Isovalent effects on the structural and electronic features of scandium-doped aluminum clusters ScmAln−m+/0/− with m = 1–2, n = 3–15

Abstract

Singly and doubly scandium-doped aluminum clusters Sc_mAl_n−m^+/0/− with m = 1–2 and n = 3–15 are systematically investigated using density functional theory (DFT) with the PBE functional and the def2-TZVP basis set. Incorporation of scandium atoms significantly enhances the thermodynamic stability of aluminum clusters, following the trend of Sc₂Al_n−2^+/0/− > ScAl_n−1^+/0/− > Al_n^+/0/−. Structural evolution of these systems is constructed from three building units, including a three-atom triangle, a six-atom octahedron and a thirteen-atom icosahedron. Owing to the isovalent nature of Sc and Al, the doped clusters exhibit extensive delocalization of valence electrons, giving rise to metal aromaticity that spans all structural centers. The bonding network consists of unpolarized Al–Al interactions combined with polarized Al^δ+–Sc^δ− or Al^δ+–Al^δ- bonds, where Sc atoms primarily act as electron acceptors. Progressive electron filling across charge states reveals the 20- and 40-electron shell closure tendencies for the six- and thirteen-atom clusters, respectively. The six-atom clusters show a progressive tendency to achieve the [(1S)²(1P)⁶(2S)²(1D)¹⁰] electron shell, whereas the thirteen-atom clusters tend toward the [(1S)²(1P)⁶(2S)²(1D)¹⁰(2P)⁶(1F)¹⁴] configuration, corresponding to their octahedral and icosahedral structures, respectively. These spherical shell fillings account for the exceptional stability, well-ordered electronic structures, and synchronization between electronic and geometric features observed in both cluster families.