Analysis of boron carbides' electronic structure. Final technical report, 18 July 1984-17 August 1986

The electronic properties of boron-rich icosahedral clusters were studied as a means of understanding the electronic structure of the icosahedral borides such as boron carbide. A lower bound was estimated on bipolaron formation energies in B/sub 12/ and B/sub 11/C icosahedra, and the associated distortions. While the magnitude of the distortion associated with bipolaron formation is similar in both cases, the calculated formation energies differ greatly, formation being much more favorable on B11C icosahedra. The stable positions of a divalent atom relative to an icosahedral borane was also investigated, with the result that a stable energy minimum was found when the atom is at the center of the borane, internal to the B/sub 12/ cage. If incorporation of dopant atoms into B/sub 12/ cages in icosahedral boride solids is feasible, novel materials might result. In addition, the normal modes of a B/sub 12/H/sub 12/ cluster, of the C/sub 2/B/sub 10/ cage in para-carborane, and of a B/sub 12/ icosahedron of reduced (D sub 3d) symmetry, such as is found in the icosahedral borides, were calculated. The nature of these vibrational modes will be important in determining, for instance, the character of the electron-lattice coupling in the borides, and in analyzingmore » the lattice contribution to the thermal conductivity.« less