Light impurity atoms as the probes for the electronic structures of actinide dioxides

- The general trends of energetics and stability of light impurities in AnO2 are firstly reported.
- The delocalization → localization transition of 5f electron is the origin of impurity effect.
- Hydrogen is very valuable in understanding the differences in the electronic structures of AnO2.

First-principles density functional theory are used to calculate the formation energies of ten light impurities X (X: H, He, Li, Be, B, C, N, O, F and Ne) in seven actinide dioxides AnO2 (An: Th, Pa, U, Np, Pu, Am and Cm), in order to elucidate the relative stability of X and to obtain some trends of impurities behaviors. The Hubbard parameter U is used to describe the strongly correlated electron behavior of An 5f electrons. The results indicate that the formation energies of X significantly depend on the properties of AnO2 and X. For X at the octahedral interstitial sites of AnO2, F is the only energetically favorable impurity for all AnO2, owing to its strong oxidability; H in PaO2, O in PaO2 and UO2, Li in PuO2, AmO2 and CmO2, Be in AmO2 and CmO2 are also energetically favorable. The oxidability or reductivity of X and the delocalization → localization transition of 5f electrons across actinide series can account for the trends of the behaviors of X in AnO2. Particularly, H, a very typical amphoteric element, is chosen to illustrate its difference existence states in AnO2. H prefers to occupy the octahedral interstitial sites of early AnO2 or form hydroxyl group in the later AnO2.

The formation energy of the amphoteric impurity H can sensitively reflect the delocalization → localization transition of 5f electrons across the AnO2 series. H tends to oxidize or reduce AnO2 with delocalized or localized 5f electrons, corresponding to form nominal [(AnO2)n]+H− and [AnnO2n−1]+[OH]−, respectively.139