INTERBAND AND GAP STATE RELATED TRANSITIONS IN BETA-RHOMBOHEDRAL BORON

The anisotropy of the electronic interband transitions of beta-rhomboh edral boron is determined by optical absorption measurements. The band gaps (indirect allowed transitions) are 1.32(1) and 1.50(1) eV for E parallel-to c, respectively, 1.29(1) and 1.46(1) eV for E perpendicula r-to c (extrapolated to T = 0 K). At low temperatures these gaps decre ase, while the thermal equilibrium develops. The temperature dependenc e of the gap is attributed to the thermal disorder caused by phonons o f 121.9 (E parallel-to c) and 94.1 meV (E perpendicular-to c) represen ting the most prominent intraicosahedral vibrations. Six intrinsic tra pping levels for electrons in distances between 0. 1 7 and 1. 1 5 eV f rom the conduction band edge are determined, the closer ones in agreem ent with different photo effects. The trapping levels are attributed t o distortion states of the icosahedra due to a dynamical Jahn-Teller e ffect. This conception allows the interpretation of other physical pro perties like internal friction, discontinuities of thermal expansion, and electrical conductivity, of the temperature dependence of the ESR linewidth, and of the density of paramagnetic centers (spin density an d magnetic susceptibility).