BIPOLARON FORMATION IN ICOSAHEDRAL AND OCTAHEDRAL BORIDES

Singlet small bipolarons, observed in boron carbides, may be associate d with paired holes in tile states that bind the atoms of boron-rich p olyhedra together. Such ''internal'' bonding electrons are well approx imated as being confined to the surface of tile sphere that circumscri bes the polyhedron. These bonding states form a succession of levels o f orbital angular moment I with 2(2l + 1)-fold degeneracy. Valence ban ds are ''filled'', yielding insulating behavior,when the lowest crysta l-field-split sub-bands are filled. By deforming the polyhedron, singl et hole: pairs, produced by removing a pair of electrons from ''filled '' internal bonding states, carl be stabilized. Our microscopic treatm ent finds carrier-induced softening, absent in standard as hoc models. This softening facilitates bipolaron formation and accounts for the a nomalously large thermoelectric powers observed from bipolarons in bor on carbides.