XAFS Measurements Research Articles

Thermodynamic and Structural Studies of Liquid and Amorphous Ge15Te85-XMX(M = Ag, Cu and In, andx=0 to 10)

The measurements of sound velocity and neutron scattering in the liquid state, and DSC and XAFS measurements in the amorphous state were carried out for Ge 15 Te 85 and Ge 15 Te 85- X M X (M = Ag, Cu and In) to investigate the correlation between the glass forming ability and local structure in the liquid and amorphous states. The results suggest that there exists Cu 2 Te like chemical short-range order in liquid and amorphous Ge 15 Te 80 Cu 5 . Similar results are obtained for liquid and amorphous Ge 15 Te 80 Ag 5 alloys. The local structure around In atoms is different from those of Ag and Cu atoms, which could be attributed to the difference in the chemical bonds between M and Te. In the Ge 15 Te 85- X M X (M = Ag, Cu and In) alloys, the structure of glasses and the glass forming ability would be closely related to the liquid structure and nature of the Te–M bonds.

In Situ XAFS Study of LiNi0.5Mn0.5O2 Cathode for Li Rechargeable Batteries.

The initial charge/discharge behavior of LiNi0.5Mn0.5O2 for a Li-ion battery has been examined by XRD, in situ XAFS and electrochemical methods. The LiNi0.5Mn0.5O2(R¯3 m:a=0.28999(7)nm, c=1.4330(7)nm in a hexagonal setting) was observed to retain its initial structure after charging to 4.5 V (vs. Li/Li+). XAFS measurements have also revealed that the majority of the nickel and manganese, respectively, exists in a divalent and a tetravalent state. Upon charging LiNi0.5Mn0.5O2, Ni2+ ions were consequently oxidized to Ni4+. A Li-ion cell constructed with LiNi0.5Mn0.5O2/graphite shows only a 15% capacity loss after 500 cycles at 60°C.

電池材料 放射光を用いたＬｉイオン二次電池用正極材料ＬｉＮｉ０．５Ｍｎ０．５Ｏ２の構造変化の研究

The cell performance of Li1-xNi0.5Mn0.5O2 Using Li exhibited a specific capacity of c.a. 150 mAh/g in the voltage range of 2.5 to 4.3 V. XRD and XAFS measurements using synchrotron radiation were carried out to observe the average and the local structural changes during a charging process. As the cell charged, Ni K-edge of XANES spectra showed the chemical shift towards higher energy in the voltage range of 2.5-4.1 V and then no remarkable shift up to 4.3 V. It was found that nickel of x=0 in Li1-xNi0.5Mn0.5O2 was located in divalent and manganese in tetravalent at the octahedral site. Lithium was removed by the oxidation of Ni, resulting in the ordering of cations in the layer, that is, appearance of the monoclinic phase at x=0.5.

Structural Change of Li1−xNi0.5Mn0.5O2 Cathode Materials for Lithium-ion Batteries by Synchrotron Radiation

Abstract Synchrotron X-ray diffraction and XAFS measurement have been employed to investigate structural change and the charging process of a layered LiNi0.5Mn0.5O2 cathode material. The structure of charged Li1−xNi0.5Mn0.5O2 (x=0.5), which corresponds to the composition for showing rechargeable capacity, was determined. The results showed that divalent nickel metal was oxidized to trivalent after charging, in association with the phase transition from hexagonal (R3-m) to monoclinic (C2/m) resulting from the ordering of cations in the layered structure.

Evidence for a new structure in a mixed metal sulphate system by EXAFS

AbstractEXAFS measurements on and above the CsLIII absorption edge were performed in LiCsSO4 (LCS) and in the system Li1.44Rb0.11Cs0.45SO4 (LRCS), aiming at the identification of the local symmetry around Cs ions in this new mixed metal sulphate LRCS. EXAFS simulations were also carried out and the results of the LRCS agrees well with recent single‐crystal x‐ray diffraction measurements, confirming that a new crystalline phase is expected for the mixed system LiCsSO4–LiRbSO4. Copyright © 2002 John Wiley & Sons, Ltd.

