Ln@B30: an endohedral borospherene with spherical aromaticity.

The structural stability and electronic delocalization of boron clusters are central topics in cluster science. Herein, we theoretically identified a highly stable D5h-symmetric Eu@B30 cage cluster as the global minimum structure. Magnetic criteria and electronic delocalization analyses reveal that the system possesses typical fullerene spherical aromaticity, where the encapsulated Eu atom contributes valence electrons to facilitate a uniform distribution of the delocalized electron cloud over the entire cage framework. Further investigation demonstrates a significant "lanthanide-specificity": the entire lanthanide series follows the same stabilization mechanism as Eu@B30, where electron transfer from the lanthanide atom to the B30 framework induces multicenter delocalized bonding to maintain the cage structure. Among them, Eu@B30 and Gd@B30 exhibit the highest binding energies due to the half-filled 4f shell, with spin distributions following a "volcano-type" trend. Comparisons with non-lanthanide elements (Sc, Y, Hf, Ta) confirm that the stability of this D5h cage is highly dependent on the unique 4f-shell polarizability and flexible valence states of lanthanides. This work elucidates the synergistic mechanism between multicenter delocalized bonding, spherical aromaticity, and structural stability in rare-earth-doped boron clusters, providing a theoretical foundation for the design of novel lanthanide-encapsulated boron-based materials.