Structural stability and electronic properties of β-tetragonal boron: A first-principles study

It is known that elemental boron has five polymorphs: alpha- and beta-rhombohedral, alpha- and beta-tetragonal, and the high-pressure gamma phase. beta-tetragonal (beta-t) boron was first discovered in 1960, but there have been only a few studies since then. We have thoroughly investigated, using first-principles calculations, the atomic and electronic structures of beta-t boron, the details of which were not known previously. The difficulty of calculation arises from the fact that beta-t boron has a large unit cell that contains between 184 and 196 atoms, with 12 partially-occupied interstitial sites. This makes the number of configurations of interstitial atoms too great to calculate them all. By introducing assumptions based on symmetry and preliminary calculations, the number of configurations to calculate can be greatly reduced. It was eventually found that beta-t boron has the lowest total energy, with 192 atoms (8 interstitial atoms) in an orthorhombic lattice. The total energy per atom was between those of alpha- and beta-rhombohedral boron. Another tetragonal structure with 192 atoms was found to have a very close energy. The valence bands were fully filled and the gaps were about 1.16 to 1.54 eV, making it comparable to that of beta-rhombohedral boron. (C) 2014 Elsevier Inc. All rights reserved.