Experimental observation and computational identification ofSc@Cu16+, a stable dopant-encapsulated copper cage

Abstract

We report a combined experimental and computational study of scandium doped copper clusters. The clusters are studied with time-of-flight mass spectrometry after laser fragmentation. Enhanced stabilities for specific cluster sizes in the mass abundance spectra are discussed using both electronic (shell closing) and geometric (symmetry) arguments. The exceptional stability observed for ${\mathrm{Cu}}_{16}{\mathrm{Sc}}^{+}$ is investigated in detail computationally. Density functional geometry optimizations at the Becke-Perdew 1986--LANL 2-double-zeta (BP86/LANL2DZ) level result in a Frank-Kasper tetrahedron, encapsulating a scandium atom in a highly coordinated position. The high stability is therefore interpreted in terms of extremely stable dopant encapsulated structures featuring a closed electron shell. The thermodynamic stability, as indicated by the stable backbone and large binding energy per atom, the relatively small ionization energy, and the moderate electron affinity of the neutral ${\mathrm{Cu}}_{16}\mathrm{Sc}$ cluster show that it has a superatom character, chemically similar to the alkaline-metal atoms.