Promotion Effect of Ceria on Pd/Al2O3 Catalyst for the Hydrogenation of Carbon Monoxide to Dimethyl Ether.

Hydrogenation of CO was carried out over Pd catalyst supported on Al2O3, MgO, silica alumina, and silica gel supports. Addition of CeO2 increased the activity of Pd/Al2O3 and the main product was dimethyl ether (DME). DME formation proceeded via dehydration of methanol on the acid sites of the supports. However, the yield of DME was much higher over Al2O3 than silica alumina. Therefore, Al2O3 has an inherently high activity for methanol production. Temperature programmed reduction experiments showed that CeO2 on Al2O3 was reduced with H2, whereas CeO2 on MgO underwent little reduction as confirmed by a XAFS measurement. XAFS analysis indicated that Pd/CeO2/MgO contained Pd0 plus Pd2+ owing to the presence of the reduction-resistant CeO2 on MgO and the Pd on CeO2/Al2O3 was in a more reduced state although Pd2+ still remained. The results of CO adsorption measurement and XAFS and TEM observation revealed that an addition of CeO2 increased the dispersion of Pd metal particles on Al2O3. The intermediate strength interactions between Pd, CeO2, and Al2O3 seems to account for the higher activity of the Pd/CeO2/Al2O3 catalyst for O-containing compounds formation than Pd/CeO2/MgO.

X-ray absorption studies of high performance Low-k dielectric materials

ABSTRACTThe XAFS measurement of the MSQ type low-k dielectrics (LKD™) was conducted to clarify the structure change with and without the EB cure. Furthermore, three different types of other MSQ films, the ladder structure, the random structure and the CVD film, have been investigated as references. We have determined Si-O-Si bond angle and Si-O(Si-C) bond length by fitting the Fourier transformed EXAFS spectra.The ladder structure and the random structure have Si-O-Si bond angle of 133 and 146, respectively.Si-O-Si bond angle of LKD™ film is among that of the ladder and the random structure, and the XANES spectrum of LKD™ film displays two broad features, corresponding to the mixture of both structures. In contrast, Si-O-Si angles of the EB cured LKD™ film and the CVD film resemble each other, and the XANES features of both films are almost identical with that of the random structure.We have confirmed that the EB cure process for LKD™ film makes drastic structure change from the mixture of ladder and random structure to the random network structure.

In situ XAFS Studies on the Effects of the Hydrophobic–Hydrophilic Properties of Ti-Beta Zeolites in the Photocatalytic Reduction of CO2 with H2O

The photocatalytic reduction of CO2 with H2O to produce CH4 and CH3OH was found to proceed in the gas phase at 323 K with different reactivities and selectivities on hydrophilic Ti-Beta(OH) and hydrophobic Ti-Beta(F) zeolites prepared in the OH− and F− media, respectively. In situ XAFS measurements have revealed that these differences are attributed to the differences in the affinity for the adsorption of H2O molecules on the highly dispersed tetrahedrally coordinated titanium oxide species depending on the hydrophobic–hydrophilic properties of zeolites.

Microscopic parameters of heterostructures containing nanoclusters and thin layers of Ge in Si matrix

Ge K XAFS measurements have been performed using the total electron yield detection mode for pseudomorphous Ge films deposited on Si(0 0 1) substrate via molecular beam epitaxy at 300°C. The samples have been produced by thrice repeating the growing procedure separated by deposition of blocking Si layers at 500°C. The local microstructure parameters (interatomic distances, Ge coordination numbers) are linked to nanostructure morphology and adequate models are suggested and discussed. It was established that pseudomorphous 4-monolayer Ge films contain 50% of Si atoms on the average. Pyramid-like, pure Ge islands formed in the Stranski–Krastanov growth are characterized by the interatomic Ge–Ge distances of 2.41 Å (by 0.04 Å less than in bulk Ge) and the Ge–Si distances of 2.37 Å. It was revealed that the pure Ge nanoclusters are covered by a 1–2-monolayer film with admixture on the average of a 50% Si atom impurity from blocking Si layers.

