Superatomic orbitals in sixteen-coordinate M@Li16 bonded by metallic bonds

Abstract

Based on density-functional calculation and genetic algorithm structure search, we propose a series of 16-coordinate core–shell clusters: M@Li₁₆(M = Ca, Sr, Ba, Ti, Zr, Hf). A tetrahedral (T_d) structure with an outer shell of 16 lithium atoms and one enclosed heavy atom is found to be the global minimum in the structural exploration of BaLi₁₆ based on genetic algorithm. This structure also has lower energy compared to the other isomers we employed in all the MLi₁₆ clusters. In this structure, the atoms are bonded together by metallic bonds with alkali (IA) and alkaline-earth (IIA) metal atoms. Their corresponding first electronic shells are closed with significant energy gaps because their total numbers of valence electrons fulfil the 18-electron rule. Such a combination could be extended to 20-electron systems by enclosing IVB elements. With simple valence electrons and highly symmetric structures, superatomic molecular orbitals are identified in all of the T_d clusters.