Shared structural motifs and bonding in new families of boron structures found via XtalOpt evolutionary algorithm

The complex chemistry of elemental boron is demonstrated in both the large number of predicted and experimentally-realized allotropes as well as the curious structural motifs that arise. As a consequence of electron deficiency, multicenter bonding is prevalent in boron-based systems, demonstrated in the combination of two- and three-center bonding in α-rhombohedral boron. In an attempt to clarify potential metastable structural adaptations of boron at high pressures, crystal structure searches were performed at 100 GPa using the XtalOpt evolutionary algorithm. In addition to the experimentally-observed Cmca α-Ga allotrope, a series of dynamically stable metastable structures were identified. Several are directly recognizable as adaptations of the α-Ga structure, with others displaying channels throughout. The SSAdNDP analysis, which generates a chemically-intuitive bonding representation within solid systems, revealed networks of four-, three- and two-center bonding interactions, with different bonding motifs reflecting the multiple structural families. The relationships among the identified structures hint at many possible variations of elemental boron structures.