XANES Study Research Articles

Extension of the “1201” Family to Strontium-Rich Chromite and Ferrite, Bi 0.4Sr 2.5Cr 1.1O 4.9 and Bi 0.4Sr 2.5Fe 1.1O 5

Two new Sr-rich “1201”-type oxides, Bi 0.4Sr 2.5Cr 1.1O 4.9 and Bi 0.4Sr 2.5Fe 1.1O 5 have been synthesized. These compounds, intergrowths of double rock-salt layers with single perovskite layers, show a 1:1 ordering between (Bi, M) and Sr species within the intermediate rock-salt layer [Bi 0.4 M 0.1Sr 0.5O]. The XANES study shows that bismuth is mainly trivalent, whereas iron is mixed valent containing 50% Fe 3+ and 50% Fe 4+ (also confirmed by Mössbauer), and chromium could be a mixture of Cr 3+ and Cr 6+ sitting in the perovskite and rock-salt-type sites, respectively. Both compounds exhibit antiferromagnetic interactions. The Cr-phase is a strong insulator, whereas the Fe-phase exhibits a semi-conductor-like resistivity whose value at room temperature is close to that of isotypic cobaltite.

Fe and Mn K-edge XANES study of ancient Roman glasses

Abstract: The X-ray Absorption Near Edge Structure technique has been successfully applied to the study of ancient glasses fromPatti Roman Villa (Messina, Sicily). From the chemical point of view, the samples are ‘ low-magnesia’ glasses, with a compositiontypicaloftheRomanperiod.FluorescencedetectedFeandMnK-edgeXANESspectraandadetailedpre-edgeanalysisconfirmthatthe color of the glass fragments depends on the oxidation state of iron and suggest the deliberate addition of manganese oxide asdecolorant during the melting procedure. This work shows that X-ray Absorption Spectroscopy is a potentially very useful non-destructive technique that can be applied in archaeological studies on many different materials. Key-words: XANES, archaeological glasses, iron, manganese. Introduction It is well known that the color exhibited by glasses can bedeterminedbythe oxidationstateandthe electronic config-uration of the metal ions in them (Doremus, 1973; Hender-son,1985;Freestone,1991).Theseareusuallyelementsbe-longingtothetransitionrowoftheperiodictable,whichab-sorb characteristic frequencies of the visible region as a re-sult of d-d electronic transitions. In particular, ancient glas-sesoftencontainironatlevelsthatcanimpartatypicalinci-dental green coloration. To minimize this problem, fromaround the middle of the first millennium BC, substanceswere added which tended to neutralize the colorant effectsof the iron. Before the Roman period, antimony was themaindecolorant,whilefromthesecondcenturyBCmanga-nese become important(SayreSSayre,1963;Newton, 1978;Freestone,1991).Sincethecharacterizationof colorant and decolorant components is important in un-derstanding the manufacturing technique of the ancientglasses, we have applied X-ray Absorption Spectroscopy(XAS) withsynchrotronradiationtothe study of theoxida-tion state of iron and manganese in a number of glass sam-ples of archaeological interest, characterized by differentcolors (from green to pale brown to uncolored).The samples are fragments of perfume bottles of the 2

The chemistry of the oxychlorination catalyst: an in situ, time-resolved XANES study.

The chemistry of the oxychlorination catalyst: an in situ, time-resolved XANES study.

XANES study of polyaniline–V 2O 5 and sulfonated polyaniline–V 2O 5 nanocomposites

In this work, two materials for secondary lithium battery cathodes formed by polyaniline–V 2O 5 and sulfonated polyaniline–V 2O 5, which have a higher charge capacity than the V 2O 5 xerogel, were synthesized. X-ray absorption and Fourier transform infrared spectroscopies were employed to analyze the short-range interactions in these materials. Based on these experiments, it was possible to observe significant differences in the symmetry of the VO 5 units, and this was attributed to the intimate contact between V 2O 5 and the polymers, and to some flexibility of the VO 5 square pyramids due to the low range order of the nanocomposites.

A XANES Study of Cu Valency in Cu-Doped Epitaxial ZnO

Near-edge X-ray absorption (XANES) at the Cu K-edge was employed to study the valency of Cu (a potential p-dopant) in Cu-doped ZnO grown by molecular beam epitaxy. For a similar chemical environment, a shift in the onset of absorption can be interpreted as being due to a change in effective valency. We have studied this shift for both as-grown and argon ambient annealed (1000 °C, 30 min) Cu-doped ZnO samples. The valency shift was measured against ab-initio standards calculated using feff8 as well as the standard samples Cu2O (+1 valence) and CuO (+2 valence). It was found for the flux region explored here that as-incorporated Cu assumes an effective valence of approximately +1 which increases towards +2 upon a 1000 °C argon ambient anneal. X-ray diffraction also shows the presence of both metallic Cu and Cu2O depending upon growth and annealing conditions.

