Abstract

There are three phases for the bulk boron: ?-rhombohedral, ?-rhombohedral [1,2], and ?-tetragonal [3]. ?-Rhombohedral is made of B₁₂ units, with lattice constant 5.30 Å and angle 58.13° [4] (Figure 47.1). Each B₁₂ icosahedron is slightly distorted due to the Jahn–Teller effect, reducing the symmetry from I_h to D_3d [4]. ?-Rhombohedral has a somewhat complicated unit cell, composed of 105 atoms, with lattice constant 10.15 Å and angle 65.17° [2,5]. A striking B₈₄ structure with icosahedral symmetry can be identified in ?-rhombohedral [6]. ?-Tetragonal unit cell is made of 50 atoms, with lattice constants a = 8.75 Å and c = 5.06 Å [4]. It contains four B₁₂ icosahedra and two isolated 4-bonded atoms. Recently, by means of ab initio studies, it has been shown that ?-rhombohedral is more stable than ?-rhombohedral [7]. ?-Rhombohedral bulk is a semiconducting material with an energy gap of ~2 eV [4].