Electronic structure of uranium halides and oxyhalides in the solid state. An X-ray photoelectron spectral study of bonding ionicity

Core and valence X-ray photoelectron spectra of 18 uranium halides (U3+ to U5+) and oxyhalides (U3+ to U6+) (fluorides, chlorides, and bromides) are described. The results are discussed, as a function of halogen nature and compounds stoichiometry, in terms of ionicity of the bonds, of U 5f participation in bonding, and effect of oxygen. Evidence is produced that UF3 has an unexpectedly strong covalent character. The experimental study of uranium halides (F, Cl, Br) and oxyhalides has brought to light some important information about the nature of chemical bonding in these compounds: (a) The combined examination of valence photoelectron peaks (particularly U 5f), core levels, and U 4f shake-up satellites is a powerful tool in the determination of the ionicity of the solid. Only core level chemical shifts are not sufficient by themselves. (b) There is evidence for U 5f participation in chemical bonding; this contribution increases when the compound becomes more covalent. (c) Ionicity is largest for the higher halides (higher uranium oxidation state) and with the more electronegative halogens; and exception to this rule is UF3, which is more covalent than UCl3. (d) The oxygen in oxyhalides shows a growing tendency to form uranyl groups, as the uranium oxidation state goes up. These uranyl groups have covalent bonding and behave as a single species vs. the halogen