Extended X-ray Absorption Fine Structure Results Research Articles

Local structures and concentration dependence of magnetic properties in Cr- and Mn-doped amorphous silicon ferromagnetic thin films

Local environments surrounding magnetic ions in Mn- or Cr-doped and hydrogenated amorphous Si films have been determined using extended x-ray absorption fine structure (EXAFS) method. These films are found to be ferromagnetic with high Curie temperatures above 250 K and therefore are of tremendous interest for practical spintronics applications. Our EXAFS results indicate that these material systems are nearly free of dopant clusters and oxides up to an unusually high dopant concentration of $20--22\text{ }\text{at}\text{.}\text{ }\mathrm{%}$. As the dopant concentration increases, the saturation magnetization in the Cr-doped samples is decreased while that in the Mn-doped sample remains practically unchanged. Antiferromagnetic coupling of magnetic ions and enhancement of carrier-mediated ferromagnetism are proposed to account for the different concentration dependence of magnetization in the Cr- and Mn-doped samples.

Characterization of water/AOT/benzene microemulsions during photoreduction to produce silver particles

Colloidal dispersions of silver (Ag) particles were prepared by the photoreduction of silver perchlorate (AgClO 4) in water/AOT/benzene water-in-oil (w/o) microemulsions consisting of sodium bis(2-ethylhexyl) sulfosuccinate (AOT), water and benzene. Formation mechanisms of Ag particles prepared in the presence of benzoin by photoreduction from Ag +-containing w/o microemulsions were investigated by UV–vis, transmission electron microscopy (TEM), extended X-ray absorption fine structure (EXAFS) and small-angle X-ray scattering (SAXS) measurements. By a combination of TEM and EXAFS results, Ag particles show a metallic nature after the photoreduction and the diameter of Ag particles ranges between 5 and 30 nm. In situ time-resolved SAXS measurements show that the integrated rate equation can be applied in the reduction process of Ag + ions to Ag 0 atoms, and the autocatalytic Ag particle growth proceeds in the association process of Ag 0 atoms during the photoreduction of Ag +-containing w/o microemulsions. The nanometer-sized water droplets microemulsified by AOT in benzene continuous-phase regulated the size of Ag particles, and the size and shape of water droplets were retained during the Ag particle formation.

Extended x-ray absorption fine structure study of arsenic in HgCdTe: p-type doping linked to nonsubstitutional As incorporation in an unknown AsHg8 structure

An extended x-ray absorption fine structure (EXAFS) investigation has been carried out on arsenic-doped Hg70Cd30Te samples. The incorporation of atomic arsenic has been achieved using a nonconventional radio-frequency plasma source in a molecular beam epitaxy reactor. Two samples from the same epitaxial wafer have been studied. One underwent a 400°C activation annealing under Hg pressure, leading to n to p-type conversion. In the commonly admitted scenario, this conversion is associated with the annealing-induced migration of As from a Hg site to a Te site. This study shows that this is not the case. Before annealing, As is found to be involved in noncrystalline structures: 50% inside an As2Te3 chalcogenide glass and 50% inside a new AsHg8 compact structure. After annealing, the As2Te3 chalcogenide glass disappears, 31% of As occupies Hg sites and 69% incorporates inside this new AsHg8 compact structure that occupies Te sites. The EXAFS results are in excellent agreement with 77K Hall-effect measurements. The new AsHg8 structure is found to have an acceptor behavior. Overall, this study provides an entirely new vision of extrinsic p-type doping of HgCdTe as well as the first experimental evidence of As site transfer induced by annealing.

Evidence for Different Surface Speciation of Arsenite and Arsenate on Green Rust: An EXAFS and XANES Study

