  The complex crystal chemistry of elemental boron has led to numerous proposed
structures with distinctive motifs as well as contradictory findings. Herein,
evolutionary structure searches performed at 100 GPa have uncovered a series of
potential new metastable phases of boron, and bonding analyses were carried out
to elucidate their electronic structure. These polymorphs, dynamically stable
at 100 GPa, were grouped into two families. The first was derived from the
thermodynamic minimum at these conditions, $\alpha$-Ga, whereas channels
comprised the second. Two additional intergrowth structures were uncovered, and
it was shown they could be constructed by stacking layers of $\alpha$-Ga-like
and channel-like allotropes on top of each other. A detailed bonding analysis
revealed networks of four-center $\sigma$-bonding functions linked by
two-center B-B bonds in the $\alpha$-Ga based structures, and networks that
were largely composed of three-center $\sigma$-bonding functions in the
channel-based structures. Seven of these high pressure phases were found to be
metastable at atmospheric conditions, and their Vickers hardnesses were
estimated to be $\sim$36 GPa.
