Bonding in boron: building high-pressure phases from boron sheets

We present the results of a study of the high pressure phase diagram of elemental boron, using full-potential density functional calculations. We show that at high pressures (P > 100 GPa) boron crystallizes in quasi-layered bulk phases, characterized by in-plane multicenter bonds and out-of-plane unidimensional sigma bonds. These structures are all metallic, in contrast to the low-pressure icosahedral ones, which are semiconducting. We show that the structure and bonding of layered bulk phases can be easily described in terms of single puckered boron sheets [1]. Our results bridge the gap between boron nanostructures and bulk phases.\\[4pt] [1] Kunstmann et al., Phys. Rev. B 74, 035413 (2006).