Nature of the Chemical Bond in Uranium Dioxide UO2

Fine structure of the X-ray photoelectron and conversion spectra of low-energy (0-40 eV) electrons of uranium dioxide UO2 was analyzed based on the electronic structure calculations for the UO812- cluster with O-h symmetry, simulating the nearest surrounding of uranium in UO2, by the relativistic X-alpha discrete variation method. It was predicted theoretically and validated experimentally that, in UO2, the U5f electrons (similar to 1 U5f electron) can directly participate in chemical bonding: similar to 2 U5f electrons weakly contributing to chemical bonding are localized at -1.9 eV; similar to 1 U5f electron participating in chemical bonding is delocalized in the range of outer valence molecular orbital energies from -4 to -9 eV; and unfilled U5f states are localized mostly at low (from 0 to 5 eV above zero) energies. It was shown experimentally that the U6p electrons actively participate in formation of not only inner valence but also outer valence (0.6 U6p electron) molecular orbitals. The density of the U6p states in UO2 was estimated experimentally. The composition and sequence of the inner valence molecular orbitals at energies within 13-40 eV were also elucidated.