The knowledge of arsenic speciation at the surface of green rusts (GRs), [Fe(II)((1-x))Fe(III)(x) (OH)(2)](x+) (CO(3), Cl, SO(4))(x-), is environmentally relevant because arsenic sorption onto GRs could contribute to arsenic retention in anoxic environments (hydromorphic soils, marine sediments, etc.). The nature of arsenic adsorption complexes on hydroxychloride green rust 1 (GR1Cl) at near-neutral pH under anoxic conditions was investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy at the As K-edge. Sorption data indicate that As(V) sorbs more efficiently than As(III) at the studied As loadings (0.27 micromol m(-2) and 2.7 micromol m(-2)). EXAFS results indicate that arsenite [As(III)] and arsenate [As(V)] form inner-sphere complexes on the surface of GR1Cl at arsenic surface coverages of 0.27 and 2.70 micromol m(-2), with distinct types of As(III) and As(V) sorption complexes, which change in relative concentration as a function of arsenic loading. For As(V), the EXAFS-derived As-Fe distances (3.34 +/- 0.02 and 3.49 +/- 0.02 A) suggest the presence of binuclear bidentate double-corner complexes ((2)C) and monodentate mononuclear corner-sharing complexes ((1)V). For As(III), EXAFS-derived As-As distance (3.32 +/- 0.02 A) and As-Fe distances (3.49 +/- 0.02 and 4.72 +/- 0.02 A) are consistent with the presence of dimers of As(III) pyramids binding to the edges of the GR1Cl layers by corner sharing with FeO(6) octahedra. However, (2)C and (1)V As(III) complexes cannot be excluded. These results improve our knowledge of the mode of As(V) and As(III) inner-sphere adsorption on green rusts, which will help to constrain sorption modeling of arsenic in soils, sediments, and aquifers.

Reduction and immobilization of chromium(VI) by iron(II)-treated faujasite

Removal of hexavalent chromium (Cr(VI)) from wastewater typically involves reduction of Cr(VI) to insoluble Cr(III) using zerovalent iron (Fe 0) or ferrous iron (Fe(II)). This study investigates the effectiveness of Fe(II)-treated faujasite (zeolite Fe(II)-Y) for reduction of Cr(VI) and immobilization (adsorption/co-precipitation) of the Cr(III) reaction product. The Fe(II)-faujasite material effectively removed high concentrations of dissolved Cr(VI) from aqueous solution resulting in Cr solid loadings as high as 0.30 mmol Cr per gram Fe(II)-faujasite or ∼1.5% Cr (w:w). Results of Cr K-edge X-ray absorption near edge spectroscopy (XANES) confirmed that the oxidation state of Cr in Cr(VI)-treated Fe(II)-faujasite was Cr(III). The local atomic structure of Cr was investigated by extended X-ray absorption fine structure (EXAFS) spectroscopy and the structure of Cr in the product was described by a Cr–O first shell of six O atoms at 1.98(±0.02) Å plus a second atomic shell of metal (Cr, Fe) at 3.13(±0.02) Å. The EXAFS results, combined with SEM imaging and X-ray diffraction analyses, suggested that the product of the reaction of Cr(VI) with Fe(II)-faujasite is primarily a poorly order Cr x Fe 1− x (OH) 3 mixed phase similar to previous investigations of the reaction of Cr(VI) with Fe 0 and not solely Cr(III) bound directly to zeolite cation exchange sites.

PAMAM Dendrimer-Derived Ir/Al2O3 Catalysts: An EXAFS Characterization

Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to characterize several synthesis stages of hydroxyl-terminated generation four (G4OH) PAMAM dendrimer-derived Ir/γ-Al2O3 catalyst. The EXAFS results indicate that Ir3+ forms complexes with dendrimer functional groups through displacement of two Cl− ion ligands. These complexes are very stable in solution, and no reduction of Ir3+ to Ir nanoparticles or clusters is observed after introduction of reducing agents such as NaBH4 or H2. These Ir3+-dendrimer complexes remain essentially intact after support impregnation. The formation of 1–2 nm particles occurs when the catalysts are treated with O2/H2 or H2, and the dendrimer-derived catalysts exhibit a lower degree of metal support interaction.

Extended X-ray Absorption Fine Structure Investigation of Arsenic in HgCdTe: the Effect of the Activation Anneal

Extended x-ray absorption fine structure (EXAFS) investigation was carried out on two arsenic-doped HgCdTe samples. Arsenic incorporation was achieved with a non-conventional radiofrequency plasma source in a molecular beam epitaxy reactor. Both samples were taken from the same epitaxial run. One of these samples followed a 400°C activation anneal under Hg pressure. EXAFS was used here to probe the local environment around arsenic, and the experimental data were fit through first-principle calculations to extract a quantitative description of the arsenic site transfer upon annealing. Arsenic neighbors are described in terms of chemical nature, numbers and distances. Arsenic was found to be involved mostly in non-crystalline structures, either an As2Te3 glass or an AsHg compact structure. The effect of annealing is to break down the chalcogenide glass, thus favoring the compact AsHg structure. EXAFS results were compared to 77 K Hall-effect measurements, and a very good correlation was found. These findings were compared to the commonly admitted scenario for arsenic site incorporation and transfer upon annealing and provided a new picture and the first experimental evidence of the site transfer.

