Structural motifs and bonding in two families of boron structures predicted at megabar pressures

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 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 approximate to 36 GPa.

Automated summary

Evolutionary structure searches at 100 GPa uncovered a series of metastable phases of boron, grouped into two families, one derived from the thermodynamic minimum alpha-Ga and the other composed of channels. Additionally, two intergrowth structures were found, constructed by stacking layers of different allotropes.