Bipolaron Formation in Icosahedral and Octahedral Borides

Abstract

Singlet small bipolarons, observed in boron carbides, may be associated with paired holes in the states that bind the atoms of boron-rich polyhedra together. Such “internal” bonding electrons are well approximated as being confined to the surface of the sphere that circumscribes the polyhedron. These bonding states form a succession of levels of orbital angular moment l with 2(2l + 1)-fold degeneracy. Valence bands are “filled”, yielding insulating behavior, when the lowest crystal-field-split sub-bands are filled. By deforming the polyhedron, singlet hole pairs, produced by removing a pair of electrons from “filled” internal bonding states, can be stabilized. Our microscopic treatment finds carrier-induced softening, absent in standard as hoc models. This softening facilitates bipolaron formation and accounts for the anomalously large thermoelectric powers observed from bipolarons in boron carbides.