Thermally activated hopping in β rhombohedral boron

The electrical conductivity of β rhombohedral boron of high purity was measured in the temperature range 140 - 300 K. Special precautions were applied in order to maintain an equilibrium distribution of electrons and to prevent surface conduction. Results are discussed using a model based on the assumption of two types of thermally activated hopping between the localized valence states. The equilibrium conductivity of boron at intermediate temperatures is ascribed to hopping via the localized valence states, the energy levels of which lie near the Fermi level. The temperature dependence of the conductivity obeys Mott's law. (Auth.)