Atom-Resolved Chemical States in the Multivalent U-TM-O (TM: Ti, V, Cr, Mn, Fe, Ni, Nb, Mo, W) Ternary Oxides from First-Principles

Abstract

Uranium (U) has attracted considerable fundamental and applied studies; among them, the rigorous determination of its chemical state is one of the crucial topics owing to its well-known multivalent feature associated with complicated chemical bonding. However, the related studies on solid-state U ternary or beyond ternary compounds are quite in lack. Herein, first-principles DFT + U calculations are performed to unambiguously determine the actual oxidation states of metal ions in U ternary oxides containing multivalent transition-metals (TMs: Ti, V, Cr, Mn, Fe, Ni, Nb, Mo, W). For the first time, the actual oxidation states of U and TM in the ternary oxides are obtained in a quantitative and atom-resolved manner, thereby providing their nominal compositions. The results disclose the variations of U oxidation states with TM's properties, chemical surrounding, local structure, and oxygen vacancy. In terms of charge transfer, electronic structure, orbital energy level, and oxygen vacancy effect, I conclude that the oxidation state is the competitive consequence of those electronic and structural factors and that electron-structure-related factors play more important roles. Because of the increasing orbital expansion from 3d to 4d and 5d TMs, the latter TMs (Nb, Mo, W) have the greater tendency to form high oxidation states and suppress the formation of high oxidation states of U. On the contrary, 3d TMs, especially Cr, Mn, Fe, and Ni, tend to adopt low oxidation states, thereby promoting the emergence of high oxidation states of U.

Valence Fluctuation of Uranium Ions in Uranium Sesquinitride Revealed by Dynamical Mean‐field Theory Merged with Density Functional Theory

“…Energetically, 7 s, 6d orbital is almost degenerate with 5f orbital in U 2 N 3 . [44,45] As illustrated in Figure 5(a), 5f orbitals scarcely hybrid-ize with 7 s, 6d, and N 2p orbitals in the vicinity of Fermi level, which is consistent with the hybridization function as illustrated in Figure 6. As illustrated in Figure 5 As illustrated in Figure 5(b), in U 2 N 3 , the spectra features, e. g., a broad peak of j = 5/2 manifold and a sharp peak of j = 7/ 2 state, demonstrate that j = 5/2 and j = 7/2 components exhibit the itinerant metallic and localized insulating behaviors, respectively, which are in accord with the spectrum functions shown in Figure 3.…”

Site‐dependent correlation strength and occupation number of 5f electrons in alpha phase plutonium metal

“…Total density of state (TDOS), spd projected density of state (PDOS) and Pu 5f PDOS in α-Pu are plotted in Figures 3A-D and 4A,B, respectively. In principle, energetically, Pu 7s and 6d orbitals are almost quasi-degenerate with Pu 5f orbitals [17,62], thus 7s, 6d states would hybridize with 5f orbitals in the conduction and semi-core bands [19], as illustrated in Figure 3C. As shown in Figure 3A, DFT + DMFT calculation reproduces the well-known three-peaked structure in α-Pu, that is, a coherent quasi-particle peak, and two broad peaks corresponding to the lower and upper Hubbard bands [4].…”

Origin of Near-Zero Thermal Expansion in A2O(PO4)2 Oxides over an Ultrawide Temperature Range

“…In particular, the strongly correlated Hubbard model was employed to describe the 5f electrons of U atoms within the GGA + U method . After a series of tests and comparison with other related studies, , the effective Hubbard U eff ( U eff = U – J , i.e., the difference between Coulomb U and exchange J values) was set to 2 eV.…”