  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.