Quantum chemical modelling of polarons and perovskite solid solutions

Following our previous study [J. Phys.: Condens. Matter 10 (1998) 6271] of a single Nb impurity and Nb clusters in KTaO 3, we present results of the calculations for a series of perovskite KNb x Ta 1− x O 3 (KTN) solid solutions ( x=0, 0.125, 0.25, 0.75, 1). The quantum chemical method of the intermediate neglect of the differential overlap (INDO) combined with the large unit cell (LUC) periodic model is used. According to the INDO calculations, Nb impurity becomes off-center in KTaO 3 already at the lowest studied concentrations ( x=0.125), in a good agreement with XAFS measurements. We compare our results with previous ab initio FP-LMTO calculations. Quantum chemical calculations confirm the existence of self-trapped electrons in KNbO 3; the corresponding lattice relaxation energy is 0.21 eV. We estimate the optical absorption energy to be 0.78 eV. An electron in the ground state occupies a t 2g orbital of the Nb 4+ ion. Its orbital degeneracy is lifted by a combination of the breathing and Jahn–Teller (JT) modes when four nearest equatorial O atoms are displaced by 1.4% a o outwards and two oxygens shift 1% inwards along the z axis.

Optically induced spin transition effect on structure of Fe(II) polymeric complexes by XAFS spectroscopy

Changes in the electronic and spatial structure of polymeric Fe(II) complexes with 1, 2, 4-triazoles and various anions upon spin transition was studied using XAFS spectroscopy. The compounds exhibit an abrupt thermally driven spin transition ( S=0–2) at temperatures varying from 175 to 400 K depending on the composition. Spin transition and structural changes were induced by variations of the anion, dilution with Zn, under the heating or the action of light. XAFS measurements were performed at the Budker Institute of Nuclear Physics in Novosibirsk. In all complexes, the spin transition is accompanied by drastic changes in the local environment of Fe atoms. High spin metastable long life states was detected and studied in the polymeric complex Fe(atrz) 3(ClO 4) 2. It was established that the changes in structure of polymeric complexes upon the transition to a metastable high spin state under the action of light differ from those in the thermally induced spin transition. Such difference is determined by mutual influence of Fe atoms in high spin and low spin states in polymeric chains.

Studies on intake of heavy metals by Bradybaena similaris, land snails, by XAFS measurement.

We have applied XAFS in order to determine both the chemical form and the place where heavy metals are stored in cultivated land snails. From Cu and Zn XANES spectra, the shells showed similar patterns as those of soft tissues and not like carbonates. This indicates that heavy metals are not completely taken into carbonate structures but are present within organic components in the shells. In addition, Cu XANES spectra of the samples showed low absorption edge-energy in the order of hepatopancreas, mantle, body, and shell. By comparing samples with standard reagents, each of which has only S- or O-ligand, it was found that the metals in hepatopancreas exist mostly as S-bound chemical components. To quantify the relative abundance of S-bound chemical component, partial least-squares (PLS) regression was applied. The PLS result indicated that for Cu, S-bound compound was higher in the order of hepatopancreas > mantle > body >shell.

Effect of the Promotion of Ceria on Pd/MgO Catalyst for Methanol Synthesis.

The hydrogenation of CO was carried out over Pd supported on MgO. The addition of various lanthanide oxides increased the activity of Pd/MgO to produce methanol. CeO2 exhibited the highest promoting effect at a temperature as low as of 250°C and the optimum molar ratio of Ce/Pd was about unity. TPR experiment showed that CeO2 on MgO (without Pd) suffered less reduction with hydrogen compared with that supported on Al2O3. Thus, the former retained the Ce(IV) state almost completely under the reducing condition, which was ascertained by XAFS measurement. XAFS analysis also indicated that Pd in Pd/MgO suffered reduction extensively while that in Pd/CeO2/MgO partly retained the oxidized state owing to the reduction-resistant CeO2 on MgO. The results of CO adsorption XPS, XAFS, and TEM measurement revealed that addition of CeO2 increased the dispersion of Pd metal particles. It was concluded that the high active of the catalyst resulted from the combination of highly dispersed Pd metal and partly oxidized Pd component with a Pd-O bond.

