Title: Metastable electronic states in uranium tetrafluoride

Abstract

Here, 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 thatmore » 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.« less

Authors:

Miskowiec, Andrew J.  ^[1]

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+ Show Author Affiliations

1. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)

Publication Date:

2018-04-03

Research Org.:

Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)

Sponsoring Org.:

USDOE

OSTI Identifier:

1435318

Grant/Contract Number:  

AC05-00OR22725

Resource Type:

Accepted Manuscript

Journal Name:

Physical Chemistry Chemical Physics. PCCP

Additional Journal Information:

Journal Volume: 20; Journal Issue: 15; Journal ID: ISSN 1463-9076

Publisher:

Royal Society of Chemistry

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY