| 1. |
- Kapaklis, Vassilios, et al.
(author)
-
Temperature dependence of the electrical resistivity and electronic structure of amorphous Fe100-xZrx films and multilayers
- 2012
-
In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 24:49, s. 495402-
-
Journal article (peer-reviewed)abstract
- The electrical resistivity of amorphous Fe100-xZrx metal alloy films and multilayers has been investigated in a wide temperature and composition range. The overall behavior of the resistivity is consistent with bulk measurements, exhibiting prominent semiconductor-like changes at low temperatures. The transition from positive (metallic) to negative temperature coefficient of resistivity behavior is accompanied by minute changes in magnetoresistance and we can therefore rule out magnetic phase changes as being the cause for the observed changes in the resistivity. Using x-ray absorption and emission spectroscopies we are able to probe the unoccupied and occupied electronic densities of states. The corresponding spectra are found to significantly overlap, as expected for a metallic-like electronic structure and the absence of a band gap. Besides a broadening of the x-ray emission lines expected from an amorphous material, remarkably small differences are observed in the electronic structures when changing the amount of Zr. The resistivity data were modeled and agreement with the Mott variable range hopping model was found, indicating localized electronic states due the disordered structure of the Fe100-xZrx alloys.
|
|
| 2. |
- Guo, J. H., et al.
(author)
-
X-ray spectroscopic study of the charge state and local ordering of room-temperature ferromagnetic Mn-doped ZnO
- 2007
-
In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 19:17
-
Journal article (peer-reviewed)abstract
- The charge state and local ordering of Mn doped into a pulsed laser deposited single-phase thin film of ZnO are investigated by using x-ray absorption spectroscopy at the O K-edge, Mn K-edge and L-edge, and x-ray emission spectroscopy at the O K-edge and Mn L-edge. This film is ferromagnetic at room temperature. EXAFS measurement shows that Mn2+ replaces the Zn site in tetrahedral symmetry, and there is no evidence for either metallic Mn or MnO in the film. Upon Mn doping, the top of O 2p valence band extends into the bandgap, indicating additional charge carriers being created.
|
|
| 3. |
- Modin, Anders, et al.
(author)
-
5 f-Shell correlation effects in dioxides of light actinides studied by O 1s x-ray absorption and emission spectroscopies and first-principles calculations
- 2015
-
In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 27:31
-
Journal article (peer-reviewed)abstract
- Soft x-ray emission and absorption spectroscopic data are reported for the O 1s region of a single crystal of UO2, a polycrystalline NpO2 sample, and a single crystal of PuO2. The experimental data are interpreted using first-principles correlated-electron calculations within the framework of the density functional theory with added Coulomb U interaction (DFT+U). A detailed analysis regarding the origin of different structures in the x-ray emission and x-ray absorption spectra is given and the effect of varying the intra-atomic Coulomb interaction-U for the 5f electrons is investigated. Our data indicate that O 1s x-ray absorption and emission spectroscopies can, in combination with DFT+U calculations, successfully be used to study 5f-shell Coulomb correlation effects in dioxides of light actinides. The values for the Coulomb U parameter in these dioxides are derived to be in the range of 4-5 eV.
|
|
| 4. |
- Rothe, J., et al.
(author)
-
The INE-Beamline for actinide science at ANKA
- 2012
-
In: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 83:4, s. 043105-
-
Journal article (peer-reviewed)abstract
- Since its inauguration in 2005, the INE-Beamline for actinide research at the synchrotron source ANKA (KIT North Campus) provides dedicated instrumentation for x-ray spectroscopic characterization of actinide samples and other radioactive materials. R&D work at the beamline focuses on various aspects of nuclear waste disposal within INE's mission to provide the scientific basis for assessing long-term safety of a final nuclear waste repository. The INE-Beamline is accessible for the actinide and radiochemistry community through the ANKA proposal system and the European Union Integrated Infrastructure Initiative ACTINET-I3. Experiments with activities up to 1 x 10(+6) times the European exemption limit are feasible within a safe but flexible containment concept. Measurements with monochromatic radiation are performed at photon energies varying between similar to 2.1 keV (P K-edge) and similar to 25 keV (Pd K-edge), including the lanthanide L-edges and the actinide M-and L3-edges up to Cf. The close proximity of the INE-Beamline to INE controlled area labs offers infrastructure unique in Europe for the spectroscopic and microscopic characterization of actinide samples. The modular beamline design enables sufficient flexibility to adapt sample environments and detection systems to many scientific questions. The well-established bulk techniques x-ray absorption fine structure (XAFS) spectroscopy in transmission and fluorescence mode have been augmented by advanced methods using a microfocused beam, including (confocal) XAFS/x-ray fluorescence detection and a combination of (micro-)XAFS and (micro-)x-ray diffraction. Additional instrumentation for high energy-resolution x-ray emission spectroscopy has been successfully developed and tested.
|
|
| 5. |
- Butorin, Sergei. M., et al.
(author)
-
Characteristics of chemical bonding of pentavalent uranium in La-doped UO2
- 2017
-
In: Chemical Communications. - : Royal Society of Chemistry (RSC). - 1359-7345 .- 1364-548X. ; 53:1, s. 115-118
-
Journal article (peer-reviewed)abstract
- The effect of La doping on the electronic structure of U in UO2 was studied using an advanced technique, namely, X-ray absorption spectroscopy (XAS) in the high-energy-resolution fluorescence-detection (HERFD) mode, at the U 3d(3/2) (M-4) edge. Thanks to a significant reduction of the core-hole lifetime broadening and distinct chemical shifts of the HERFD-XAS lines, the U(v) formation as a result of La doping was identified. The isolated contribution of U(v) in the M-4 HERFD-XAS spectrum reveals the so-called charge-transfer satellites due to the U 5f-O 2p hybridization. The analysis of the experimental data within the framework of the Anderson impurity model (AIM) indicates a significant change in the characteristics and degree of covalency for the chemical bonding in the U(v) subsystem of UO2 as compared to undoped UO2, which is a Mott-Hubbard system. The results are also supported by AIM calculations of X-ray photoelectron and optical absorption data.
|
|
| 6. |
- Butorin, Sergei M., et al.
(author)
-
Crystal-Field and Covalency Effects in Uranates : An X-ray Spectroscopic Study
- 2016
-
In: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 22:28, s. 9693-9698
-
Journal article (peer-reviewed)abstract
- The electronic structure of U-V- and U-VI-containing uranates NaUO3 and Pb3UO6 was studied by using an advanced technique, namely X-ray absorption spectroscopy (XAS) in high-energy-resolution fluorescence-detection (HERFD) mode. Due to a significant reduction in core-hole lifetime broadening, the crystal-field splittings of the 5f shell were probed directly in HERFD-XAS spectra collected at the U3d edge, which is not possible by using conventional XAS. In addition, the charge-transfer satellites that result from U5f-O2p hybridization were clearly resolved. The crystal-field parameters, 5f occupancy, and degree of covalency of the chemical bonding in these uranates were estimated by using the Anderson impurity model by calculating the U3d HERFD-XAS, conventional XAS, core-to-core (U 4f-3d transitions) resonant inelastic X-ray scattering (RIXS), and U4f Xray photoelectron spectra. The crystal field was found to be strong in these systems and the 5f occupancy was determined to be 1.32 and 0.84 electrons in the ground state for NaUO3 and Pb3UO6, respectively, which indicates a significant covalent character for these compounds.
|
|
| 7. |
- Butorin, Sergei M., et al.
