A first principles investigation of the electronic structure of actinide oxides.
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A first principles investigation of the electronic structure of actinide oxides. Leon Petit1, Axel Svane2, Zdzislawa Szotek1, Walter Temmerman1 and Malcolm Stocks3 1 Daresbury Laboratory, Daresbury, Warrington WA4 4AD, United Kingdom. 2 Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark. 3 Materials Science and Technology Division and Center for Defect Physics, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, U.S.A. ABSTRACT The ground state electronic structures of the actinide oxides AO, A2O3 and AO2 (A=U, Np, Pu, Am, Cm, Bk, Cf) are determined from first-principles calculations using the selfinteraction corrected local spin-density approximation. Our study reveals a strong link between preferred oxidation number and degree of localization. The ionic nature of the actinide oxides emerges from the fact that those oxides where the ground state is calculated to be metallic do not exist in nature, as the corresponding delocalized f-states favour the accommodation of additional O atoms into the crystal lattice. INTRODUCTION Actinide oxides play a dominant role at all stages of the nuclear fuel cycle. With respect to both fuel performance and interactions with the environment it is crucial to understand the electronic properties of these materials. Accordingly, electronic structure calculations can provide fundamental insights at the level not achievable through experiments alone. Here we wish to focus specifically on the f-electrons, their contributions to the ground state electronic properties of the actinide oxides, the role they play with regards to stability towards oxidation, and their behaviour under ionic bonding conditions. When modeling the electronic structure of actinide materials, the most distinguishing feature is the increasing importance of correlations across the series form U to Cf, as the nature of the f-electrons changes from delocalized in the early actinides to localized in the later actinides.[1, 2] Electronic structure methods have been developed that augment the standard band structure framework to include the effects of strong correlations. The self-interaction corrected local spin density (SIC-LSD) approximation [3] used in the current work is an approach which improves on the LSD treatment of correlations by removing an unphysical electron self-interaction inherent in the LSD. The SIC-LSD approach gives rise to a split 5f electron manifold, describing a dual character of the electrons, represented by localized/occupied and hybridized/unoccupied subsets. [4, 5] Different localized/delocalized configurations can be realized by assuming different numbers of localized states—here f states on actinide-atom sites—with the relevant configurations ranging from the LSD scenario, where the electrons, including all the f-electrons, are treated as itinerant electron states, to the fully localized scenario, where all the f-electrons are treated as localized. Since the different localization scenarios constitute distinct local minima of
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