Structure and properties of boron-rich solids constructed of icosahedral and soccer-ball clusters

Abstract

Boron-rich solids are semiconducting materials which have unique three-centered covalent bonds and crystalline structures with icosahedral B 12 or soccer-ball B 84 clusters. The icosahedral cluster B 12 is two-electron deficient and is distorted by the Jahn-Teller effect. Photoluminescence from the intrinsic acceptor level, which is induced by the Jahn-Teller effect, in β-rhombohedral boron is observed. The crystalline structure of the β-rhombohedral boron is essentially the same as the f.c.c. structure of C 60. However, the electrical conductivity and magnetic susceptibility of metal-doped β-boron remain semiconductive up to a metal dopant content of several per cent and the conductivity has a variable-range hopping type temperature dependence. The Fermi level is pinned at the localized intrinsic acceptor level and a carrier may form a bipolaron. Boron-rich solids are candidates for a semiconducting quasicrystal. Based on the calculated cohesive energy, the α-rhombohedral boron type quasicrystal can be realized. A metastable phase was found after annealing the B 95C 5 amorphous phase.