A XANES Study of Cu Valency in Cu-Doped Epitaxial ZnO

Near-edge X-ray absorption (XANES) at the Cu K-edge was employed to study the valency of Cu (a potential p-dopant) in Cu-doped ZnO grown by molecular beam epitaxy. For a similar chemical environment, a shift in the onset of absorption can be interpreted as being due to a change in effective valency. We have studied this shift for both as-grown and argon ambient annealed (1000 °C, 30 min) Cu-doped ZnO samples. The valency shift was measured against ab-initio standards calculated using feff8 as well as the standard samples Cu2O (+1 valence) and CuO (+2 valence). It was found for the flux region explored here that as-incorporated Cu assumes an effective valence of approximately +1 which increases towards +2 upon a 1000 °C argon ambient anneal. X-ray diffraction also shows the presence of both metallic Cu and Cu2O depending upon growth and annealing conditions.

XANES studies of CdS nano-structures on porous silicon

Abstract CdS nano-structures were produced electrochemically using porous silicon (PS) as a template/substrate. AFM and SEM images show that the CdS deposit are grains of ∼100 nm, each of which is an aggregate of smaller particles of several nanometers. XANES studies of the CdS nano-structures together with bulk CdS were conducted. The electronic behavior and optical properties of the CdS/PS composite, particularly those relevant to nano-size effects, were discussed.

Chromium-containing muscovite: crystal chemistry and XANES spectroscopy

To verify chromium enrichment of the muscovite layer, a crystal chemical and XANES study on chromium-containing muscovite crystals from the South Island of New Zealand was carried out. The crystals studied differ from those of end-member muscovite in that they display variable levels of octahedral substitutions and homovalent and heterovalent substitution of K in interlayer sites. Single-crystal X-ray diffraction data were collected for three crystals in the space group C 2/ c to an agreement factor (R obs ) from 0.025 to 0.033. Tetrahedral cation disorder was found for each sample and the values of mean bond length for both tetrahedra do not depart significantly from that of the end-member muscovite-2 M 1 . Electron density at the M2 site is greater than that required for the ideal muscovite-2 M 1 structure, and a small excess of electron density is found for two crystals in M1. As the octahedral substitution of larger cations for Al increases in the octahedral sites, the match between tetrahedral and octahedral sheets improves and tetrahedral rotation angle, α, decreases. XANES spectra at the Cr K-edge in these chromium-containing muscovite samples exhibit octahedral symmetry. Moreover, a careful analysis of the pre-edge region shows at least two features. A qualitative fitting procedure of the pre-edge region indicates that no more than 0.5% of total Cr(III), if any, may occupy the tetrahedral site.

X-ray magnetic circular dichroism in cobalt-iron spinels and electronic states of Co ions.

Synchrotron X-ray experiments on magnetic circular dichroism (XMCD) have been performed at Co K and Fe K absorption edges for CoFe2O4, Fe(1.5)Co(1.5)O4 and FeCo2O4 with the spinel structure. The XANES studies have clarified that only Fe3+ exists in all compounds and Co2+ and Co3+ coexist in the latter two cobaltites. A systematic change in XMCD spectra has concluded that the Co2+ ions in the B sites of FeCo2O4 are in the low-spin state. Since the Co2+ ions in CoFe2O4 are in the high-spin state, the spectra of the intermediate cobaltite may be explained as a mixture of low-spin and high-spin states.

XANES study on Ruddlesden-Popper phase, La(n+1)Ni(n)O(3n+1) (n=1,2 and infinity).

Ruddlesden-Popper phase, La(n+1)Ni(n)O(3n+1 (n=1, 2, and infinity) compounds were prepared by citrate sol-gel method. We revealed the origin of the variation of the electrical conductivities in La(n+1)Ni(n)O(3n+1 (n=1, 2, and infinity) using resistivity measurements, Rietveld analysis, and X-ray absorption spectroscopy. According to the XANES spectra, it is found that the degree of 4p(pi) - 4psigma energy splitting between 8345 eV and 8350 eV is qualitatively proportional to the elongation of the out-of-plane Ni-O bond length. With the decrease of 4p(pi) - 4psigma splitting, the strong hybridization of the sigma-bonding between Ni-3d and O-2p orbitals creates narrow antibonding sigma* bands, which finally results in the lower electrical resistivity.