(author)
-
Local Symmetry Effects in Actinide 4f X-ray Absorption in Oxides
- 2016
-
In: Analytical Chemistry. - : American Chemical Society (ACS). - 0003-2700 .- 1520-6882. ; 88:8, s. 4169-4173
-
Journal article (peer-reviewed)abstract
- A systematic X-ray absorption study at actinide N-6,(7) (4f -> 6d transitions) edges was performed for light-actinide oxides including data obtained for the first time for NpO2, PuO2 and UO3. The measurements were supported by ab initio calculations based on local-density approximation. with added 5f-5f Coulomb interaction (LDA+U). Improved energy resolution compared to common experiments at actinide L-2,L-3 (2p -> 6d transitions) edges allowed us to resolve the major structures of the 13 unoccupied 6d density of states (DOS) and estimate the crystal-field 116) splittings in the 6d shell directly from the spectra of light-actinide dioxides. The measurements demonstrated an enhanced sensitivity of the N-6,N-7, spectral shape to changes in the compound crystal structure. For nonstoichiometric NpO2-x, the filling of the entire band gap with Np 6d states was observed thus supporting a phase coexistence of Np metal and stoichiometric NpO2 which is in agreement with the tentative Np-O phase diagram.
|
|
| 8. |
- Ghosh, Anirudha, et al.
(author)
-
Magnetic circular dichroism in the dd excitation in the van der Waals magnet CrI3 probed by resonant inelastic x-ray scattering
- 2023
-
In: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 107:11
-
Journal article (peer-reviewed)abstract
- We report on a combined experimental and theoretical study on CrI3 single crystals by employing the polarization dependence of resonant inelastic x-ray scattering (RIXS). Our investigations reveal multiple Cr 3d orbital splitting (dd excitations) as well as magnetic dichroism (MD) in the RIXS spectra. The dd excitation energies are similar on the two sides of the ferromagnetic transition temperature, T-C similar to 61 K, although MD in RIXS is predominant at 0.4 T magnetic field below TC. This demonstrates that the ferromagnetic superexchange interaction that is responsible for the interatomic exchange field is vanishingly small compared with the local exchange field that comes from exchange and correlation interaction among the interacting Cr 3d orbitals. The recorded RIXS spectra reported here reveal clearly resolved Cr 3d intraorbital dd excitations that represent transitions between electronic levels that are heavily influenced by dynamic correlations and multiconfiguration effects. Our calculations taking into account the Cr 3d hybridization with the ligand valence states and the full multiplet structure due to intra-atomic and crystal field interactions in Oh and D3d symmetry clearly reproduced the dichroic trend in experimental RIXS spectra.
|
|
| 9. |
- Kurian, R., et al.
(author)
-
Intrinsic deviations in fluorescence yield detected x-ray absorption spectroscopy : The case of the transition metal L 2,3 edges
- 2012
-
In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 24:45, s. 452201-
-
Journal article (peer-reviewed)abstract
- Fluorescence yield (FY) detected x-ray absorption spectra (XAS) of 3d transition metal ions are calculated from the integrated 2p3d resonant x-ray emission spectra. The resulting FY-XAS spectra are compared with the normal XAS spectra corresponding to the absorption cross section and significant deviations between the two spectra are found. This implies that the assumption that the FY-XAS spectrum identifies with the XAS spectrum is disproved. Especially for the early transition metal systems the differences between the FY-XAS and XAS are large, due to the opening of inelastic decay channels from selected x-ray absorption final states. The theoretical calculations show that the difference between FY detection and XAS is largest for the detection in depolarized geometry. The calculations are compared with experimental spectra for oxides and coordination compounds for Fe 2+, Co 2+ and Ni 2+ systems. The implications for the sum rules in XAS and magnetic circular dichroism experiments are discussed.
|
|
| 10. |
- Kvashnina, Kristina, et al.
(author)
-
Sensitivity to Actinide Doping of Uranium Compounds by Resonant Inelastic X-ray Scattering at Uranium L-3 Edge
- 2015
-
In: Analytical Chemistry. - 0003-2700 .- 1520-6882. ; 87:17, s. 8772-8780
-
Journal article (peer-reviewed)abstract
- Valence-to-core resonant inelastic X-ray scattering (RIXS) and high energy resolution fluorescence detection (HERFD) X-ray absorption measurements were performed at the U (L)3 edges of UO2 and UO2(NO3)(2)(H2O)(6). The results are compared with model calculations based on the local-density-approximation formalism, taking into account Coulomb interaction U (LDA + U). We show that despite strong 5f-5f electronic correlations in the studied systems and the use of core-level excitations in the intermediate stage of the spectroscopic process, the RIXS technique probes a convolution of the single-particle densities of states in the valence and conduction bands. For UO2, the detected crystal-field splitting between the U 6d e(g) and t(2g) orbitals from the RIXS spectra (similar to 3.5 eV) is larger than that previously derived from optical spectroscopy. Furthermore, by using an example of the U0.75Pu0.25O2 mixed oxide, we show that the RIXS technique at the U L-3 edges is sensitive to the substitution of U with other actinide, in contrast to conventional X-ray absorption methods. That is, due to changes in the occupied part rather than in the unoccupied part of the U 6d states caused by the substitution.
|
|
| 11. |
- Phuyal, Dibya, et al.
(author)
-
Ferroelectric properties of BaTiO3 thin films co-doped with Mn and Nb
- 2019
-
In: AIP Advances. - : American Institute of Physics. - 2158-3226. ; 9:9
-
Journal article (peer-reviewed)abstract
- We report on properties of BaTiO3 thin films where the bandgap is tuned via aliovalent doping of Mn and Nb ions co-doped at the Ti site. The doped films show single-phase tetragonal structure, growing epitaxially with a smooth interface to the substrate. Using piezoforce microscopy, we find that both doped and undoped films exhibit good ferroelectric response. The piezoelectric domain switching in the films was confirmed by measuring local hysteresis of the polarization at several different areas across the thin films, demonstrating a switchable ferroelectric state. The doping of the BaTiO3 also reduces the bandgap of the material from 3.2 eV for BaTiO3 to nearly 2.7 eV for the 7.5% doped sample, suggesting the viability of the films for effective light harvesting in the visible spectrum. The results demonstrate co-doping as an effective strategy for bandgap engineering and a guide for the realization of visible-light applications using its ferroelectric properties.
|
|
| 12. |
|
|
| 13. |
|
|
| 14. |
|
|
| 15. |
- Sterling, Cody M., 1993-, et al.
