In-situ X-ray-absorption Spectroscopy Study of Hydrogen Absorption by Nickel-Magnesium Thin Films

Structural and electronic properties of co-sputtered Ni-Mg thin films with varying Ni to Mg ratio were studied by in-situ x-ray absorption spectroscopy in the Ni L-edge and Mg K-edge regions. Co-deposition of the metals led to increased disorder and decreased coordination around Ni and Mg compared to pure metal films. Exposure of the metallic films to hydrogen resulted in formation of hydrides and increased disorder. The presence of hydrogen as a near neighbor around Mg caused a drastic reduction in the intensities of multiple scattering resonances at higher energies. The optical switching behavior and changes in the x-ray spectra varied with Ni to Mg atomic ratio. Pure Mg films with Pd overlayers were converted to MgH2: the H atoms occupy regular sites as in bulk MgH2. Although optical switching was slow in the absence of Ni, the amount of H2 absorption was large. Incorporation of Ni in Mg films led to an increase in the speed of optical switching but decreased maximum transparency. Significant shifts in the Ni L3 and L2 peaks are consistent with strong interaction with hydrogen in the mixed films

Near-Edge X-ray Absorption Fine Structure Study of Ion-beam-induced Phase Transformation in Gd2(Ti1-yZry)2O7

The structural and electronic properties of Gd2(Ti1−yZry)2O7 (y=0–1) pyrochlores following a 2.0-MeV Au2+ ion-beam irradiation (~5.0 X 1014 Au2+/cm2) have been investigated by Ti 2p and O 1s near-edge x-ray absorption fine structure (NEXAFS). The irradiation of Gd2(Ti1−yZry)2O7 leads to the phase transformation from the ordered pyrochlore structure (Fd3m) to the defect fluorite structure (Fm3m) regardless of Zr concentration. Irradiated Gd2(Ti1−yZry)2O7 with y≤0.5 are amorphous, although significant short-range order is present. Contrasting to this behavior, compositions with y≥0.75 retain crystallinity in the defect fluorite structure following irradiation. The local structures of Zr4+ in the irradiated Gd2(Ti1−yZry)2O7 with y≥0.75 determined by NEXAFS are the same as in the cubic fluorite-structured yttria-stabilized zirconia (Y–ZrO2), thereby providing conclusive evidence for the phase transformation. The TiO6 octahedra present in Gd2(Ti1−yZry)2O7 are completely modified by ion-beam irradiation to TiOx polyhedra, and the Ti coordination is increased to eight with longer Ti–O bond distances. The similarity between cation sites and the degree of disorder in Gd2Zr2O7 facilitate the rearrangement and relaxation of Gd, Zr, and O ions/defects. This inhibits amorphization during the ion-beam-induced phase transition to the radiation-resistant defect fluorite structure, which is in contrast to the ordered Gd2Ti2O7

Performance Characteristics of Beamline 6.3.1 from 200 eV to 2000 eV at the Advanced Light Source

Bend magnet beamline 6.3.1 at the Advanced Light Source operates from 200 eV to 2000 eV, primarily used for x-ray absorption fine structure investigations. The beamline optics consist of a compact, entrance-slitless, Hettrick-Underwood type variable-line-spacing plane-grating monochromator and refocusing mirrors to provide a 25 μm × 500 μm spot at the focal point in the reflectometer end station. Wavelength is scanned by the simple rotation of the grating and illuminates a fixed exit slit. The LabView based beamline control and data acquisition computer code has been implemented to provide a convenient interface to the user. The dedicated end station is a reflectometer that is isolated from the beamline by a differential ion pump. The reflectometer can position samples to within 4 μm with an angular position of 0.002°, has total electron and fluorescence yield detectors, and pumps down in about 30 minutes. External end stations can be mounted downstream of the reflectometer as well. The versatility and simplicity of beamline 6.3.1 have made it useful for a wide range of applications such as the characterization of optical components, reflective coatings, and the investigation of a diverse range of materials in both the solid state and in solution

Probing Cation Antisite Disorder in Gd2Ti2O7 Pyrochlore by Site-specific NEXAFS and XPS

Disorder in Gd2Ti2O7 is investigated by near-edge x-ray-absorption fine structure (NEXAFS) and x-ray photoelectron spectroscopy (XPS). NEXAFS shows Ti4+ ions occupy octahedral sites with a tetragonal distortion induced by vacant oxygen sites. O 1s XPS spectra obtained with a charge neutralization system from Gd2Ti2O7(100) and the Gd2Ti2O7 pyrochlore used by Chen et al. [Phys. Rev. Lett. 88, 105901 (2002)], both yielded a single peak, unlike the previous result on the latter that found two peaks. The current results give no evidence for an anisotropic distribution of Ti and O. The extra features reported in the aforementioned communication resulted from charging effects and incomplete surface cleaning. Thus, a result confirming the direct observation of simultaneous cation-anion antisite disordering and lending credence to the split vacancy model has been clarified

