Unexpected Reconstruction of the α-Boron (111) Surface

We report a novel reconstruction of the alpha-boron (111) surface, discovered using ab initio evolutionary structure prediction, and show that this unexpected neat structure has a much lower energy than the recently proposed (111)-I-R, ((alpha)) surface. In this reconstruction, all single interstitial boron atoms bridge neighboring B-12 icosahedra by polar covalent bonds, and this satisfies the electron counting rule, leading to the reconstruction-induced metal-semiconductor transition. The peculiar charge transfer between the interstitial atoms and the icosahedra plays an important role in stabilizing the surface.