Metastable electronic states in uranium tetrafluoride

• Published in: Physical chemistry chemical physics : PCCP 2018-04, Vol.2 (15), p.1384-1395
• Main Author: Miskowiec, Andrew
• Format: Article
• Language: English
• Subjects: Actinides; Axes (reference lines); Collinearity; Concavity; Crystals; Eigenvectors; Electron states; Ferromagnetism; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Metastable state; Occupation; Orbitals; Transition metal oxides; Uranium; Uranium dioxide
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c7cp07970b

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, UO 2 , benefits from the + U approach whereby the bare LDA method predicts UO 2 to be a ferromagnetic metal, whereas LDA+ U correctly predicts UO 2 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 (UF 4 ) represents a more complex analogy to UO 2 in that the crystal field has lower symmetry and the unit cell contains two symmetrically distinct U atoms. We explore the metastable states in UF 4 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 UF 4 , 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, UF 4 also shows significant hybridization in U-F bonding, which involves non-trivial contributions from s, p, d, and f orbitals. We present two methods for exploring metastable electronic states in a model low-symmetry actinide system.