Structure determination of W-capsulated Si cage clusters by x-ray absorption fine structure spectra

Local structure of transition metal (TM)-encapsulated Si clusters TMSin is investigated by x-ray absorption fine structure. For a series of W-encapsulated Si cluster (WSin) films, the extended x-ray absorption fine structure (EXAFS) results indicate that the W atoms are coordinated by 9–10 Si atoms with a large structural disorder. The x-ray absorption near-edge structure (XANES) spectra exhibit intense white lines for the cluster films, which distinguishes the different local structures from that of amorphous W–Si alloy. Multiple-scattering XANES calculations indicate that the intense white lines are reproduced by the WSin cage clusters with a W atom in the Sin cage centre. Combining the XANES and EXAFS results, it is revealed that the WSin cage structure (n = 8 or 12) is formed, characterized by strengthened covalency consistent with considerable bond contraction.

Modeling Copper(II) Complexation in a Peat Soil Based on Spectroscopic Structural Information

The speciation of Cu in soils and surface waters is largely influenced by complexation reactions with natural organic matter (NOM). In this study, ion selective electrode data for the binding of Cu2+ to a forest peat soil were collected as a function of equilibration time, pH (2.4–6.6), and total Cu(II) concentration (1–54g Cu kg−1 dry soil). As a first step, a one‐site Langmuir isotherm was successfully fitted to the Cu adsorption data for the complete concentration range at pH 4.6. In a second step, structural information extracted from extended x‐ray absorption fine structure (EXAFS) spectroscopy, showing that Cu(II) forms five‐membered rings with possible combinations of amine, carboxyl, and carbonyl functional groups in NOM, were used as input for chemical speciation calculations (using the chemical equilibrium model MINTEQA2). In agreement with the EXAFS results, a model consisting of one RNH2, forming monodentate complexes (Cu2+ + RNH2 ↔ RH2NCu2+; log stability constant KRH2NCu2+ = 9.2; −log acid dissociation constant [pKa] = 9.0 for RNH3+), and two adjacent RCOO− groups, forming bidentate complexes (Cu2+ + 2RCOO− ↔ Cu(OOCR)2; log stability constant β(RCOO)2Cu = 4.7; pKa = 4.5 for RCOOH), gave the best fit to the experimental data. Determined stability constants for Cu(II)–amine and Cu(II)–carboxyl complexes were in good agreement with well‐defined Cu complexes with amino acids and carboxyls, respectively.

Synthesis and characterization of γ-Al 2O 3-supported Pt catalysts from Pt 4 and Pt 6 clusters formed in aqueous solutions

Highly dispersed Pt catalysts were prepared by deposition of Pt 4 and Pt 6 clusters, initially formed in unprotected and poly(vinyl alcohol) (PVA)-protected colloidal Pt suspensions, onto a γ-Al 2O 3 surface. These catalysts were characterized by extended X-ray absorption fine structure (EXAFS) and Fourier transform infrared (FTIR) spectroscopies. The EXAFS results indicate that the supported Pt species formed were very similar in structure to those of the original clusters in the corresponding colloidal suspensions. The FTIR results further indicate that the γ-Al 2O 3-supported Pt 4 clusters have significantly lower chemisorptive properties compared with larger supported Pt nanoparticles; nevertheless, the Pt 4/ γ-Al 2O 3 sample was active for the oxidation of CO with no need for additional activation treatment. In fact, treatment of this sample with H 2 at 150–200 °C led to the formation of Pt aggregates with sizes of 1.0–1.6 nm, demonstrating that the surface Pt 4 species readily sintered in this temperature range under reducing conditions.

Electrospun MnFe 2O 4 nanofibers: Preparation and morphology

The composite fibers were prepared by an electrospinning the solutions mixed manganese ferrite sol with polyvinyl acetate (PVAc). The manganese ferrite sol was obtained from manganese(II) nitrate and iron(III) nitrate nonahydrate molecular precursor based on a sol–gel procedure. Manganese ferrite (MnFe 2O 4) nanofibers were prepared by calcining the above composite fibers in air at 400, 600 and 800 °C, respectively. The obtained nanofibers were characterized by SEM, FT-IR, XRD, and extended X-ray absorption fine structure (EXAFS). After being calcined at 800 °C, the fibers became continuous crystallites as a result of the complete decomposition of PVAc. The morphology of manganese ferrite was found to be cubic spinel structure containing eight oxygens and 12 metal atoms in the unit cell, referred from the results of EXAFS. Also, the magnetic properties of the calcined samples were characterized by using a vibrating sample magnetometer (VSM).

On the structure of the manganese complex of photosystem II: extended-range EXAFS data and specific atomic-resolution models for four S-states

The water-oxidizing manganese complex bound to the proteins of photosystem II (PSII) was studied by X-ray absorption spectroscopy on PSII membrane particles. An extended range for collection of extended X-ray absorption fine-structure (EXAFS) data was used (up to 16.6A(-1)). The EXAFS suggests the presence of two Mn-Mn distances close to 2.7A (per Mn4Ca complex); the existence of a third Mn-Mn distance below 2.9A is at least uncertain. Interestingly, a distance of 3.7A is clearly resolved in the extended-range data and tentatively assigned to a Mn-Mn distance. Taking into account the above EXAFS results (inter alia), we present a model for the structure of the PSII manganese complex, which differs from previous atomic-resolution models. Emphasizing the hypothetical character, we propose for all semi-stable S-states: (i) a structure of the Mn4Ca(mu-O)n core, (ii) a model of the amino acid environment, and (iii) assignments of distinct Mn oxidation states to all the individual Mn ions. This specific working model may permit discussion, verification and invalidation of its various features in comparison with experimental and theoretical findings.

Local environment surrounding Co in MBE-grown Co-dopedHfO2thin films probed by EXAFS

Local structure around Co impurity atoms in $\mathrm{Hf}{\mathrm{O}}_{2}:\mathrm{Co}$ thin films prepared by molecular beam epitaxy has been investigated using the extended x-ray absorption fine structure (EXAFS) technique. These films were found to be ferromagnetic at room temperature. Progressive formation of Co clusters was observed accompanied by interstitial doping of Co in samples with Co concentration of 1%--20% grown at a substrate temperature of $\ensuremath{\sim}700\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. However, films with a relatively high Co concentration of $\ensuremath{\sim}10%$ can be prepared without clustering of Co metal in a low-temperature growth mode with a substrate temperature $\ensuremath{\sim}100\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. These low-temperature-grown films were found to be thermally stable up to an annealing temperature of $\ensuremath{\sim}700\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Our EXAFS results have therefore verified the feasibility for preparing Co-cluster-free room-temperature-ferromagnetic $\mathrm{Hf}{\mathrm{O}}_{2}:\mathrm{Co}$-diluted magnetic oxide thin films with great potential for possible spintronic applications.

X-ray Diffraction and X-ray Absorption Spectroscopy Studies on the Chemical Transformation and Formation of Nanoscale LaMnO3.12

Nanoscale lanthanum manganites (LaMnO 3+δ ) were prepared with thermal decomposition of precursors derived from hydrothermal synthetic method. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) revealed that the as-prepared sample is composed of nanofilaments in an amorphous form. The as-prepared nanoparticles were hydrothermally treated at 180 °C for 12 h and then were annealed at 600, 800, and 900 °C for 4 h to form LaMnO 3+δ . With increasing annealing temperature, a shape change from filaments to rods to spheres was found, and their diameters increased from 4.5 to 52.9 nm. XRD showed a chemical transformation from La(OH) 3 to LaMnO 3+δ . Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) are used to investigate the change of the of local atomic structure around La/Mn with increasing annealing temperature. Combining the TEM, XRD, XANES and EXAFS results, the La-compound is found to be embedded in an amorphous Mn compound with medium range order. As annealed at different temperature, the chemical transformation process from La(OH) 3 to LaMnO 3+δ was discussed. It was the chemical transformation that drives the shape change of these nanomaterials.

