INTERBAND AND GAP STATE RELATED TRANSITIONS IN BETA-RHOMBOHEDRAL BORON

The anisotropy of the electronic interband transitions of beta-rhombohedral 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 perpendicular-to c (extrapolated to T = 0 K). At low temperatures these gaps decrease, while the thermal equilibrium develops. The temperature dependence of the gap is attributed to the thermal disorder caused by phonons of 121.9 (E parallel-to c) and 94.1 meV (E perpendicular-to c) representing the most prominent intraicosahedral vibrations. Six intrinsic trapping levels for electrons in distances between 0. 1 7 and 1. 1 5 eV from the conduction band edge are determined, the closer ones in agreement with different photo effects. The trapping levels are attributed to distortion states of the icosahedra due to a dynamical Jahn-Teller effect. This conception allows the interpretation of other physical properties 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 and magnetic susceptibility).