Molecular crystals vs. superatomic lattice: a case study with superalkali-superhalogen compounds

Using a first-principles approach, we study the assembly of atomically-precise cluster solids with atomic precision. The aims are to create binary assemblies of clusters through charge transfer between neutral molecular clusters, and employing intercluster electrostatic attraction as a driving force for co-assembly. We combined pairs of complementary clusters in which one cluster is electron-donating (superalkali) and the other is electron-accepting (superhalogen). From the analysis of the binding energy between superatomic counterparts, charge transfer, and the relative size of the clusters, we analyze the resulting structures as either molecular crystals or superatomic lattices. We demonstrate that the substitution of a single atom can result in minor changes to the crystal structure of the binary solids or entirely new packing structures. The [N₄Mg₆Li]⁺[AlCl₄]^-, [N₄Mg₆Na]⁺[AlCl₄]^-, [N₄Mg₆K]⁺[AlCl₄]^-, [N₄Mg₆Li]⁺[AlF₄]^-, [N₄Mg₆Na]⁺[AlF₄]^-, and [N₄Mg₆K]⁺[AlF₄]^- compounds all form the same close-packed superatomic lattice structure through halogen bonding, with subtle differences in the orientation of the superatoms. These salts may also form molecular crystals where clusters are held to one another by electrostatic interactions. Our results emphasize how the structure of superatomic solids can be tuned upon single atom substitution.