Apparatus for in situ x-ray absorption fine structure studies on catalytic systems in the energy range 1000 eV<E<3500 eV

A new apparatus for in situ x-ray absoprtion fine structure measurements in the medium energy range of 1000–3500 eV has been developed. Measurements can be performed in a gaseous environment (max. pressure 1 bar) at temperatures ranging from 80 to 750 K. Pre-treatments can be performed at 5 bar and 750 K in the same cell, after which XAFS measurements can be done without exposing the sample to ambient air. In a modular set-up several detector systems can be used: fluorescence detection using a gas proportional counter, a photodiode or a microstrip detector. All detectors are highly integrated into the cell, gaining solid angle for detection. Electron yield detection can be used simultaneously using conversion electron yield or total electron yield. The performance of the new apparatus is demonstrated by a study of the K edge of Al in Zeolite Beta. The Al content is as low as 2 wt%. It will be shown that octahedral framework Al is formed while adding gaseous water at room temperature after ammonia removal (at 450 °C) of an NH4-Beta.

ＳＰｒｉｎｇ‐８での構造研究 今何ができるか 原研ビームライン（ＢＬ１４Ｂ１）における高温高圧下Ｘ線回折実験

Current status of the high pressure research on BL14B1 JAERI beamline at the SPring-8 is presented. A cubic type multi-anvil high pressure device is currently used for an in situ X-ray diffraction studies of materials under high pressure and high temperature conditions. By switching the optics, either white or monochromatic X-rays is available for powder X-ray diffraction experiments with energy dispersive or angle dispersive methods as well as XAFS measurements.

XAFS measurements on zinc chloride aqueous solutions from ambient to supercritical conditions using the diamond anvil cell.

XAFS measurements on zinc chloride aqueous solutions from ambient to supercritical conditions using the diamond anvil cell.

Structural studies on colloidal tetraalkylammonium manganese oxides.

t. Introduction Mixed valent manganese oxides are of interest due to their diverse range of electrical, magnetic, and catalytic properties. Recently, novel colloids of manganese oxide have been prepared using tetraalkylammonium cations (allcyl = methyl, ethyl, propyl, and butyl) to prevent agglomerization (Brock et al., 1998). These materials are of interest due to unique optical properties associated with quantum confinement and their potential as starting materials for novel manganese oxide materials. In this work Mn K-edge XAFS studies were performed on colloidal MnOx samples prior and upon sol to gel transition and after different heat treatment procedures to reveal structural and electronic changes in the colloidal particles. Theoretical XAFS calculations known to yield reliable backscattering phases and amplitudes are applied in combination with suitable fitting routines to the analysis of colloidal manganese oxide systems. 2. Experimental section 2.1 Preparation of colloidal manganese oxides A detailed description of the synthesis of tetraalkylammonium (TAA) manganese oxide colloids appears elsewhere (Brock et al., 1999). The materials are prepared via reduction of tetraalkylanunonium permanganate salts (alkyl = ethyl, prowl) with 2-butanol in aqueous solutions. XAFS measurements have been obtained on the colloids as-prepared (TAA-A), with the residual 2-butanol removed (TAA-H), and on the gels (TAA-G). Measurements have been made on both tetraethylammonium (TEA) and tetrapropylammonium (TPA) manganese oxide colloids and gels with manganese concentrations from 0.1-0.4 M.