(author)
-
Electronic Structure and Chemical Bonding in Methylammonium Lead Triiodide and Its Precursor Methylammonium Iodide
- 2022
-
In: The Journal of Physical Chemistry C. - : American Chemical Society. - 1932-7447 .- 1932-7455. ; 126:47, s. 20143-20154
-
Journal article (peer-reviewed)abstract
- A detailed examination of the electronic structures of methylammonium lead triiodide (MAPI) and methylammonium iodide (MAI) is performed with ab initio molecular dynamics (AIMD) simulations based on density functional theory, and the theoretical results are compared to experimental probes. The occupied valence bands of a MAPI single crystal and MAI powder are probed with X-ray photoelectron spectroscopy, and the conduction bands are probed from the perspective of nitrogen K-edge X-ray absorption spectroscopy. Combined, the theoretical simulations and the two experimental techniques allow for a dissection of the electronic structure unveiling the nature of chemical bonding in MAPI and MAI. Here, we show that the difference in band gap between MAPI and MAI is caused chiefly by interactions between iodine and lead but also weaker interactions with the MA+counterions. Spatial decomposition of the iodine p levels allows for analysis of Pb-I σ bonds and πinteractions, which contribute to this effect with the involvement of the Pb 6p levels. Differences in hydrogen bonding between the two materials, seen in the AIMD simulations, are reflected in nitrogen valence orbital composition and in nitrogen K-edge X-ray absorption spectra.
|
|
| 16. |
- Sterling, Cody M., et al.
(author)
-
Sensitivity of Nitrogen K-Edge X-ray Absorption to Halide Substitution and Thermal Fluctuations in Methylammonium Lead-Halide Perovskites
- 2021
-
In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:15, s. 8360-8368
-
Journal article (peer-reviewed)abstract
- The performance of hybrid perovskite materials in solar cells crucially depends on their electronic properties, and it is important to investigate contributions to the total electronic structure from specific components in the material. In a combined theoretical and experimental study of CH3NH3PbI3-methylammonium lead triiodide (MAPI)-and its bromide cousin CH3NH3PbBr3 (MAPB), we analyze nitrogen K-edge (N Is-to-2p*) X-ray absorption (XA) spectra measured in MAPI and MAPB single crystals. This permits comparison of spectral features to the local character of unoccupied molecular orbitals on the CH3NH3+ (MA(+)) counterions and allows us to investigate how thermal fluctuations, hydrogen bonding, and halide-ion substitution influence the XA spectra as a measure of the local electronic structure. In agreement with the experiment, the simulated spectra for MAPI and MAPB show close similarity, except that the MAPB spectral features are blue-shifted by +0.31 eV. The shift is shown to arise from the intrinsic difference in the electronic structure of the two halide atoms rather than from structural differences between the materials. In addition, from the spectral sampling analysis of molecular dynamics simulations, clear correlations between geometric descriptors(N-C, N-H, and H center dot center dot center dot I/Br distances) and spectral features are identified and used to explain the spectral shapes.
|
|
| 17. |
- Vitova, T., et al.
(author)
-
High energy resolution x-ray absorption spectroscopy study of uranium in varying valence states
- 2010
-
In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:23, s. 235118-
-
Journal article (peer-reviewed)abstract
- A high energy resolution x-ray absorption near-edge structure (XANES) spectroscopy study on U4+ (UO2), U5+ ([UO2Py5][KI2Py2]), and U6+ [UO2(NO3)(2)(H2O)(6)] demonstrates the potential of this experimental technique for qualitative/semiqualitative and quantitative actinide speciation investigations. We observe a pre-edge feature with quadrupole nature in a U L-3 edge partial fluorescence yield-XANES spectrum. This feature is a tool for characterizing the participation of 5f orbitals in U-O bonding. The feature origin is explained by performing calculations with the finite difference method near-edge structure code based on the multiple-scattering theory and the finite difference method.
|
|
| 18. |
- Amidani, Lucia, et al.
(author)
-
Oxygen K-Edge X-ray Absorption Spectra of ThO2 and CeO2 : Experiment, Interpretation, and Structural Effects
- 2023
-
In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 127:6, s. 3077-3084
-
Journal article (peer-reviewed)abstract
- Experimental oxygen K-edge spectra of ThO2 and CeO2 are presented and interpreted based on density functional theory (DFT). The contribution of d and f orbitals to the O Kedge spectrum is identified as well-distinguished peaks, the presence of which evidences the strong hybridization of Th and Ce metal centers with O orbitals. The sensitivity of the O K-edge to both f- and d-states in the absence of a core-hole on the metal ion results in an insightful overview of the electronic structure involved in the chemical bond. In particular, the large bandwidth of the Th 5f band as compared to the Ce 4f band is observed as a set of wider and more substantial set of peaks in the O K-edge, confirming the stronger hybridization of the former with O orbitals. The peak ascribed to the 5f band of ThO2 is found at higher energy than the 6d band, as predicted from DFT calculations on actinide dioxides. To highlight the sensitivity and the potential use of the O K-edge for the characterization of ThO2-based systems, the sensitivity of the spectrum to structural changes such as lattice expansion and size reduction are calculated and discussed.
|
|
| 19. |
- Amidani, Lucia, et al.
(author)
-
Understanding the size effects on the electronic structure of ThO2 nanoparticles
- 2019
-
In: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 21:20, s. 10635-10643
-
Journal article (peer-reviewed)abstract
- Developing characterization techniques and analysis methods adapted to the investigation of nanoparticles (NPs) is of fundamental importance considering the role of these materials in many fields of research. The study of actinide based NPs, despite their environmental relevance, is still underdeveloped compared to that of NPs based on stable and lighter elements. We present here an investigation of ThO2 NPs performed with High-Energy Resolution Fluorescence Detected (HERFD) X-ray Absorption Near-Edge Structure (XANES) and with ab initio XANES simulations. The first post-edge feature of Th L-3 edge HERFD XANES disappears in small NPs and simulations considering non-relaxed structural models reproduce the trends observed in experimental data. Inspection of the simulations of Th atoms in the core and on the surface of the NP indeed demonstrates that the first post-edge feature is very sensitive to the lowering of the number of coordinating atoms and therefore to the more exposed Th atoms at the surface of the NP. The sensitivity of the L-3 edge HERFD XANES to low coordinated atoms at the surface stems from the hybridization of the d-Density of States (DOS) of Th with both O and Th neighboring atoms. This may be a common feature to other oxide systems that can be exploited to investigate surface interactions.
|
|
| 20. |
|
|
| 21. |
- Butorin, Sergei, et al.
(author)
-
Effect of Ag Doping on Electronic Structure of Cluster Compounds AgxMo9Se11 (x = 3.4, 3.9)
- 2018
-
In: ACS Applied Energy Materials. - : American Chemical Society (ACS). - 2574-0962. ; 1:8, s. 4032-4039
-
Journal article (peer-reviewed)abstract
- The electronic structure of AgxMo9Se11 as a potential material for thermoelectric applications was studied using high-energy-resolution fluorescence-detection X-ray absorption spectroscopy (HERFD-XAS) and the resonant inelastic X-ray scattering (RIXS) technique. The experiments were supported by first-principle calculations using density functional theory (DFT). The analysis of obtained spectra indicate the presence of subvalent (less than 1+) Ag in AgxMo9Se11. The advanced HERFD-XAS measurements allowed us to resolve the contribution of the electronic states at the Fermi level of AgxMo9Se11 and to monitor its dependence on the x value. A comparison of the experimental data with the results of the DFT calculations suggests the importance of the Ag2-type sites with the shortest Ag–Se distance for affecting the properties of AgxMo9Se11.
|
|
| 22. |
- Butorin, Sergei M., et al.
