Structures, stability, mechanical and electronic properties of a-boron and its twined brother a*-boron

The structures, stability, mechanical and electronic properties of a-boron and its twined brother a*-boron have been studied by first-principles calculations. Both a-boron and a*-boron consist of equivalent icosahedra B12 clusters in different connecting configurations of "3S-6D-3S" and "2S-6D-4S", respectively. The total energy calculations show that a*-boron is less stable than a-boron but more favorable than beta-boron and Gamma-boron at zero pressure. Both a-boron and a*-boron are confirmed dynamically and mechanically stable. The mechanical and electronic properties of a-boron and a*-boron indicate that they are potential superhard semiconducting phases of element boron.