Single-valence uranium compounds are much less widespread, the most common being uranium(IV) oxide (UO 2) known for its application as nuclear fuel in light-water reactors. Uranium occurs in numerous ores and minerals, as well as in seawater, but most often it is present in a hexavalent U(VI) state, or in a mixture of tetra-, penta- or hexavalent states (Grenthe et al., 2006 ). Indeed, the U(V) valence state corresponds to a simpler electronic configuration 5 f 1, where no electron–electron repulsion interactions are expected, at least within the 5 f shells. The influence of the latter effect can be significantly reduced by exploring pure pentavalent U(V) compounds. Experimental studies typically require dedicated laboratories, owing to the natural radioactivity of uranium, and, in addition, theoretical calculations are revealed to be extremely complicated because of comparable magnitudes of the crystal field, spin–orbit coupling and the electron–electron repulsion interactions. Despite the large amount of crystallographic, physical, chemical and thermodynamic data available today (Grenthe et al., 2006 ), understanding of the uranium electronic structure is far from complete. One consequence is the ability of the 5 f electrons to participate in the conduction band and to remain localized simultaneously, producing rather complex and still unclear behaviour of the uranium bonding nature with a mixed covalent/ionic character (Kaltsoyannis, 2013 ). Additionally, uranium 5 f electrons show an apparent duality in localization, being often found in radially dispersed and hybridized bands in the vicinity of the Fermi level, whereas sometimes they remain localized (Guziewicz et al., 2004 Teterin et al., 1981 Teterin & Teterin, 2004 ). In particular, uranium can induce very versatile physico-chemical properties due to the wide range of possible oxidation states offered by its 7 s 26 d 15 f 3 electronic ground state configuration. Uranium compounds are usually associated with nuclear applications, such as nuclear fission for energy generation, but for many decades they also triggered interest from more fundamental aspects.
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