Valence-band photoemission in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">UO</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>(111) near the<i>5d</i>resonant photon energy

Cox, L.E.; Ellis, W. P.; Cowan, Robert D.; Allen, J. W.; Oh, S.h.; Lindau, I.; Pate, Bradford B.; Arko, A. J.

doi:10.1103/physrevb.35.5761

Cited by 54 publications

(41 citation statements)

References 18 publications

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“…The calculation suggests that the U formal oxidation state is +4 with two unpaired 5f electrons. The calculated density of states shows that at the top of the valence band is mainly U-5f band, and the O-2p band is $3-6 eV below the Fermi level, consistent with the result from photoemission spectroscopic experiments [57]. The calculated unit cell constant is 5.54 Å with the GGA+U (U = 3.8, J = 0.4) methods, $1% higher than 5.47 Å of an experimental value at ambient temperature [58].…”

Section: Dft+u Description Of Uo 2+xsupporting

confidence: 86%

Simulations of the Thermodynamic and Diffusion Properties of Actinide Oxide Fuel Materials

Becker¹

2013

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Short summary of achievementsNEUP grant DE-AC07-05ID14517 concluded on 08/31/12. The objectives of this grant were to (1) determine the solid solution thermodynamics of actinide dioxides and (2) evaluate actinide diffusion in UO 2 . During the three-year grant period, we published six peer-reviewed articles, two peer-reviewed conference proceedings, and disseminated eleven presentations. The papers are attached at the end of this report. In both major objectives and in all sub-objectives of these, we have succeeded to do the necessary method development and derived the thermodynamic mixing properties of a number of fuel-relevant solid solutions and the associated phase diagrams for objective 1. In addition, we developed the basis for actinide diffusion involving different vacancy mechanism and for interstitial He diffusion in fuels and potential waste forms. Solid SolutionsThe major advances in the method development for deriving the mixing thermodynamics of solid solutions were done by modeling different oxide solutions, not only with different compositions but also using different structure types (up to three in the same solid solution series).A fairly standardized approach was used to first develop cation-cation interaction parameters from quantum-mechanical energies of small configuration arrangements with subsequent Monte Carlo calculations of large systems. Further thermodynamic integration allows for the study of temperature effects on the free energy and entropy of mixing of the solid solutions. In the framework of this objective, we analyzed the results beyond what has been done before. We were able to evaluate the apparent discrepancy in the temperature of exsolution onset from the Gibbs free energy of mixing diagrams and the evidence from the projection of the cation ordering. By measuring the height of the Gibbs free energy peak with respect to the tangent of the curve, we were able to determine the exsolution driving force, which was further compared to the kinetic hindrances to exsolution. The primary hindrance to exsolution is the surface energy built up between exsolved lamellae. While the final deliverable was a temperature-composition phase diagram (as described in the proposal), we went beyond by analyzing the effects of cation ordering based on different cation-cation interaction parameter fit models (i.e., fitting with and without a configuration independent energy term). The different fit options allow the Monte Carlo simulations to more closely model an exsolving system. Thus, by comparing the two different approaches, the two extremes -disorder and exsolution -could be simulated. ImplicationsTh x U 1-x O 2 solid solution: this is the first atomic-scale investigation of the energetics and cation ordering in the Th x U 1-x O 2 system has not been conducted. However, according to the computational analysis exsolution temperatures are so low that exsolution in natural uranothorianite and synthetic ThxU1_xO2 is not expected. This has potential implications for fuel properties of the thorium fu...

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Section: Dft+u Description Of Uo 2+xsupporting

confidence: 86%

Simulations of the Thermodynamic and Diffusion Properties of Actinide Oxide Fuel Materials

Becker¹

2013

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“…3). Previously, Cox et al [15] observed only a change in the intensity of the spectrum of OVMO electrons at electron binding energies from 0 to 13 eV. In [8], we observed for the first time the resonance in the spectra of UO 2 and ThO 2 for OVMO and IVMO electrons at electron binding energies within 0340 eV (Fig.…”