The Electronic Structure of Crystalline and Aqueous Solutions of NaBr and NaBrO3 Using in-situ Na K and Br L Edge X-ray Absorption Spectroscopy

The electronic structures of crystalline and aqueous solutions of NaBr and NaBrO3 were studied using in-situ Na K and Br L edge x-ray absorption spectra measured under ambient conditions. Analyses of Na K-edge x-ray absorption spectra show that NaBr and NaBrO3 strongly dissociate in aqueous solution and form Na+ hydration structures, [Na(H2O)n]+ (n = 0–6). However, the size of the anions significantly affects the formation of the Na+ hydration structures in aqueous solutions. The ligand field potential (10 Dq) observed in the Br L-edge x-ray absorption spectra allows in determining their dependence not only on the coordination structure between the Br and the first near neighbor but also the second near neighbor in crystalline state, and, the hydration structure and the counter cations in aqueous solutions. DV-Xα molecular-orbital calculations predict that the transitions in the Br L-edge NEXAFS occur from Br 2p to the unoccupied states containing mainly 4d orbitals of Br for both crystalline and aqueous solution. However, 5s and 5p of Br and 3p of Na in addition to the 4d orbitals of neighbor Br for crystalline NaBr and 5s and 5p of Br, 2p of O and 1s of H for aqueous NaBr also contribute to the unoccupied states

The Electronic Structure of Crystalline and Aqueous Solution of LiBr, NaBr, KBr, and KBrO3: in-situ Br L-edge NEXAFS Study

The electronic structures of crystalline and aqueous solutions of LiBr, NaBr, KBr, and KBrO3 were studied using in-situ Br L-edge near-edge X-ray absorption fine structure (NEXAFS) under ambient conditions. The direct observation of the ligand-field potential (10Dq) allows the determination of their dependence on the interatomic distances between Br and the first near neighbor in crystalline LiBr, NaBr, KBr, and KBrO3 and the effect of hydration in the corresponding aqueous solutions. DV-Xa molecular-orbital calculations show that for both crystalline and aqueous solutions of KBr, the transitions occur from Br 2p to the unoccupied states containing mainly 4d orbitals of Br. The 5s and 5p orbitals of Br and 3d orbitals of K also contribute to the unoccupied states in addition to the 4d orbitals of neighbor Br due to the orbital mixing in crystalline KBr

Analysis of Soft-x-ray Absorption Spectra Measured by a Total-fluorescence X-ray-yield Mode Using Electronic State Calculations: Structure Analysis of Powder and Aqueous Solution of Sodium Aluminate

X-ray absorption spectra near Na and Al K-edges of powder and aqueous solution of sodium aluminate have been measured with a total fluorescence yield mode using our experiment setup under in-situ conditions. The coordination structures to Al in powder and aqueous solution are found from the spectral analysis by means of electronic state calculations for the corresponding structure models. The proportion of some complex species in powder and aqueous solution can be also estimated

Oxidative dissolution potential of biogenic and abiogenic TcO2 in subsurface sediments

Technetium-99 (Tc) is an important fission product contaminant associated with sites of nuclear fuels reprocessing and geologic nuclear waste disposal. Tc is highly mobile in its most oxidized state [Tc(VII)O4−] and less mobile in the reduced form [Tc(IV)O2•nH2O]. Here we investigate the potential for oxidation of Tc(IV) that was heterogeneously reduced by reaction with biogenic Fe(II) in two sediments differing in mineralogy and aggregation state; unconsolidated Pliocene-age fluvial sediment from the upper Ringold (RG) Formation at the Hanford Site and a clay-rich saprolite from the Field Research Center (FRC) background site on the Oak Ridge Site. Both sediments contained Fe(III) and Mn(III/IV) as redox active phases, but FRC also contained mass-dominant Fe–phyllosilicates of different types. Shewanella putrefaciens CN32 reduced Mn(III/ IV) oxides and generated Fe(II) that was reactive with Tc(VII) in heat-killed, bioreduced sediment. After bioreduction and heat-killing, biogenic Fe(II) in the FRC exceeded that in RG by a factor of two. More rapid reduction rates were observed in the RG that had lower biogenic Fe(II), and less particle aggregation. EXAFS measurements indicated that the primary reduction product was a TcO2-like phase in both sediments. The biogenic redox product Tc(IV) oxidized rapidly and completely in RG when contacted with air. Oxidation, in contrast, was slow and incomplete in the FRC, in spite of similar molecular scale speciation of Tc compared to RG. X-ray microprobe, electron microprobe, X-ray absorption spectroscopy, and micro X-ray diffraction were applied to the whole sediment and isolated Tc-containing particles. These analyses revealed that non-oxidizable Tc(IV) in the FRC existed as complexes with octahedral Fe(III) within intra-grain domains of 50–100 µm sized, Fe-containing micas presumptively identified as celadonite. The markedly slower oxidation rates in FRC as compared to RG were attributed to mass-transfer-limited migration of O2 into intra-aggregate and intraparticle domains where Tc(IV) existed; and the formation of unique, oxidation-resistant, intragrain Tc(IV)–Fe(III) molecular species