Charge transfer and distortion in systems of boron icosahedra

It has been proposed that electrical transport in boron carbides (and possibly in similar borides) is accomplished by hopping of singlet electron pairs between bipolaronic states localized on the icosahedra. Formation of a bipolaron will be accompanied by a distortion of the associated icosahedron. We have used quantum-chemical calculational techniques to consider the relation between charge state and distortion for a system consisting of a B12 icosahedron and 12 tieoff atoms. It is found that localization of two extra electrons on an icosahedron results in a contraction of the icosahedral cage and a reduction of the total energy of the cluster by -1.3 eV compared to that of the neutral cluster. The reduction in energy corresponds to the formation energy of a bipolaron in our model. This result depends critically on the presence of the tieoff atoms, which serve to simulate the lattice surrounding the icosahedron in a boride compound. Refinements and extensions of the model are planned for the future, including calculations of polaron vs bipolaron formation energies. 8 refs