Actinide-induced structural transformation of B₁₈ clusters: from quasi-planar to crown-like configurations

Boron clusters exhibit a wide variety of structures, and most small- and medium-sized clusters usually adopt quasi-planar or planar configurations. However, the introduction of metal dopants can modulate the structure and electronic properties of boron clusters. In this work, we introduced actinides (An = Pa, U, Np, and Pu) into B₁₈ clusters and predicted that their structures can transition from quasi-planar to crown-like structures. Meanwhile, the electronic structures and thermodynamic stabilities of these AnB₁₈ clusters were systematically investigated using density functional theory calculations. The results show that the An-B interactions gradually weaken from Pa to Pu, as reflected by increasing An-B bond distances, decreasing Wiberg bond indices, and reduced dissociation energies. Among the studied clusters, PaB₁₈ is predicted to be the most stable, with the strongest An-B bonding and the highest dissociation energy. This indicates that covalent actinide-boron interactions play a key role in the stability of these clusters. Molecular orbital analysis demonstrates that the covalency of the An-B bonds mainly originates from the interaction between the An 5f/6d orbitals and the B₁₈ delocalized orbitals. These results indicate that actinide dopants play a significant role in modulating the structures and bonding properties of boron clusters. The present work provides an in-depth elucidation of the bonding mechanisms between actinides and boron atoms in the boron clusters and offers theoretical guidance for the rational design of stable boron-based nanomaterials.