POSSIBILITY OF SEMICONDUCTING QUASI-CRYSTAL IN BORON-RICH SOLIDS

Analyzing the crystalline structure of alpha- and beta-rhombohedral boron as approximants of a quasicrystal, atomic structures of two unit cells (prolate and oblate rhombohedra) of an icosahedral quasicrystal are constructed. Based on the molecular orbital calculation, as the cohesive energy for the alpha-boron type quasicrystal has a value between those for alpha- and beta-boron, such a structure can be realized. A neutral B12H12 icosahedral cluster is distorted to a cubic or rhombohedral one by the Jahn-Teller effect. The distortions of B-12 cluster in the K2B12H12 and beta-rhombohedral boron have the same signs as the lowest energy distortions in the calculation. Among the alpha-rhombohedral-type structures, the distortion of B-12 is correlated with the rhombohedral axis angle, and the angle is the closest to the icosahedral angle for B60 and the distortion is the smallest for B-C. A search for semiconducting quasicrystal has been experimentally done in the B-C system. A meta-stable phase has been found on the way to crystallization of the B95C5 amorphous phase.