Whether
PuO2 could be further oxidized is a very important
topic in Pu chemistry. Experiments on its oxidation have not completely
demonstrated the existence of higher oxides. Here, the reaction energies
of PuO2 with a series of molecules and their radiation-induced
radicals and products are systematically predicted using first-principles
calculations. The results show that F2 can react with PuO2, indicating the existence of higher-valence Pu in PuO2-based materials. Moreover, the formations of PuO2.25, PuO2.25H0.25, and PuO2.5H0.5 by the reactions of PuO2 with O• and OH• radials are exothermic. However, from
the reaction energies of the reactions between hyperstoichiometric
Pu oxides and two radicals, PuO2+x
H
x
, instead of PuO2+x
, are deduced to be the experimentally observed products in
the kinetically favorable environments. The widely discussed PuO2+x
act only as the intermediate products.
Electronic structures of these hyperstoichiometric Pu oxides also
reveal that PuO2+x
H
x
are more stable than PuO2+x
.
First-principles DFT + U methods are performed
to calculate the formation energy and to determine the relative stability
of hydrogen at the different sites of UO2 and PuO2. Twenty-one incorporation sites for hydrogen, i.e., along the pathway
from its first nearest-neighboring oxygen to the octahedral interstitial
site, are considered. The results indicate that hydrogen in UO2 energetically prefers to exist as a hydride ion ([(UO2)
n
]+H–) rather than forming a hydroxyl group ([U
n
O2n–1]+[OH]−). The negative formation energy of hydrogen at the
octahedral interstitial site of UO2 shows that hydrogen
is soluble and can oxidize uranium ion to the higher valence states.
However, hydrogen in PuO2 is relatively stable in the form
of [Pu
n
O2n–1]+[OH]− with comparison to [(PuO2)
n
]+H–. The slightly positive formation energy of hydrogen in the form
of the hydroxyl group in PuO2 reveals that hydrogen is
either insoluble or just lies on the edge of solubility. The differences
in the existence states of atomic hydrogen in the two dioxides are
proposed to be dependent on the nature of 5f electrons of uranium
and plutonium; that is, uranium 5f electrons are more delocalized
and more favorable to participate in chemical bonding than plutonium
5f electrons.
The energetics of some typical non-metallic impurity atoms (H, Ne, Cl, Ar, Kr and Xe) in PuO 2 are calculated using a projector augmented-wave method under the framework of density functional theory. The Hubbard parameter U and van der Waals corrections are used to describe the strongly correlated electronic behavior of f electrons in Pu and weak interactions of rare gases, respectively. Three incorporation sites of for impurity atoms, i.e., octahedral interstitial, O vacancy, and Pu vacancy sites, are considered. The results indicate that the energetics of impurity atoms depend significantly on the incorporation sites and on atomic properties such as atomic radius and electron affinity. Almost all impurity atoms considered here are energetically unfavorable at the three incorporation sites, with the exception of the F atom at the octahedral interstitial and O vacancy sites. The trends of incorporation energies of rare gas atoms generally reflect a size effect. Furthermore, charge-transfer analysis reveals that the valence electrons can be polarized more easily with increasing atomic number of rare gas elements. Finally, electronic structures of these systems containing impurity atoms also exhibit general trends in their relative stability and chemical bonding character.
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