Cluster Calculations of the Effects of Local Bonding on the Electronic Properties of B6 Octahedra

B6 octahedra, which consist of six boron atoms in an octahedral arrangement, are important structural units in such materials as the metal hexaborides and the recently discovered boron-metal WANGNOL alloys. The X(alpha) Scattered Wave Method has been used to calculate the electronic properties of both an isolated octahedron and the effects of external environment. The latter has been investigated by surrounding a B6 octahedron by various configurations of atoms representing the local structure of the metal hexaboride and alloy materials. Results obtained from these calculations have been used to provide an explanation of the difference in properties between the metal hexaborides and the WANGOL alloys in terms of the difference in local bonding properties of the two materials. The bonding in the metal hexaborides is shown to be primarily ionic-covalent, while that of the WANGNOL alloys is found to be more ionic-metallic. This difference in bonding is shown to lead to quite different electronic and mechanical properties in the two classes of materials.