Helmut Werheit

Abstract:

Boron and boron-rich solids are distinguished by outstanding characteristics like extraordinary high melting points, great hardness, low density and high chemical stability. The neutron absorption cross section of the 10B isotope is very high. The complex icosahedral boron-rich structures vary from the boron allotrope alpa-rhombohedral boron with 12 atoms to YB66 type crystals with 1584 atoms per elementary cell, including boron carbide, the hitherto technically most important and one of the most intensively investigated boron compounds. All of them are essentially composed of nearly regular B12 icosahedra, sometimes containing one carbon atom substituting for boron, forming open networks with hollow spaces in between. These allow accommodating foreign atoms, thus offering a basis for tailoring the individual properties according to technical requirements. Fundamental discrepancies occurred between experimental results proving semiconducting behavior and theoretical calculations predicting metallic character. Reason is the not yet understood tendency of these structures to avoid metallic behavior by generating structural defects in required concentrations. These are high, sometimes in the order of 1 to 10 %, thus explaining that the theoretical calculations based on idealized undistorted structures necessarily failed. According to Ogitsu et al., the partial occupancies of specific regular sites in the beta-rhombohedral boron structure result from a geometrical frustration originating from the intrinsic instability of the B28 subunits. They can be described by an antiferromagnetic Ising model on an expanded Kagome lattice. The experimentally gained electronic band scheme contains numerous acceptor-like gap states and a series of electron traps determining essentially the electronic transport properties. Apart from high concentration of vacancies in the center site of the elementary cell, the structures of boron carbides in the entire homogeneity range (B4.3C to B~11C) consist of B12 and B11C icosahedra and CBC and CBB chains, whose shares depend on the actual chemical composition. As the differently composed elementary cells are statistically distributed, X-ray and neutron diffraction, averaging comparably large volumes, failed in determining the components quantitatively. NMR failed as well. However, phonon spectroscopy on isotope-enriched boron carbide proved suitable to solve this problem. The experimentally determined band scheme of boron carbide, describing the electronic properties consistently, contains high concentrations of various gap states, which are generated by the structural defects. These gap states are responsible for the complex electronic properties.