| Nuclear waste and spent fuel are constantly generated with the development of nuclear power and nuclear industry, so how to safely dispose nuclear waste becomes one of the hot topics for a sustained development of nuclear industry. There has been great interest in using materials with fluorite and fluorite-related structures as potential host phases for the immobilization of actinides. Crystalline oxides (δ)-phase compounds Re6MO12(δ-Re6MO12) with fluorite (CaF2) derivative structures as radiation-tolerant materials in immobilization of nuclear waste have been received considerable attention in recent years. Experimental studies have shown that δ-phase Re6MO12exhibit both high chemical durability and solubility for radionuclides.First-principles calculations have been performed to study the structural, energetic and electronic properties of δ-Re6MO12(Re=Ho, Gd, Y; M=U, W). The optimized structural parameters suggested that only the radius ratio and lattice parameters cannot generally predict the O-D transformation (order to disorder) or amorphization in δ-A6BO12. The calculated results indicated that the energetic tendencies for the formation of Frenkel-pair defects of δ-Re6MO12are consistent with the experimental results, and the Frenkel-pair defects have a significant influence on radiation-induced phase transformation. Density of states (DOS) analysis showed hybridizations between W5d and O2p states in Y6WO12and between U6d and O2p states in δ-Re6UO12(Re=Ho, Gd, Y) are observed, but the covalence of <M-O> bond is much stronger in δ-Re6UO12(Re=Ho, Gd, Y) than that in Y6WO12. Bader charge analysis revealed that<U-O> bond in δ-Re6MO12(Re=Ho, Gd, Y) is more covalent than<W-O> bond in Y6WO12.It was proposed that<M-O> bond may play a more significant role in determining their radiation tolerance.To further study the effects of the electronic structure of the material stability, The band structure, density of states and Bader charge of δ-Re6MO12(Re=Ho, Gd, Y; M=U, W) have been calculated by the method based on GGA-PW91functional, GGA-PBE functional and GGA+Ueff, respectively. The calculated band structures showed that the compounds δ-Re6MO12(Re=Ho, Gd, Y; M=U, W) are semiconductor of indirect band gap behavior, and density functional theory plus Hubbard U correction (U corrects the intraband Coulomb interaction) will corrects the energy gap of Gd6UO12and Y6UO12. The comparative analysis of density of states (DOS) and Bader charge showed that the spin polarization of δ-Re6UO12(Re=Ho, Gd) is caused by4f electron of lanthanide, and4f electron of lanthanide leads to changing distribution of electron density of states and Bader charge, which indicated that the Re-site4f electrons do take part in the chemical bonding. Density functional theory plus Hubbard Ueff correction was employed to study electronic and bonding properties in Gd6UO12and Y6UO12. The reductions of hybridizations of orbital electron between U, Re and O from density of states distribution and the increases of Bader charges of U and Re were found to be due to the strong correlation effect of f orbital electron. The calculations results of electronic and bonding properties revealed that the key factor in the stability and their radiation resistance to amorphization may be not relating to f orbital electron and interaction of strong association for4f and5f electron in δ-Re6MO12(Re=Ho, Gd, Y; M=U, W). |
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