(author)
-
Chemical Reduction of Actinides Probed by Resonant Inelastic X-ray Scattering
- 2013
-
In: Analytical Chemistry. - : American Chemical Society (ACS). - 0003-2700 .- 1520-6882. ; 85:23, s. 11196-11200
-
Journal article (peer-reviewed)abstract
- The study addresses the possibilities of immobilizing the mobile species of actinides in the geosphere using metallic iron. Sorption on corroding iron is well-known, but there have been uncertainties with regard to the possibilities of reducing the actinyl species to sparingly soluble oxides and, thereby, permanently immobilizing them. Resonant inelastic X-ray scattering (RIXS) measurements at the actinide 5d edges on Fe foils exposed to uranium(VI) and neptunium(V) solutions in groundwater unambigiously indicate reduction of actinides to, respectively, uranium(IV) and neptunium(IV) on iron surfaces. The reduction manifests itself in an appearance of distinct specific signatures of uranium(IV) and neptunium(IV) in the RIXS profile of 5f-5f excitations. Such signatures and RIXS intensity/cross-section behavior with varying energy of incident photons can be reproduced by model atomic-multiplet calculations of the RIXS spectra. By normalizing the RIXS signal of corresponding 5f-5f excitations to core-to-core 6p-to-5d characteristic fluorescence transitions of actinides, their reduction rates on Fe samples with different exposure to actinide solutions can be estimated. Observed reduction implies similar processes in the nuclear waste canister thus suggesting reduced probability of nuclear waste release with ground waters from the canister.
|
|
| 23. |
- Butorin, Sergei M., et al.
(author)
-
High-resolution X-ray absorption spectroscopy as a probe of crystal-field and covalency effects in actinide compounds
- 2016
-
In: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 113:29, s. 8093-8097
-
Journal article (peer-reviewed)abstract
- Applying the high-energy resolution fluorescence-detection (HERFD) mode of X-ray absorption spectroscopy (XAS), we were able to probe, for the first time to our knowledge, the crystalline electric field (CEF) splittings of the 5f shell directly in the HERFD-XAS spectra of actinides. Using ThO2 as an example, data measured at the Th 3d edge were interpreted within the framework of the Anderson impurity model. Because the charge-transfer satellites were also resolved in the HERFD-XAS spectra, the analysis of these satellites revealed that ThO2 is not an ionic compound as previously believed. The Th 6d occupancy in the ground state was estimated to be twice that of the Th 5f states. We demonstrate that HERFD-XAS allows for characterization of the CEF interaction and degree of covalency in the ground state of actinide compounds as it is extensively done for 3d transition metal systems.
|
|
| 24. |
- Butorin, Sergei. M., et al.
(author)
-
Probing Chemical Bonding in Uranium Dioxide by Means of High- Resolution X-ray Absorption Spectroscopy
- 2016
-
In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:51, s. 29397-29404
-
Journal article (peer-reviewed)abstract
- A systematic X-ray absorption study at the U 3d, 4d, and 4f edges of UO2 was performed, and the data were analyzed within framework of the Anderson impurity model. By applying the high-energy-resolution fluorescence-detection (HERFD) mode of X-ray absorption spectroscopy (XAS) at the U 3d(3/2) edge and conducting the XAS measurements at the shallower U 4f levels, fine details of the XAS spectra were resolved resulting from reduced core-hole lifetime broadening. This multiedge study enabled a far more effective analysis of the electronic structure at the U sites and characterization of the chemical bonding and degree of the 5f localization in UO2. The results support the covalent character of UO2 and do not agree with the suggestions of rather ionic bonding in this compound as expressed in some publications.
|
|
| 25. |
- Conradson, Steven D., et al.
(author)
-
Closure of the Mott gap and formation of a superthermal metal in the Frohlich-type nonequilibrium polaron Bose-Einstein condensate in UO2+x
- 2017
-
In: Physical Review B. - 2469-9950 .- 2469-9969. ; 96:12
-
Journal article (peer-reviewed)abstract
- Mixed valence O-doped UO2+x. and photoexcited UO2 containing transitory U3+ and U5+ host a coherent polaronic quantum phase (CPQP) that exhibits the characteristics of a Frohlich-type, nonequilibrium, phononcoupled Bose-Einstein condensate whose stability and coherence are amplified by collective, anharmonic motions of atoms and charges. Complementary to the available, detailed, real space information from scattering and EXAFS, an outstanding question is the electronic structure. Mapping the Mott gap in UO2, U4O9, and U3O7 with O XAS and NIXS and UM5 RIXS shows that O doping raises the peak of the U5f states of the valence band by similar to 0.4 eV relative to a calculated value of 0.25 eV. However, it lowers the edge of the conduction band by 1.5 eV vs the calculated 0.6 eV, a difference much larger than the experimental error. This 1.9 eV reduction in the gap width constitutes most of the 2-2.2 eV gap measured by optical absorption. In addition, the XAS spectra show a tail that will intersect the occupied U5f states and give a continuous density-of-states that increases rapidly above its constricted intersection. Femtosecond-resolved photoemission measurements of UO2, coincident with the excitation pulse with 4.7 eV excitation, show the unoccupied U5f states of UO2 and no hot electrons. 3.1 eV excitation, however, complements the O-doping results by giving a continuous population of electrons for several eV above the Fermi level. The CPQP in photoexcited UO2 therefore fulfills the criteria for a nonequilibrium condensate. The electron distributions resulting from both excitations persist for 5-10 ps, indicating that they are the final state that therefore forms without passing through the initial continuous distribution of nonthermal electrons observed for other materials. Three exceptional findings are: (1) the direct formation of both of these long lived (> 3-10 ps) excited states without the short lived nonthermal intermediate; (2) the superthermal metallic state is as or more stable than typical photoinduced metallic phases; and (3) the absence of hot electrons accompanying the insulating UO2 excited state. This heterogeneous, nonequilibrium, Frohlich BEC stabilized by a Fano-Feshbach resonance therefore continues to exhibit unique properties.
|
|
| 26. |
- Gerber, Evgeny, et al.
(author)
-
Insight into the structure-property relationship of UO2 nanoparticles
- 2021
-
In: Inorganic Chemistry Frontiers. - : Royal Society of Chemistry (RSC). - 2052-1545 .- 2052-1553. ; 8:4, s. 1102-1110
-
Journal article (peer-reviewed)abstract
- Highly crystalline UO2 nanoparticles (NPs) with sizes of 2–3 nm were produced by fast chemical deposition of uranium(IV) under reducing conditions at pH 8–11. The particles were then characterized by microscopy and spectroscopy techniques including high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), high-energy resolution fluorescence detection (HERFD) X-ray absorption spectroscopy at the U M4 edge and extended X-ray absorption fine structure (EXAFS) spectroscopy at the U L3 edge. The results of this investigation show that despite U(IV) being the dominant oxidation state of the freshly prepared UO2 NPs, they oxidize to U4O9 with time and under the X-ray beam, indicating the high reactivity of U(IV) under these conditions. Moreover, it was found that the oxidation process of NPs is accompanied by their growth in size to 6 nm. We highlight here the major differences and similarities of the UO2 NP properties to PuO2, ThO2 and CeO2 NPs.
|
|
| 27. |
- Gerber, Evgeny, et al.
