Pressure-induced metallization in solid boron

Different phases of solid boron under high pressure are studied by first principles calculations. The α-B₁₂ structure is found to be stable up to 270 GPa. Its semiconductor band gap (1.72 eV) decreases continuously to zero around 160 GPa, where the material transforms to a weak metal. The metallicity, as measured by the density of states at the Fermi level, enhances as the pressure is further increased. The pressure-induced metallization can be attributed to the enhanced boron-boron interactions that cause band overlap. Limited calculations on β boron indicate a similar trend of gap closure.