Metallization of α-boron by hydrogen doping

A theoretical assessment of doping α-boron with hydrogen is presented aiming at high Tc superconductor. Dissimilar to other impurities, hydrogen is unique because T site is the most stable site. This position is well suited to the three-center bond. The bottom of conduction band has a large component at this center. Insertion of H atom increases the bonding character of this band through the three-center bond, which is otherwise a weak bond. Enhancement of bonding character of the three-center bond merges the bottom conduction and the top valence bands, yielding a relatively large density of states (DOS) at the Fermi level. This large DOS suggests that H doped α-boron is a promising high Tc material. When the gaseous state H2 is used as the starting material, the formation energy is above 2 eV at the normal condition, so that heavy doping is not easy. High-pressure process can avoid this difficulty. It is predicted that hydrogen is incorporated easily at p > 18 GPa. A thermodynamic investigation has been made for high-pressure doping method when gases are used as the starting materials.