 K, while the transformation temperatures of A _s to A _f were found to be in the range of 250.42 to 365.8 K. By increasing the proportion of Sn, the compressive fracture strength and shape memory behaviour were enhanced; however, the corrosion resistance tended to be decreased. Based on the bioactivity results, antibacterial activity was improved with the addition of Sn. In conclusion, the existent results indicate that Ti–30 at.% Ta SMAs may be a convenient alternative to Ni–Ti for certain biomedical applications.