XANES study on the valence transitions in cerium oxide nanoparticles.

The aim of this work is the determination of Ce environment and valence state in Cerium oxide nanoparticles prepared by the microemulsion method. X-ray absorption near-edge structure measurements at Ce L3 edge were performed on the nanoparticles as a function of annealing temperature, ranging from 298K to 873K under air condition. The experimental results support the conclusion that Ce ion, in the investigated systems, is in trivalence state when the annealing temperature is below 473K. As the temperature increases up to 623K, the XANES spectrum shows the coexistence of Ce3+ and Ce4+ states. When the temperature is higher than 623K, the spectra become identical to that of CeO2 with a distinct double-peak structure, corresponding to the Ce4+ state.

Experimental and theoretical XANES study of the effects of Fe-Mg solid solution in the enstatite-ferrosilite series.

Synthetic orthopyroxenes in the join enstatite - ferrosilite (Mg2Si2O6-Fe2Si2O6) have been studied by XANES with the aim to interpret the variations found in the spectra and evaluate the effects of the Mg-Fe substitution in the orthopyroxene (OPX) structure. The experimental Fe- and Mg-K edge XANES spectra show variations of the peaks intensity as a function of the chemical composition of the samples along the EN-FS join. In the region of the Fe edge, the intensity ratio between peak A and B (IB/IA) varies linearly (R=0.99) with the Mg/Fe substitution. Multiple scattering calculation have been performed both at the Fe and Mg K-edge, to study the geometrical modifications of the M1 and M2 octahedral sites induced by the Mg-Fe substitutions. The spectra, obtained locating Fe (or Mg) in both M1 and M2 and weighing the contributions using the occupancy data for the M1 and M2 sites determined by XRD, allow to discriminate the effects to the total spectrum due to Fe (Mg) located in each sites separately.

In situ and ex situ XANES study of nanodispersed Mo species in zeolites used in fine chemistry catalysis.

Mo K-edge XANES experiments on Mo-containing zeolites at low Mo loading (1 and 2 wt% of Mo on H-ZSM-11, H-BETA and H-ZSM-5 catalysts), active in fine chemistry reactions, were performed ex situ as function of sample calcination temperature in air (in the range 773-973 K) or in situ at 873 and 973K under N2 flow. The results showed a 4-fold oxygen coordination for the incorporated Mo species in the activated (dehydrated) state. Combining these results with additional data evidences an almost total Mo exchange inside the zeolite channels.

XANES study of charge ordering on the spin-Peierls phase transition of alpha'-NAV2O5.

alpha'-NaV2O5 transforms at Tc = 34K, and the origin of the phase transition seems to be caused by the charge order and spin-Peierls mechanism. However the experiments which reveal the existence of charge order are very little. We measured XANES of V atoms which relates to the valence of V. It was found that the valences of V atoms in NaV2O5 were V4.5+ in the high temperature phase by comparing pre-edge and XANES of NaV2O5 with that of LiV2O5 CaV2O5 and V2O5. The analysis of EXAFS also shows the environment of the V4.5+ state.

O K and Cu LIII edge study of itinerant holes in I2-, Hgl2- and HgBr2- intercalated BSCCO(2212) single crystals.

Intercalation effect on BSCCO (2212) system, although it increases the interlayer distance and the c-axis remarkably, produces only a small change in the transition temperature. Thus, amongst other things, intercalation provides an effective method to investigate the influence of the interblock coupling. Electrons are transferred from the host Cu-O2 layers to the guest molecules I2, HgBr2, HgI2 leading to evolution of the Tc. For this we have made high resolution XANES study on the O K and Cu L3 edges to estimate the density of the doping holes. We attempt on basis of our and earlier results the evolution of Tc in these as also the much larger decrease produced in Tc for I2-intercalation for which the increase in basal spacing is the smallest of the three halides.

Sulfur K-edge XANES study of local electronic structure in ternary monosulfide solid solution [(Fe, Co, Ni) 0.923 S