(author)
-
The missing pieces of the PuO2 nanoparticle puzzle
- 2020
-
In: Nanoscale. - : ROYAL SOC CHEMISTRY. - 2040-3364 .- 2040-3372. ; 12:35, s. 18039-18048
-
Journal article (peer-reviewed)abstract
- The nanoscience field often produces results more mystifying than any other discipline. It has been argued that changes in the plutonium dioxide (PuO2) particle size from bulk to nano can have a drastic effect on PuO2 properties. Here we report a full characterization of PuO2 nanoparticles (NPs) at the atomic level and probe their local and electronic structures by a variety of methods available at the synchrotron, including extended X-ray absorption fine structure (EXAFS) at the Pu L-3 edge, X-ray absorption near edge structure (XANES) in high energy resolution fluorescence detection (HERFD) mode at the Pu L-3 and M-4 edges, high energy X-ray scattering (HEXS) and X-ray diffraction (XRD). The particles were synthesized from precursors with different oxidation states of plutonium (III, IV, and V) under various environmentally and waste storage relevant conditions (pH 8 and pH > 10). Our experimental results analyzed with state-of-the-art theoretical approaches demonstrate that well dispersed, crystalline NPs with a size of similar to 2.5 nm in diameter are always formed in spite of diverse chemical conditions. Identical crystal structures and the presence of only the Pu(IV) oxidation state in all NPs, reported here for the first time, indicate that the structure of PuO2 NPs is very similar to that of the bulk PuO2. All methods give complementary information and show that investigated fundamental properties of PuO2 NPs, rather than being exotic, are very similar to those of the bulk PuO2.
|
|
| 28. |
- Kadas, Krisztina, et al.
(author)
-
Structural properties of amorphous metal carbides : Theory and experiment
- 2012
-
In: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 60:12, s. 4720-4728
-
Journal article (peer-reviewed)abstract
- By means of theoretical modeling and experimental synthesis and characterization, we investigate the structural properties of amorphous Zr-Si-C. Two chemical compositions are selected: Zr0.31Si0.29C0.40 and Zr0.60Si0.33C0.07. Amorphous structures are generated in the theoretical part of our work by the stochastic quenching (SQ) method, and detailed comparison is made regarding the structure and density of the experimentally synthesized films. These films are analyzed experimentally using X-ray absorption spectroscopy, transmission electron microscopy and X-ray diffraction. Our results demonstrate a remarkable agreement between theory and experiment concerning bond distances and atomic coordination of this complex amorphous metal carbide. The demonstrated power of the SQ method opens up avenues for theoretical predictions of amorphous materials in general.
|
|
| 29. |
- Kotani, A., et al.
(author)
-
A new method of directly determining the core-hole effect in the Ce L-3 XAS of mixed valence Ce compounds : An application of resonant X-ray emission spectroscopy
- 2011
-
In: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048 .- 1873-2526. ; 184:3-6, s. 210-215
-
Journal article (peer-reviewed)abstract
- Using a simplified model, we calculate resonant X-ray emission (RXE) spectra with Ce 2p to 5d excitation and Ce nl to 2p de-excitation for mixed valence Ce compounds, where nl is 3d or valence states. If the core-hole effect is large to play an important role in the Ce L-3 XAS, the RXE spectral features for nl = 3d and valence states are significantly different, but if it is negligibly small, they are essentially the same. Therefore, these RXE spectra are useful as a new powerful method of directly determining the core-hole effect in the L-3 XAS. More detailed theoretical calculations of these RXE spectra are made for CeO2, and compared with new experimental data, indicating the importance of the core-hole effect. Similar investigations are desirable for some metallic mixed valence Ce compounds in which the core-hole effect is controversial. Finally, we point out that for nl = 5d we can observe the resonant inelastic X-ray scattering spectra corresponding to the charge-transfer excitations or Kondo resonance excitations.
|
|
| 30. |
- Kotani, A., et al.
(author)
-
Spectator and participator processes in the resonant photon-in and photon-out spectra at the Ce L-3 edge of CeO2
- 2012
-
In: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 85:8, s. 257-
-
Journal article (peer-reviewed)abstract
- We study both theoretically and experimentally the photon-in and photon-out spectra of CeO2, which are caused by the Ce 2p to Ce 5d excitation followed by the three different de-excitation channels: (i) Ce 3d to Ce 2p (denoted by 3d-RXES), (ii) O 2p to Ce 2p (v-RXES), and (iii) Ce 5d to Ce 2p (RIXS). In 3d- and v-RXES, the 5d electron plays a role of a spectator, but in RIXS it is a participator. By extending our single impurity Anderson model (SIAM), which was used recently for our calculations of v-and 3d-RXES spectra of CeO2, we study the polarization dependence in the spectator and participator spectra, and we perform more detailed calculations for 3d- and v-RXES spectral features, as well as new calculations for the RIXS spectrum with charge transfer excitations. The polarization dependence is different for the spectator and participator spectra; we have no polarization correlation between the incident and emitted photons for the spectator spectra but a strong polarization correlation for the participator spectrum. The theoretical calculations predict that the charge transfer excitations in RIXS occur in the transfer-energy range overlapped with v-RXES, but the RIXS and v-RXES spectra can be discriminated by taking advantage of the different polarization dependence. The overlapped RIXS and v-RXES spectra are observed successfully by our experiments and well reproduced by our SIAM calculations.
|
|
| 31. |
- Kvashnina, Kristina O., et al.
(author)
-
A Novel Metastable Pentavalent Plutonium Solid Phase on the Pathway from Aqueous Plutonium(VI) to PuO2 Nanoparticles
- 2019
-
In: Angewandte Chemie International Edition. - : John Wiley & Sons. - 1433-7851 .- 1521-3773. ; 58:49, s. 17558-17562
-
Journal article (peer-reviewed)abstract
- Here we provide evidence that the formation of PuO2 nanoparticles from oxidized PuVI under alkaline conditions proceeds through the formation of an intermediate PuV solid phase, similar to NH4PuO2CO3, which is stable over a period of several months. For the first time, state‐of‐the‐art experiments at Pu M4 and at L3 absorption edges combined with theoretical calculations unambiguously allow to determine the oxidation state and the local structure of this intermediate phase.
|
|
| 32. |
- Kvashnina, K. O., et al.
(author)
-
Chemical State of Complex Uranium Oxides
- 2013
-
In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 111:25, s. 253002-
-
Journal article (peer-reviewed)abstract
- We report here the first direct observation of U(V) in uranium binary oxides and analyze the gradual conversion of the U oxidation state in the mixed uranium systems. Our finding clarifies previous contradicting results and provides important input for the geological disposal of spent fuel, recycling applications, and chemistry of uranium species.
|
|
| 33. |
- Kvashnina, Kristina O., et al.
