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Issue 15, 2018
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Metastable electronic states in uranium tetrafluoride

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Abstract

The DFT+U approach, where U is the Hubbard-like on-site Coulomb interaction, has successfully been used to improve the description of transition metal oxides and other highly correlated systems, including actinides. The secret of the DFT+U approach is the breaking of d or f shell orbital degeneracy and adding an additional energetic penalty to non-integer occupation of orbitals. A prototypical test case, UO2, benefits from the +U approach whereby the bare LDA method predicts UO2 to be a ferromagnetic metal, whereas LDA+U correctly predicts UO2 to be insulating. However, the concavity of the energetic penalty in the DFT+U approach can lead to a number of convergent “metastable” electronic configurations residing above the ground state. Uranium tetrafluoride (UF4) represents a more complex analogy to UO2 in that the crystal field has lower symmetry and the unit cell contains two symmetrically distinct U atoms. We explore the metastable states in UF4 using several different methods of selecting initial orbital occupations. Two methods, a “pre-relaxation” method wherein an initial set of orbital eigenvectors is selected via the self-consistency procedure and a crystal rotation method wherein the x, y, z axes are brought into alignment with the crystal field, are explored. We show that in the case of UF4, which has non-collinearity between its crystal axes and the U atoms' crystal field potentials, the orbital occupation matrices are much more complex and should be analyzed using a novel approach. In addition to demonstrating a complex landscape of metastable electronic states, UF4 also shows significant hybridization in U–F bonding, which involves non-trivial contributions from s, p, d, and f orbitals.

Graphical abstract: Metastable electronic states in uranium tetrafluoride

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Publication details

The article was received on 27 Nov 2017, accepted on 08 Mar 2018 and first published on 03 Apr 2018


Article type: Paper
DOI: 10.1039/C7CP07970B
Citation: Phys. Chem. Chem. Phys., 2018,20, 10384-10395
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    Metastable electronic states in uranium tetrafluoride

    A. Miskowiec, Phys. Chem. Chem. Phys., 2018, 20, 10384
    DOI: 10.1039/C7CP07970B

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