Extended X-ray absorption fine structure, X-ray diffraction and Raman analysis of nickel-doped Ba(Mg 1/3Ta 2/3)O 3

Raman, X-ray diffraction and extended X-ray absorption fine structure (EXAFS) measurements of xBa(Ni 1/3Ta 2/3)O 3 + (1 − x)Ba(Mg 1/3Ta 2/3)O 3 samples with x = 0–0.03 were performed to reveal the nickel doping effect on the microwave properties. EXAFS result clearly shows that the nickel is located on the Mg lattice site. We also found that, as the nickel concentration increases, microwave dielectric constant decreases with the Ta O and Ni O bond distances. X-ray diffraction shows that the 1:2 ordered structure is degraded with the increasing of nickel concentration. The stretching phonon of the TaO 6 octahedra, that is A 1g(O) phonon near 800 cm −1, are strongly correlated to the microwave properties of xBa(Ni 1/3Mg 2/3)O 3 + (1 − x)Ba(Mg 1/3Ta 2/3)O 3 samples. The large Raman shift and the large width of the A 1g(O) imply rigid but distorted oxygen octahedral structure, therefore, the effect of nickel doping lowers the dielectric constant and the Q × f value of Ba(Mg 1/3Ta 2/3)O 3 ceramic.

EXAFS Characterization of Dendrimer−Pt Nanocomposites Used for the Preparation of Pt/γ-Al2O3 Catalysts

Pt/gamma-Al2O3 catalysts were prepared using hydroxyl-terminated generation four (G4OH) PAMAM dendrimers as the templating agents and the various steps of the preparation process were monitored by extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS results indicate that, upon hydrolysis, chlorine ligands in the H(2)PtCl(6) and K(2)PtCl(4) precursors were partially replaced by aquo ligands to form [PtCl3(H2O)3]+ and [PtCl2(H2O)2] species, respectively. The results further suggest that, after interaction of such species with the dendrimer molecules, chlorine ligands from the first coordination shell of Pt were replaced by nitrogen atoms from the dendrimer interior, indicating that complexation took place. This process was accompanied by a substantial transfer of electron density from the dendrimer to platinum, indicating that the dendrimer plays the role of a ligand. Following treatment of the H(2)PtCl(6)/G4OH and K(2)PtCl(4)/G4OH complexes with NaBH4, no substantial changes were observed in the electronic or coordination environment of platinum, indicating that metal nanoparticles were not formed during this step under our experimental conditions. However, when the reduction treatment was performed with H2, the formation of extremely small platinum clusters, incorporating no more than four Pt atoms was observed. The nuclearity of these clusters depends on the length of the hydrogen treatment. These Pt species remained strongly bonded to the dendrimer. Formation of larger platinum nanoparticles, with an average diameter of approximately 10 A, was finally observed after the deposition and drying of the H(2)PtCl(6)/G4OH nanocomposites on a gamma-Al(2)O(3) surface, suggesting that the formation of such nanoparticles may be related to the collapse of the dendrimer structure. The platinum nanoparticles formed appear to have high mobility because subsequent thermal treatment in O2/H2, used to remove the dendrimer component, led to further sintering.

Investigation of phase miscibility of CoCrPt thin films using anomalous x-ray scattering and extended x-ray absorption fine structure

The phase miscibility of Co, Cr and Pt in oriented nanostructured CoCrPt magnetic thin films was investigated using anomalous x-ray scattering (AXS) from the (002) reflection and extended x-ray absorption fine structure (EXAFS) at CoK, CrK and PtLIII edges. The AXS measurements at CoK edge clearly showed the presence of Co in the crystalline region. However, Cr was not detected in the lattice. The EXAFS at CoK edge indicated that the nearest neighboring atoms of Co were mixed with 80% Co and 20% Pt, consistent with the results of EXAFS at PtLIII edge. Our observations suggested that only Pt and Co were at the Co (002) lattice of the nanotextured CoCrPt thin films. This indicated that the AXS alone may not be reliable to determine the phase miscibility in textured thin films. Complementary information from the EXAFS was useful to understand the phase miscibility of nanoscale materials.

