First-principles calculations of elemental crystalline boron phases under high pressure: Orthorhombic B28 and tetragonal B48

The structural, electronic, mechanical properties and hardness of orthorhombic B-28 and tetragonal B-48 boron phases have been studied by first-principles of pseudopotential calculations. The results indicated that both boron phases are energetically and also mechanically stable. In addition to electronic properties of highly directional covalent bonds, mechanical properties, and also the Debye temperatures of structures support that both are superhard materials. Calculated electronic band structures and density of states revealed that orthorhombic B-28 crystal is a semiconductor, and the tetragonal B-48 is metallic. The pressure-dependent behaviors of both structures are different, and both are ultra-incompressible and anisotropic materials. The physical parameters of the structures such as lattice parameters, bond lengths, and also energy gaps between valance and conduction bands are closely sensitive to applied external pressures. By means of pressure-volume graphs, obtained EOSs for alpha-rhombohedral B-12, orthorhombic B-28 and tetragonal B-48 boron phases are compared with available data. However, energetically possible pressure-induced phase transitions among the purposed structures are predicted on the pressure range of 0-460 GPa. Furthermore, our calculations showed that for the pressures from 0 GPa to 24 GPa energetically the more stable elemental boron phase is alpha-rhombohedral B-12, and from 24 GPa to 106 GPa is orthorhombic B-28, and from 106 GPa to 460 GPa is alpha-Ga-type boron. (C) 2011 Published by Elsevier B.V.