(author)
-
Direct study of the f-electron configuration in lanthanide systems
- 2011
-
In: Journal of Analytical Atomic Spectrometry. - : Royal Society of Chemistry (RSC). - 0267-9477 .- 1364-5544. ; 26:6, s. 1265-1272
-
Journal article (peer-reviewed)abstract
- The valence shell electron configurations within a few electron volts above the Fermi level in cerium, ytterbium, europium and samarium compounds were probed by resonant X-ray emission spectroscopy (RXES) at the L-3 absorption pre-edge. The rare earth systems show distinct spectral signatures depending on the f-electron configuration. The high energy resolution experimental results reported here are well reproduced by atomic multiplet calculations confirming the localized character of the 4f electrons. The magnitude of the electron-electron interactions within the 4f shell and between 3d and 4f electrons is analyzed. The present technique is a powerful tool for the study of the 4f valence electron configuration that, unlike L-3 absorption spectroscopy at the main edge, is little influenced by valence electron relaxation following core hole creation.
|
|
| 34. |
- Kvashnina, Kristina O., et al.
(author)
-
In situ X-ray absorption study of copper films in ground water solutions
- 2007
-
In: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 447:1-3, s. 54-57
-
Journal article (peer-reviewed)abstract
- This study illustrates how the damage from copper corrosion can be reduced by modifying the chemistry of the copper surface environment. The surface modification of oxidized copper films induced by chemical reaction with Cl− and in aqueous solutions was monitored by in situ X-ray absorption spectroscopy. The results show that corrosion of copper can be significantly reduced by adding even a small amount of sodium bicarbonate. The studied copper films corroded quickly in chloride solutions, whereas the same solution containing 1.1 mM prevented or slowed down the corrosion processes.
|
|
| 35. |
- Kvashnina, Kristina O., et al.
(author)
-
Trends in the valence band electronic structures of mixed uranium oxides
- 2018
-
In: Chemical Communications. - : ROYAL SOC CHEMISTRY. - 1359-7345 .- 1364-548X. ; 54:70, s. 9757-9760
-
Journal article (peer-reviewed)abstract
- The valence band electronic structures of mixed uranium oxides (UO2 , U4O9 , U3O7, U3O8 , and beta-UO3) have been studied using the resonant inelastic X-ray scattering (RIXS) technique at the U M-5 edge and computational methods. We show here that the RIXS technique and recorded U 5f-O 2p charge transfer excitations can be used to test the validity of theoretical approximations.
|
|
| 36. |
- Man, Gabriel, et al.
(author)
-
Electronic coupling between the unoccupied states of the organic and inorganic sublattices of methylammonium lead iodide : A hybrid organic-inorganic perovskite single crystal
- 2021
-
In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:4
-
Journal article (peer-reviewed)abstract
- Organic-inorganic halide perovskites have been intensively reinvestigated due to their applications, yet the optoelectronic function of the organic cation remains unclear. Through organic-selective resonant Auger electron spectroscopy measurements on well-defined single-crystal surfaces, we find evidence for electronic coupling in the unoccupied states between the organic and inorganic sublattices of the prototypical hybrid perovskite, which is contrary to the notion based on previous studies that the organic cation is electronically inert. The coupling is relevant for electron dynamics in the material and for understanding optoelectronic functionality.
|
|
| 37. |
- Modin, A., et al.
(author)
-
Electronic structure of Cu3N films studied by soft x-ray spectroscopy
- 2008
-
In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 20:23
-
Journal article (peer-reviewed)abstract
- Soft x- ray emission spectroscopy was used to characterize the electronic structure of seven copper nitride films, one synthesized with atomic layer deposition ( ALD) and six grown with chemical vapor deposition ( CVD) at different preparation temperatures. Interpretation of the x- ray emission spectra was supported by calculations of the electronic structure for bulk pure Cu3N and Cu3N with: an excess of Cu atoms, oxygen or carbon impurities, and N vacancies. The calculations are shown to describe the experimental spectra quite well. Analysis of the x- ray spectra suggests that films grown in copper rich environments and above a cut- off temperature of approximately 360 degrees C have a growing fraction of copper enriched areas, while films prepared below this temperature do not have these areas with excess copper.
|
|
| 38. |
- Modin, Anders, et al.
(author)
-
Indication of single-crystal PuO2 oxidation from O 1s x-ray absorption spectra
- 2011
-
In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 83:7, s. 075113-
-
Journal article (peer-reviewed)abstract
- The electronic structure of single-crystal PuO2 is studied using O 1s x-ray absorption (XA) and x-ray emission. Interpretation of the experimental data is supported by extensive first-principles calculations on the basis of the density functional theory + U approach. The measured XA spectra show a significant difference in intensity for the first two peaks between different spots or areas on the single crystal. Our theoretical simulations show that the first peak, at similar to 531 eV, can be attributed to O 2p-Pu 5f hybridization, while the second peak, at similar to 533.4 eV, is due to hybridization of O 2p with Pu d states. The reasons for the observed differences in the O 1s XA spectra are explored by calculating a number of defect structures PuO2+x as well as by simulating the existence of Pu(V) sites. Our results indicate the presence of oxidation states higher than Pu(IV) in some areas of the single crystal. The findings also suggest that plutonium oxide with a Pu fraction in an oxidation state higher than Pu(IV) consists of inequivalent Pu sites with (PuO2)-O-(IV) and (PuO2)-O-(V) rather than representing a system where the Pu oxidation state is constantly fluctuating between Pu(IV) and Pu(V).
|
|
| 39. |
- Phuyal, Dibya, 1985-, et al.
(author)
-
Electronic structure of the charge disproportionate and metallicity in LaFe1-xMoxO3 studied by resonant x-ray spectroscopies
-
Other publication (other academic/artistic)abstract
- A metal-insulator transition and its intriguing properties were recently reported based on doping of the antiferromagnetic insulator LaFeO3. Through doping at the B site by Mo to the insulating perovskite LaFeO3 exhibits a coexistence of an antiferromagnetic ordering with room-temperature metallicity at higher doping levels, an unusual ground state for an oxide perovskite. Replacing Fe by Mo in the Fe-O6 octahedra enhances the separation of the two 2 Fe-O bonds relative to the orthorhombic LaFeO3, largely retaining the centrosymmetric character of the Fe sites, as confirmed through extended x-ray absorption fine structure. Mo ions appear to be homogeneously doped, with average valency of both metal sites monotonically decreasing with increasing Mo concentration. Resonant photoemission was recorded for both Fe and Mo to obtain element specific spectral information on the projected 3d and 4d partial density of states. The data shows gradual increase of spectral weight at the Fermi level that is attributed to Mo d states, while the Fe d states primarily reside at higher binding energy in the valence band, with significant hybridization with O 2p states. The RIXS spectra show strong electron correlation effects from d-d inelastic features and broad charge-transfer excitations for x = 0.2 where the compound becomes metallic. This is in contrast to the insulating parent LaFeO3 compound and insulating samples with lower Mo content. The coexistence of both bound and continuum excitations observed in the RIXS spectra provides strong evidence for charge disproportionation with features that are linked to the disproportionate Fe sites.
|
|
| 40. |
- Phuyal, Dibya, 1985-, et al.