Section: Conversion Electron Spectroscopy (Ces)mentioning

confidence: 57%

X-Ray Spectral Methods in Investigation of the UO2 Electronic Structure

Teterin

2005

Radiochemistry

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Fine structure of the X-ray photoelectron spectrum of UO 2 at electron binding energies from 0 tõ 40 eV is primarily due to electrons of outer (0 315 eV) and inner (153 40 eV) valence molecular orbitals formed from unoccupied U5f,6d,7s and O2p and occupied low-energy U6p and O2s shells of the neighboring uranium and oxygen atoms, respectively. This is consistent with the results of the relativistic calculation of the electronic structure of the UO 8 12! cluster with O h symmetry, simulating the nearest surrounding of uranium in UO 2 , and is confirmed by the data of X-ray spectroscopy (conversion electron, nonresonance and resonance X-ray O 4, 5 (U) emission, O 4, 5 (U) XANES, photoelectron resonance, and Auger spectroscopy of oxygen). The fine structure of the X-ray photoelectron spectra, associated with electrons from outer valence and inner valence molecular orbitals, allows estimation of the degree of participation of U6p,5f electrons in chemical bonding, as well as the structure of the nearest surrounding of the uranium atom and the bond length in its oxides. The total contribution from electrons of inner valence molecular orbitals to the absolute value of the chemical bonding energy can be compared with the corresponding contribution of the electrons from outer valence molecular orbitals to bonding of the atoms. Inner valence molecular orbitals can be formed in compounds of any elements, and this is an important new fact in chemistry and physics of condensed state.It was traditionally believed that chemical bonds in compounds are formed mostly from electrons of unoccupied low-energy (from 0 to~15 eV) shells of the neighboring atoms. This was favored to a certain extent by a wide use of sources of excitation in the visible and UV regions in spectroscopy of atoms and molecules. X-ray emission spectroscopic studies of various substances were also oriented mostly at these electron energy regions. Although participation of electrons from relatively deep-lying occupied atomic shells (from~15 to~50 eV) in formation of inner valence molecular orbitals (IVMOs) is justified from the quantum-mechanical viewpoint, it was believed that their influence on the interatomic bonds in compounds is negligible compared to the most weakly bound electrons of the outer valence molecular orbitals (OVMOs) (energies from 0 to~15 eV). However, we found recently [133] that, under certain conditions, inner valence molecular orbitals can be formed in compounds of any elements [133]. This is a new, extremely important, scientific fact in chemistry and physics of condensed state, which can hardly be overestimated.In this study, we consider new data on the nature of chemical bonding with UO 2 as an example. These data were obtained using modern X-ray spectral methods such as X-ray photoelectron, conversion electron, nonresonance and resonance X-ray O 4, 5 (U) emission, O 4, 5 (U) XANES, photoelectron resonance, and Auger spectroscopy of oxygen, taking into account the results of the relativistic calculation of the electronic structure of th...

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“…Resonance photoemission based on 5d-5f Fano resonance revealed the 5f nature of this feature. 50 Dispersion in the O 2p manifold between 24 and 28 eV was also observed. 12 Here we report dispersion in the dominant 5f feature.…”

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confidence: 71%

Dispersion in the Mott insulator UO₂: A comparison of photoemission spectroscopy and screened hybrid density functional theory

et al. 2008

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We present a comparison between the screened hybrid density functional theory of Heyd, Scuseria, and Enzerhof (HSE06) and high-resolution photoemission (PES) measurement on a single crystal of UO(2). Angle-resolved photoemission data show a slight dispersion in the f-orbital derived bands in good agreement with the HSE band structure. The effect of spin-orbit coupling on the HSE band gap has also been calculated and found to be negligible.

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Valence-band photoemission inUO2(111) near the5dresonant photon energy

Cited by 54 publications

References 18 publications

Simulations of the Thermodynamic and Diffusion Properties of Actinide Oxide Fuel Materials

Simulations of the Thermodynamic and Diffusion Properties of Actinide Oxide Fuel Materials

X-Ray Spectral Methods in Investigation of the UO2 Electronic Structure

Dispersion in the Mott insulator UO₂: A comparison of photoemission spectroscopy and screened hybrid density functional theory

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Valence-band photoemission inUO2(111) near the5dresonant photon energy

Cited by 54 publications

References 18 publications

Simulations of the Thermodynamic and Diffusion Properties of Actinide Oxide Fuel Materials

Simulations of the Thermodynamic and Diffusion Properties of Actinide Oxide Fuel Materials

X-Ray Spectral Methods in Investigation of the UO2 Electronic Structure

Dispersion in the Mott insulator UO2: A comparison of photoemission spectroscopy and screened hybrid density functional theory

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Dispersion in the Mott insulator UO₂: A comparison of photoemission spectroscopy and screened hybrid density functional theory