New Ground-State Crystal Structure of Elemental Boron

Elemental boron exhibits many polymorphs in nature based mostly on an icosahedral shell motif, involving stabilization of 13 strong multicenter intraicosahedral bonds. It is commonly accepted that the most thermodynamic stable structure of elemental boron at atmospheric pressure is the beta rhombohedral boron (beta-B). Surprisingly, using high-resolution transmission electron microscopy, we found that pure boron powder contains grains of two different types, the previously identified beta-B containing a number of randomly spaced twins and what appears to be a fully transformed twinlike structure. This fully transformed structure, denoted here as tau-B, is based on the Cmcm orthorhombic space group. Quantum mechanics predicts that the newly identified tau-B structure is 13.8 meV/B more stable than beta-B. The tau-B structure allows 6% more charge transfer from B-57 units to nearby B-12 units, making the net charge 6% closer to the ideal expected from Wade's rules. Thus, we predict the tau-B structure to be the ground state structure for elemental boron at atmospheric pressure.