Quantitative Adsorption and Local Structures of Gallium(III) at the Water-α-FeOOH Interface

The adsorption of Ga(III) at the water-alpha-FeOOH (goethite) interface has been investigated by means of quantitative adsorption experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy, and surface complexation modeling. Under the conditions studied, pH range 3-11 and surface coverages of 0.9-3.2 micromol/m2, Ga(III) was found to adsorb strongly to alpha-FeOOH, and the surface species were more resistant toward hydrolysis and formation of soluble Ga(OH)4- than either solid gallium hydroxides or soluble polynuclear complexes. The EXAFS measurements revealed the presence of octahedral Ga(III) complexes at the water-alpha-FeOOH interface, with practically no structural variations as a function of pH or total gallium concentration. Analysis of the first coordination shell required an anharmonic model indicating a distorted geometry of the GaO6 octahedra, with mean Ga-O distances at 1.96-1.98 angstroms. A method based on the continuous Cauchy wavelet transforms (CCWT) was used to identify backscattering atoms in the higher coordination shells. This analysis indicated predominately Fe backscattering, and the quantitative data fitting resulted in three Ga-Fe paths at 3.05, 3.2, and 3.55 angstroms, which correspond to two edge-sharing and one corner-sharing linkage, respectively. The collective results from EXAFS spectroscopy showed that Ga(III) adsorbs to Fe equivalent sites at the surface alpha-FeOOH as an extension of the rows of Fe octahedra in the bulk structure. This interpretation was further corroborated by a Ga-Fe-Fe multiple scattering path at 6.13 angstroms. The quantitative adsorption and proton data were modeled using a surface complexation formalism based on a 1 pK(a) constant capacitance model. In agreement with the EXAFS results, the model obtained included one predominating surface complex with the stoichiometry [triple bond]FeOGa(OH)2(-0.5) and the stability constant log beta(intr.) = -2.55 +/- 0.04 ([triple bond]FeOH(-0.5) + Ga3+ + 2H2O <--> [triple bond]FeOGa(OH)2(-0.5) + 3H+).

Sulfur K-edge extended X-ray absorption fine structure spectroscopy of homoleptic thiolato complexes with Zn(II) and Cd(II)

The sulfur K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy is applied to homoleptic thiolato complexes with Zn(II) and Cd(II), (Et 4N)[Zn(SAd) 3] ( 1), (Et 4N) 2[{Zn(ScHex) 2} 2(μ-ScHex) 2] ( 2), (Et 4N) 2[{Cd(ScHex) 2} 2(μ-ScHex) 2] ( 3), (Et 4N) 2[{Cd(ScHex)} 4(μ-ScHex) 6] ( 4), [Zn(μ-SAd) 2] n ( 5), and [Cd(μ-SAd) 2] n ( 6) (HSAd = 1-adamantanethiol, HScHex = cyclohexanethiol). The EXAFS results are consistent with the X-ray crystal data of 1– 4. The structures of 5 and 6, which have not been determined by X-ray crystallography, are proposed to be polynuclear structures on the basis of the sulfur K-edge EXAFS, far-IR spectra, and elemental analysis. Clear evidences of the S⋯S interactions (between bridging atoms or neighboring sulfur atoms) and the S⋯C far interactions (in which C far atom is next to carbon atom directly bonded to sulfur atom) were observed in the EXAFS data for all complexes and thus lead to the reliable determination of the structures of 5 and 6 in combination with conventional zinc K-edge EXAFS analysis for 5. This new methodology, sulfur K-edge EXAFS, could be applied for the structural determination of in vivo metalloproteins as well as inorganic compounds.

Intact and Fragmented Triosmium Clusters on MgO: Characterization by X-ray Absorption Spectroscopy and High-Resolution Transmission Electron Microscopy

Oxidative fragmentation of the clusters Os(3)(CO)(12) adsorbed on MgO powder was investigated by X-ray absorption spectroscopy and scanning transmission electron microscopy (STEM). Exposure of the clusters to air leads to their fragmentation, oxidation of the osmium, and formation of ensembles consisting of three Os atoms. X-ray absorption near-edge spectra demonstrate the oxidative nature of the fragmentation process. Extended X-ray absorption fine structure (EXAFS) spectra indicate an average Os-Os distance of 3.33 Angstrom and an Os-Os coordination number of 2, consistent with the formation of ensembles of three Os atoms on the support. STEM images confirm the presence of such trinuclear ensembles, and the diameters of the observed scattering centers (6.0 Angstrom) match that indicated by the EXAFS results.