(author)
-
Electronic Structure of Two-Dimensional Lead(II) Iodide Perovskites : An Experimental and Theoretical Study
- 2018
-
In: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 30:15, s. 4959-4967
-
Journal article (peer-reviewed)abstract
- Layered two-dimensional (2D) hybrid organic-inorganic perovskites (HOP) are promising materials for light-harvesting applications because of their chemical stability, wide flexibility in composition and dimensionality, and increases in photovoltaic power conversion efficiencies. Three 2D lead iodide perovskites were studied through various X-ray spectroscopic techniques to derive detailed electronic structures and band energetics profiles at a titania interface. Core-level and valence band photoelectron spectra of HOP were analyzed to resolve the electronic structure changes due to the reduced dimensionality of inorganic layers. The results show orbital narrowing when comparing the HOP, the layered precursor PbI2, and the conventional 3D (CH3NH3)PbI3 such that different localizations of band edge states and narrow band states are unambiguously due to the decrease in dimensionality of the layered HOPs. Support from density functional theory calculations provide further details on the interaction and band gap variations of the electronic structure. We observed an interlayer distance dependent dispersion in the near band edge electronic states. The results show how tuning the interlayer distance between the inorganic layers affects the electronic properties and provides important design principles for control of the interlayer charge transport properties, such as the change in effective charge masses as a function of the organic cation length. The results of these findings can be used to tune layered materials for optimal functionality and new applications.
|
|
| 41. |
- Phuyal, Dibya, 1985-, et al.
(author)
-
Ferroelectric properties of doped BaTiO3 thin film by pulsed laser deposition
-
Other publication (other academic/artistic)abstract
- BaTiO3 thin films co-doped at the Ti site with Mn and Nb were grown on 0.5% Nb-doped (001) oriented SrTiO3 substrates by pulsed laser deposition. The films show single-phase tetragonal structure, growing epitaxially with a smooth interface to the substrate. Using piezoforce microscopy, we find that both doped and undoped films exhibit good ferroelectric response. The piezoelectric domain switching in the films was confirmed by measuring local hysteresis of the polarization at several different areas across the thin films, demonstrating a switchable ferroelectric state for these films. The doping of the BaTiO3 also reduces the bandgap of the material from 3.18 eV for BaTiO3 to nearly 2.7 eV for the 7.5% doped sample, potentially making the films effective light-harvesters in the visible spectrum. The results demonstrate co-doping as an effective strategy for bandgap engineering and a guide for the realization of visible-light applications of related thin film systems.
|
|
| 42. |
- Phuyal, Dibya, et al.
(author)
-
The origin of low bandgap and ferroelectricity of a co-doped BaTiO3
- 2018
-
In: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 124:2
-
Journal article (peer-reviewed)abstract
- We recently demonstrated the lowest bandgap bulk ferroelectric material, BaTi1-x(Mn1/2Nb1/2)xO3, a promising candidate material for visible light absorption in opto- electronic devices. Using a combination of x-ray spectroscopies and density functional theory (DFT) calculations, we here elucidate this compound’s electronic structure and the modifications induced by Mn doping. In particular, we are able to rationalize how this compound retains its ferroelectricity even through a significant reduction of the optical gap upon Mn doping. The local electronic structure and atomic coordination are investigated using x-ray absorption at the Ti K, Mn K, and O K edges, which suggests only small distortions to the parent tetragonal ferroelectric system, BaTiO3, thereby providing a clue to the substantial retention of ferroelectricity in spite of doping. Features at the Ti K edge, which are sensitive to local symmetry and an indication of Ti off-centering within the Ti-O6 octahedra, show modest changes with doping and strongly corroborates with our measured polarization values. Resonant photoelectron spectroscopy results suggest the origin of the reduction of the bandgap in terms of newly created Mn d bands that hybridize with O 2p states. X-ray absorption spectra at the O K-edge provide evidence for new states below the conduction band of the parent compound, illustrating additional contributions facilitating bandgap reduction.
|
|
| 43. |
- Phuyal, Dibya, 1985-, et al.
(author)
-
Valence electronic structure of the double perovskite Sr2FeMoO6 through resonant x-ray spectroscopies
-
Other publication (other academic/artistic)abstract
- Resonant x-ray spectroscopies were used to investigate the double perovskite Sr2FeMoO6 (SFMO) in order to identify the compounds valence electronic structure known for its half-metallicity and ferrimagnetism above room temperature. A comprehensive resonant photoelectron spectroscopy (RPES) and resonant inelastic x-ray scattering (RIXS) study was carried out to acquire a more detailed understanding and interpretation of the valence band electronic structure and hybridization between Fe and Mo d bands. The RPES spectrum clearly show enhancement of Mo d states at the Fermi level that are partly responsible for the metallic behavior. The RPES data at the Fe L3 edge shows strong hybridization with O 2p states as well hybridization with Mo d states. At the same time, characteristic d-d excitations are observed in the RIXS spectra at the Fe L edge indicative of Fe d states of more localized character. Although there are many valid definitions to the valency of the Fe and Mo ions and magnetic moments, the total electronic structure represented through element-resolved x-ray spectroscopies studies distinctly identifies the electronic states that results in its established correlated behavior and half-metallicity.
|
|
| 44. |
- Pidchenko, Ivan, et al.
(author)
-
Synthesis, Structural, and Electronic Properties of K4PuVIO2(CO3)3(cr) : An Environmentally Relevant Plutonium Carbonate Complex
- 2020
-
In: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 59:17, s. 11889-11893
-
Journal article (peer-reviewed)abstract
- The chemical properties of actinide materials are often predefined and described based on the data available for isostructural species. This is the case for potassium plutonyl (PuVI) carbonate, K4PuVIO2(CO3)3(cr), a complex relevant for nuclear technology and the environment, of which the crystallographic and thermodynamic properties of which are still lacking. We report here the synthesis and characterization of PuVI achieved by single-crystal X-ray diffraction analysis and high-energy-resolution fluorescence-detected X-ray absorption near-edge structure at the Pu M4-edge coupled with electronic structure calculations. The crystallographic properties of PuVI are compared with isostructural uranium (U) and neptunium (Np) compounds. Actinyl (AnVI) axial bond lengths, [O–AnVI–O]2+, are correlated between solid, K4AnVIO2(CO3)3(cr), and aqueous, [AnVIO2(CO3)3]4–(aq) species for the UVI–NpVI–PuVI series. The spectroscopic data are compared to KPuVO2CO3(cr) and PuIVO2(cr) to tackle the trend in the electronic structure of PuVI regarding the oxidation state changes and local structural modifications around the Pu atom.
|
|
| 45. |
- Plakhova, Tatiana V., et al.
(author)
-
Towards the surface hydroxyl species in CeO2 nanoparticles
- 2019
-
In: Nanoscale. - : ROYAL SOC CHEMISTRY. - 2040-3364 .- 2040-3372. ; 11:39, s. 18142-18149
-
Journal article (peer-reviewed)abstract
- Understanding the complex chemistry of functional nanomaterials is of fundamental importance. Controlled synthesis and characterization at the atomic level is essential to gain deeper insight into the unique chemical reactivity exhibited by many nanomaterials. Cerium oxide nanoparticles have many industrial and commercial applications, resulting from very strong catalytic, pro- and anti-oxidant activity. However, the identity of the active species and the chemical mechanisms imparted by nanoceria remain elusive, impeding the further development of new applications. Here, we explore the behavior of cerium oxide nanoparticles of different sizes at different temperatures and trace the electronic structure changes by state-of-the-art soft and hard X-ray experiments combined with computational methods. We confirm the absence of the Ce(III) oxidation state at the surface of CeO2 nanoparticles, even for particles as small as 2 nm. Synchrotron X-ray absorption experiments at Ce L-3 and M-5 edges, combined with X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and small angle X-ray scattering (SAXS) and theoretical calculations demonstrate that in addition to the nanoceria charge stability, the formation of hydroxyl groups at the surface profoundly affects the chemical performance of these nanomaterials.
|
|
| 46. |
- Safdari, Majid, et al.