Soft X-ray XAFS Beamline for Advanced Materials Research at Compact Superconducting Ring

This paper describes a soft x-ray XAFS beamline constructed at the compact superconducting storage ring AURORA (575 MeV, 300 mA) at Ritsumeikan University. It consists of a pre-focusing torroidal mirror (located 4.5 m from the source point) and a double-crystal monochromator, 1:1 focusing geometry being adopted. Radiation of an energy in the range from 1000 to 4000 eV covered by four kinds of monochromator crystals can be used with a flux of 5×109 photons/sec/mm2 at the sample position. The beamline optics have been designed to get as high photon flux as possible from the compact ring. XAFS measurements can be made either in a transmission, total electron yield or fluorescent x-ray yield mode. The beamline is dedicated for advanced materials research but also for education of undergraduate students in the course of synchrotron radiation science.

The mechanism of copper activation of sphalerite

On the basis of recent SIMS and XAFS measurements in conjunction with already published XPS results, a mechanism for the adsorption/absorption of Cu onto sphalerite is proposed. Under conditions of high pH and high nominal surface coverage of the sphalerite by the Cu, Cu(OH) 2 colloidal particles are observed on the sphalerite surfaces using SIMS. Under other conditions, SIMS measurements have indicated that adsorption of the Cu is essentially uniform over the sphalerite surface and is not related to low coordination sites on the surface of the sphalerite. Depth profiling of sphalerite surfaces with Cu adsorbed under conditions that do not result in Cu(OH) 2 colloidal particles show that the Cu adsorbed/absorbed on the sphalerite surface is largely in the first few atomic layers. XAFS analysis of Cu activated sphalerite has indicated that the Cu occupies a distorted trigonal planar geometry, coordinated to three S atoms, in both surface and bulk sites. In addition Cu(1s), absorption edges in XAFS show that both bulk and surface adsorbed copper have an oxidation state less than +1 with the surface Cu being slightly more oxidised than the bulk absorbed Cu. On the basis of the combined XPS, SIMS, XAFS and solution studies, a model is proposed that, on surface adsorption of Cu, the surface Zn(II) atoms are replaced by Cu(II) atoms which are then reduced in situ to Cu(I). This reduction is accompanied by the oxidation of the three neighbouring S atoms to an oxidation state of approximately −1.5. On bulk absorption of Cu atoms into the sphalerite lattice a distorted trigonal planar configuration is achieved through the breakage of a formerly tetrahedral Zn–S bond. The breakage of this bond results in a 3-fold coordinated Cu plus one S 3-fold coordinated to Zn atoms. The breakage of this bond leads to a greater reduction of the Cu than on surface absorption and also oxidation of the 3-fold coordinated S atom to an approximately −0.5 oxidation state. This model does not invoke any polysulfite or S–S bonded species to explain the higher binding energy components of the S(2p) XPS spectra.

XAFS study of interchain and intrachain order inSe1−xTexglasses: Nearest neighbors

XAFS measurements (LURE and NSLS) of ${\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{Te}}_{x}$ glasses have been carried out on both the Se and Te $K$ edges at low and room temperatures. Using a multishell best-fit analysis procedure, we have reconstructed the Se and Te local environment: first-shell intrachain nearest neighbors $(\mathrm{Se}\ensuremath{-}{\mathrm{Se}}_{1}, \mathrm{Se}\ensuremath{-}{\mathrm{Te}}_{1}, \mathrm{Te}\ensuremath{-}{\mathrm{Se}}_{1},$ and $\mathrm{Te}\ensuremath{-}{\mathrm{Te}}_{1}).$ We conclude that the intrachain chemical order (preferential of Se-Te pairs) increases with Te content in the glasses. On the other hand, we suggest a random distribution of Se and Te atoms between the chains. The thermal disorder dominates for the first-shell pairs $\mathrm{Se}\ensuremath{-}{\mathrm{Se}}_{1}, \mathrm{Se}\ensuremath{-}{\mathrm{Te}}_{1} (\mathrm{Te}\ensuremath{-}{\mathrm{Se}}_{1}),$ and $\mathrm{Te}\ensuremath{-}{\mathrm{Te}}_{1}$ pairs. The obtained results provide insight on intrachain structure as well as on the structural and thermal disorder of mixed ${\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{Te}}_{x}$ glasses.