New Structural Motifs in Boron at Megabar Pressures
Apart from its fascinating crystal structures and their structural chemistry, the high pressure behavior of boron is of critical importance in high-energy density research, particularly due to its role as an ablator in inertial confinement fusion experiments.
New theoretical and computational results from a collaboration led by CDAC Postdoctoral Associate Katerina Hilleke and CDAC Academic Partner Eva Zurek at the University at Buffalo, and including colleagues at Lawrence Livermore National Laboratory and the University of Rochester/Laboratory for Laser Energetics, sheds new light on the behavior of boron at megabar pressures, and provides structural models for understanding the stability of several predicted high-pressure polymorphs containing unique structural motifs.
Using evolutionary algorithm routines implemented in the XtalOpt code, the group predicts several metastable phases of boron that are dynamically stable at 100 GPa, and which can be sorted into two different types. One is based on the structure of α-Ga, and the other consists of channels along the c direction of a monoclinic lattice (Fig. 1). In addition, two intergrowth structures are predicted that are comprised of both a-Ga and channel-based structural units. These phases contain a combination of 2 center – 2 electron, 3 center – 2 electron and 4-center – 2 electron boding motifs, showing that not only are the complex structural features of α-B12 retained at high pressures, but new motifs are generated.
Several of the structures predicted in this work are calculated to be metastable at ambient pressure. With Vickers hardnesses in the range of 36 GPa, these phases, if they could be synthesized by a high P-T route and quenched, have the potential to be useful in a variety of practical applications.
Hilleke, K. P., et al., Structural motifs and bonding in two families of boron structures predicted at megabar pressures. Physical Review Materials 5, 053605 (2021).
Figure 1. Structures of boron polymorphs stable at 100 GPa. Left, a structure based on the alpha-Ga arrangment. Right, a structure based on the channel motif.