The same in the bulk but different as clusters: X3Y3 (X=B, Al, Ga; Y=P, As)

•Binary materials of group 13 and 15 elements are the same in the bulk but different as small clusters.•Hybridization of atomic orbitals governs cluster shapes.•Hybridization opposes delocalized bonding in defining cluster structures. Most binary materials formed by early group 13 and 15 elements are semiconductors identical in their structures and properties. We show that in the ultra-small cluster regime these materials are markedly different. Among X3Y3 (X=B, Al, Ga; Y=P, As), lighter clusters are planar, and heavier clusters are compact and three-dimensional. The difference is owed to the interplay between covalency and delocalized non-directional bonding in these systems. If the sp-hybridization in the constituent elements is energetically affordable and leads to strong directional overlap, the bonding is covalent and the cluster is flat. Otherwise, the cluster is three-dimensional.