Theoretical insight into the unique structural stabilization mechanism in actinide-centered boride

Abstract

The exploration of metal-doped boron clusters has emerged as an effective strategy for constructing boron-based three-dimensional nanomaterials, while a comprehensive understanding of the coordination chemistry and electronic structure of lanthanide (Ln) and actinides (An) is essential for elucidating their reactivity and behavior in nuclear fuel corrosion. Using a quantum-mechanical methodology at both scalar-relativistic and spin–orbit coupling levels, in conjunction with global minimization of the UB₁₆ potential energy surface, we systematically investigate the geometries and electronic structures of MB₁₆ (M = Sc, Ti, V, Co, Rh, Ir, Eu, and An = Th to Cm) species. Three distinct structural motifs emerge: C₂ drum-like structures for metals with atomic radii <130 pm (Co, Rh, Ir), a C_8v bilayer structure for intermediate radii (130 pm < radii < 140 pm, Ti), and C_5v half-cage geometries for larger radii > 140 pm (Sc, Eu, and actinides Th–Cm). Bonding analysis reveals that actinide–boron interactions are dominated by orbital contributions, contrasting with the predominantly electrostatic bonding in transition metal analogues. In particular, the strong π_An–B bonding interactions, as well as the substantial 5f participant to the π_An–B bonds, contribute to the stability of the C_5v half-cage structure of PaB₁₆ and UB₁₆, as well as their relatively strong delocalized An–B bonds. These findings establish atomic radius as the primary determinant of structural preference and demonstrate the distinctive 5f-covalent character of actinide–boron bonding, providing fundamental insights for designing actinide-containing compounds.