Structural Motifs and Bonding in Two Families of Boron Structures Predicted at Megabar Pressures

TL;DR

This study used evolutionary structure searches at 100 GPa to discover new metastable boron phases, revealing distinct bonding motifs and potential high hardness, advancing understanding of boron's complex crystal chemistry under extreme conditions.

Contribution

It uncovers new metastable boron structures at high pressure, classifies them into two families, and analyzes their bonding and stability, providing insights into boron's complex chemistry.

Findings

Discovered new metastable boron phases at 100 GPa.

Identified two main structural families with distinct bonding networks.

Estimated high hardness (~36 GPa) for several phases.

Abstract

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.