Synchrotron radiation S K-edge XANES spectra and unit-cell parameters are used to investigate the local electronic structure of non-stoichiometric binary and ternary Fe-Co-Ni monosulfide solid solution (mss; M0.923S, M = Fe, Co, Ni) quenched from 800 °C and low pressure. The prominent absorption edge feature of the XANES spectra represents transition of S 1s core level electrons to unoccupied S 3p σ* antibonding orbitals hybridized with empty metal 3d(eg) orbitals. There is a progressive increase in area of the edge peak from Fe0.923S to Ni0.923S and Co0.923S, which correlates with progressive decrease in c and a parameters for the NiAs-type subcell and increase in metallic character, and reflects increase in the number and availability of empty eg β orbitals and covalence of metal-S bonds. More generally, the area of the edge peak exhibits an inverse linear correlation with a, c and unit-cell volume of binary and ternary mss. This inverse linear correlation is attributed to progressive increase in covalence and M-S-M bonding interaction in the c-axis direction, through metal-S [M 3d(eg) - S 3p (or 3d)] π bonding. However, the area of the edge peak does not correlate very well with the average number of 3d electrons per metal atom in these solid solutions, showing that the absorption of synchrotron radiation reflects the local electronic structure of individual absorber atoms (i.e. the SM6 cluster), and is not a group (crystal energy band) effect.

EXAFS Study of the Structure of Desolvated Cluster Compounds of Re and Mo with Cyanide and Thiocyanate Ligands

This paper reports on EXAFS and XANES studies of structural changes in complex compounds (H3O)2Co3[Re6Se8(CN)6]2⋅14.5H2O, Cs2Co[Re6S8(CN)6]⋅2H2O, and Co(DMF)6[Mo6Br8(NCS)6] containing the octahedral cluster anions of rhenium [Re6X8(CN)6]4– (X = S, Se) or molybdenum [Mo6Br8(NCS)6]2–, induced by vacuum annealing at temperatures of up to 250°C. According to EXAFS and XANES data, the complex cluster anions do not undergo any pronounced changes during annealing. The parameters of the local environment of the cobalt cations have been determined. Structural models of the environment of the cations in various compounds after desolvation have been constructed. For Co(DMF)6[Mo6Br8(NCS)6], vacuum annealing at 250°C leads to a complete removal of DMF and formation of a new compound, Co[Mo6Br8(NCS)6], in which the cobalt atoms are coordinated by four sulfur atoms of the SCN groups.

In Situ XANES Study of Pt/Mordenite during Benzene Hydrogenation in the Presence of Thiophene

The influence of the Na+/H+ ratio on the benzene hydrogenation activity and the sulfur tolerance of Pt supported on mordenite (MOR) was studied by in situ XANES. An increase of the Pt white line was observed with increasing concentrations of thiophene up to 400 ppm, which indicates that an equilibrium exists between the thiophene concentration in the gas phase and the sulfur compounds present on the metal surface. Above a certain concentration of thiophene the Pt white line was constant, indicating a saturation of the metal surface with thiophene. The effect of sulfur poisoning on the metal surface was more effective for Pt supported on H-MOR compared to the more basic Pt/Na-MOR catalysts, which is attributed to the more pronounced decomposition of thiophene and the formation of H2S on Brønsted acid sites. During benzene hydrogenation, a decrease of the Pt white line resulting from the adsorption of benzene and other reaction intermediates on the Pt surface was observed. In the presence of sulfur, the increase of the Pt white line intensity during benzene hydrogenation suggests a higher sulfur coverage of the metal for Pt/H-MOR compared to Pt/NaH-MOR. An optimum concentration of acid sites was found with respect to the benzene hydrogenation rate and sulfur tolerance of the catalysts, which is attributed to the presence of two reaction pathways for benzene hydrogenation, i.e., one which is catalyzed by Pt and another in which acid sites close to Pt particles are the catalitycally active sites.

A XANES study of the Cu K-edge in A2CuCl4 perovskite layers under pressure. Influence of antiferrodistortive structure

This work investigates the X-ray absorption spectra of the Cu K-edge in the layered compounds (C, H;?, +, NH&CuC14 (n=1-3) and their variation with pressure. The XANES spectra show a three peaked structure around 8.98 KeV at zero pressure which is characteristic of Cu2+ complexes in D4., symmetry. Application of hydrostatic pressure induces a progressive reduction of the energy separation between the second and third peaks that has been interpreted in terms of a progressive disappearance of the antiferrodistortive structure exhibited by the Cu2+ complexes in this compound series. The results are compared with recent pressure experiments on Raman and Charge Transfer spectroscopy as well as with Multiple Scattering calculations performed on these layered systems.

XANES study on coordination geometry of implanted Cu + ions in silica glass: dependence on doses

The coordination structure of Cu atoms has been studied in Cu +-ion-implanted glasses and (O + + Cu +)-ion-implanted glasses at doses from 1 × 10 16 to 1 × 10 17 Cu + ions/cm 2 and from 5 × 10 15 to 5 × 10 16 O + ions/cm 2. It was found that Cu atoms were present as Cu(I) and their coordination geometry depended on dose through the concentration of oxygen deficiency per silicon atom.