Vibrational properties of uranium fluorides

Andrew Miskowiec; Ashley E. Shields; J. L. Niedziela; Yongqiang Cheng; Paul Taylor; Guillermo DelCul; Rodney Hunt; Barry Spencer; John Langford; Douglas Abernathy

doi:10.1016/j.physb.2019.06.049

Vibrational properties of uranium fluorides

Andrew Miskowiec, Ashley E. Shields, J. L. Niedziela, Yongqiang Cheng, Paul Taylor, Guillermo DelCul, Rodney Hunt, Barry Spencer, John Langford, Douglas Abernathy

Research output: Contribution to journal › Article › peer-review

7 Scopus citations

Abstract

Multiphase mixtures of the uranium fluoride compounds UFx with x = 3, 4, 4.5, 5, whose local U–F bonding geometry is conserved, may result from UF₆ reduction. One method for identifying multiphase mixtures is optical vibrational spectroscopy, but experimental Raman and IR spectra for UFx compounds are difficult to obtain. To supplement those experimental measurements, we use density functional perturbation theory (DFPT) with the on-site Coulomb correction (Hubbard + U) to calculate the phonon normal modes, their IR intensity, and their Raman activity for UF₃, UF₄, U₂F₉, and UF₅. In addition, we use neutron spectroscopy to measure the phonon density of states of the most stable UF₄. Our measurements on the Wide-Angular Range Chopper Spectrometer at the Spallation Neutron Source indicate that UF₄ has a broad phonon spectrum centered at about 30 meV, but modeling of this spectrum indicates significant multiphonon scattering is present. DFPT calculations for U₂F₉, which shares structural features with UF₄ and UF₅, suggest a dynamic instability and we speculate that previous crystallographic measurements of this compound were possible only in multiphase mixtures. Analysis of the detailed atomic motions of phonons in UF₅ indicate that a series of high energy modes (between 69 and 75 meV) are described by motion of single-coordinated F atoms (bonded to only one U atom). These single-coordinated F atoms are unique to the UF₅ structure and those modes could be used to distinguish UF₅ in a multiphase mixture using optical spectroscopy.

Original language	English
Pages (from-to)	194-205
Number of pages	12
Journal	Physica B: Physics of Condensed Matter
Volume	570
DOIs	https://doi.org/10.1016/j.physb.2019.06.049
State	Published - Oct 1 2019

Funding

A portion of this research used resources at the Spallation Neutron Source, a Department of Energy, Office of Science User Facility operated by the Oak Ridge National Laboratory. This research also used resources of the Compute and Data Environment for Science (CADES) at Oak Ridge National Laboratory, which is supported by the U.S. Department of Energy, Office of Science under Contract No. DE-AC05-00OR22725 . A portion of this work was performed by a postdoctoral fellow (A. E. S.), who is supported by the Department of Homeland Security . The authors would like to thank Drs. Heath Huckabay and Douglas Duckworth for a critical reading of the manuscript. This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).A portion of this research used resources at the Spallation Neutron Source, a Department of Energy, Office of Science User Facility operated by the Oak Ridge National Laboratory. This research also used resources of the Compute and Data Environment for Science (CADES) at Oak Ridge National Laboratory, which is supported by the U.S. Department of Energy, Office of Science under Contract No. DE-AC05-00OR22725. A portion of this work was performed by a postdoctoral fellow (A. E. S.), who is supported by the Department of Homeland Security. The authors would like to thank Drs. Heath Huckabay and Douglas Duckworth for a critical reading of the manuscript.☆ This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

Keywords

Density functional theory
Inelastic neutron scattering
Lattice dynamics
Phonons
Uranium

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title = "Vibrational properties of uranium fluorides",

abstract = "Multiphase mixtures of the uranium fluoride compounds UFx with x = 3, 4, 4.5, 5, whose local U–F bonding geometry is conserved, may result from UF6 reduction. One method for identifying multiphase mixtures is optical vibrational spectroscopy, but experimental Raman and IR spectra for UFx compounds are difficult to obtain. To supplement those experimental measurements, we use density functional perturbation theory (DFPT) with the on-site Coulomb correction (Hubbard + U) to calculate the phonon normal modes, their IR intensity, and their Raman activity for UF3, UF4, U2F9, and UF5. In addition, we use neutron spectroscopy to measure the phonon density of states of the most stable UF4. Our measurements on the Wide-Angular Range Chopper Spectrometer at the Spallation Neutron Source indicate that UF4 has a broad phonon spectrum centered at about 30 meV, but modeling of this spectrum indicates significant multiphonon scattering is present. DFPT calculations for U2F9, which shares structural features with UF4 and UF5, suggest a dynamic instability and we speculate that previous crystallographic measurements of this compound were possible only in multiphase mixtures. Analysis of the detailed atomic motions of phonons in UF5 indicate that a series of high energy modes (between 69 and 75 meV) are described by motion of single-coordinated F atoms (bonded to only one U atom). These single-coordinated F atoms are unique to the UF5 structure and those modes could be used to distinguish UF5 in a multiphase mixture using optical spectroscopy.",

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author = "Andrew Miskowiec and Shields, \{Ashley E.\} and Niedziela, \{J. L.\} and Yongqiang Cheng and Paul Taylor and Guillermo DelCul and Rodney Hunt and Barry Spencer and John Langford and Douglas Abernathy",

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AU - Shields, Ashley E.

AU - Niedziela, J. L.

AU - Cheng, Yongqiang

AU - Taylor, Paul

AU - DelCul, Guillermo

AU - Hunt, Rodney

AU - Spencer, Barry

AU - Langford, John

AU - Abernathy, Douglas

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AB - Multiphase mixtures of the uranium fluoride compounds UFx with x = 3, 4, 4.5, 5, whose local U–F bonding geometry is conserved, may result from UF6 reduction. One method for identifying multiphase mixtures is optical vibrational spectroscopy, but experimental Raman and IR spectra for UFx compounds are difficult to obtain. To supplement those experimental measurements, we use density functional perturbation theory (DFPT) with the on-site Coulomb correction (Hubbard + U) to calculate the phonon normal modes, their IR intensity, and their Raman activity for UF3, UF4, U2F9, and UF5. In addition, we use neutron spectroscopy to measure the phonon density of states of the most stable UF4. Our measurements on the Wide-Angular Range Chopper Spectrometer at the Spallation Neutron Source indicate that UF4 has a broad phonon spectrum centered at about 30 meV, but modeling of this spectrum indicates significant multiphonon scattering is present. DFPT calculations for U2F9, which shares structural features with UF4 and UF5, suggest a dynamic instability and we speculate that previous crystallographic measurements of this compound were possible only in multiphase mixtures. Analysis of the detailed atomic motions of phonons in UF5 indicate that a series of high energy modes (between 69 and 75 meV) are described by motion of single-coordinated F atoms (bonded to only one U atom). These single-coordinated F atoms are unique to the UF5 structure and those modes could be used to distinguish UF5 in a multiphase mixture using optical spectroscopy.

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KW - Inelastic neutron scattering

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ER -

Vibrational properties of uranium fluorides

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