(author)
-
Impact of synthetic routes on the structural and physical properties of butyl-1,4-diammonium lead iodide semiconductors
- 2017
-
In: Journal of Materials Chemistry A. - : Royal Society of Chemistry. - 2050-7488 .- 2050-7496. ; 5:23, s. 11730-11738
-
Journal article (peer-reviewed)abstract
- We report the significant role of synthetic routes and the importance of solvents in the synthesis of organic-inorganic lead iodide materials. Through one route, the intercalation of dimethylformamide in the crystal structure was observed leading to a one-dimensional (1D) [NH3(CH2)4NH3]Pb2I6 structure of the product. This product was compared with the two-dimensional (2D) [NH3(CH2)4NH3]PbI4 recovered from aqueous solvent based synthesis with the same precursors. UV-visible absorption spectroscopy showed a red-shift of 0.1 eV for the band gap of the 1D network in relation to the 2D system. This shift primarily originates from a shift in the valence band edge as determined from photoelectron-and X-ray spectroscopy results. These findings also suggest the iodide 5p orbital as the principal component in the density of states in the valence band edge. Single crystal data show a change in the local coordination around iodide, while in both materials, lead atoms are surrounded by iodide atoms in octahedral units. The conductivity of the one-dimensional material ([NH3(CH2)4NH3]Pb2I6) was 50% of the two-dimensional material ([NH3(CH2)4NH3]PbI4). The fabricated solar cells reflect these changes in the chemical and electronic structure of both materials, although the total light conversion efficiencies of solar cells based on both products were similar.
|
|
| 47. |
- Soroka, Inna L., et al.
(author)
-
Structural stability and oxidation resistance of amorphous Zr-Al alloys
- 2010
-
In: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 401:1-3, s. 38-45
-
Journal article (peer-reviewed)abstract
- We investigated the structural stability and oxidation resistance of Zr-Al films upon annealing in air. The concentration of Zr was varied from 0 to 100 at.%, with a step of 10 at.%. The films were fabricated using ultra-high vacuum based magnetron sputtering. The as-deposited films with Zr content from 17.3 at.% to 70.7 at.% were found to be X-ray amorphous at room temperature. When exposed to air a thin oxide layer, typically less than 6 nm, is formed. The thickness of the oxide layers increases when the samples are annealed in air and most of these are found to be fully oxidized at 700 degrees C with the formation of crystalline and amorphous oxides on the top of crystalline and amorphous metal films, respectively. The amorphous oxide layers are found to be dense, with well defined thicknesses. An experimental non-equilibrium phase diagram is provided, covering the whole concentration range of the Zr-Al system. (C) 2010 Elsevier B.V. All rights reserved.
|
|
| 48. |
- Tobin, J. G., et al.
(author)
-
FEFF analysis of americium oxides
- 2024
-
In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Vacuum Society. - 0734-2101 .- 1520-8559. ; 42:2
-
Journal article (peer-reviewed)abstract
- The Am N4,5 (4d3/2 and 4d5/2) and Am O4,5 (5d3/2 and 5d5/2) x-ray absorption spectroscopy (XAS) of americium sesquioxide (Am2O3) and americium dioxide (AmO2) has been evaluated with FEFF, a Green's function–based, multiple scattering code. Taking guidance from the intermediate coupling model (ICM), applicable to local and nonmagnetized samples, it is possible to completely reconstruct the experimental results for the N4,5 spectra, including the observed differences between the Am2O3 and the AmO2 cases. Although complicated by a more asymmetric line shape and difficult background variations, the FEFF analysis confirms the absence of core hole angular momentum coupling in Am O4,5 spectroscopy.
|
|
| 49. |
- Vegelius, Johan, et al.
(author)
-
Atomic and electronic structure of amorphous Al-Zr alloy films
- 2011
-
In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 23:26, s. 265503-
-
Journal article (peer-reviewed)abstract
- Amorphous Al73Zr27 alloy film, grown and then subjected to heat treatments at 400 and 700 degrees C, was studied using a combination of x-ray diffraction and soft x-ray spectroscopic techniques. The Al L-2,L-3 and Al K x-ray absorption spectroscopy (XAS) and Al L-2,L-3 x-ray emission spectroscopy (XES) used allowed probing the unoccupied and occupied Al 3s, d states and unoccupied Al 3p states in the sample studied. An irreversible transition from amorphous alloy to a mixture of polycrystalline alloy and amorphous alloy, and then to an amorphous oxide phase was observed. After the annealing at 400 degrees C the Al L-2,L-3 spectra obtained by XAS could be explained as sums of spectra from amorphous Al73Zr27 alloy and (poly) crystalline Al. This indicates that the sample consists of a mixture of Al-rich crystalline and Zr-enhanced amorphous alloys, as compared to the stoichiometry of the as-deposited Al73Zr27 sample, and that the electronic wavefunctions in the crystalline and amorphous regions can be considered to be confined within the respective regions. The relative amounts of Al atoms were found to be around 1: 3 in the crystalline and amorphous phases, respectively, as deduced from the analysis of changes in the electronic structure using Al L-2,L-3 XAS data. The interpretation was confirmed by the Al K XAS and Al L-2,L-3 XES. Upon further annealing at 700 degrees C the polycrystalline phase transformed into amorphous oxide, while the amorphous alloy phase underwent gradual oxidation. The important finding was that the greater part of the sample remained in the amorphous state throughout the temperature regimes described.
|
|
| 50. |
- Vegelius, J. R., et al.
(author)
-
X-ray Spectroscopic Study of Cu2S, CuS, and Copper Films Exposed to Na2S Solutions
- 2012
-
In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 116:42, s. 22293-22300
-
Journal article (peer-reviewed)abstract
- The electronic structure of copper sulfides was studied using X-ray absorption at Cu K and L-2,L-3 and S L-2,L-3 edges and X-ray emission spectroscopy at S L-2,L-3 thresholds. The experimental data were compared with results of local density approximation calculations taking into account the calculated valence band contributions and core-level chemical shifts between inequivalent sites. The presence of divalent Cu was detected in CuS with X-ray absorption spectroscopy in the high-energy-resolution fluorescence detection (HERFD) mode. The S L-2,L-3 emission spectrum of CuS was found to be significantly broader than that of Cu2S due to contributions from inequivalent S sites. This difference in the spectral width was used to distinguish between sulfide species formed on the Cu foil exposed to Na2S solutions. The Cu-S interaction processes for Cu films of different thicknesses exposed to Na2S solutions were monitored in situ using X-ray absorption spectroscopy in the HERFD mode at the Cu K edge. It was found that Cu ions were diffusing in the sulfide and entering the solution. The rate at which Cu ions enter the solution via diffusion was estimated on the basis of the results for a 1000 angstrom thick film.
|
|
