Bipolarons in boron-rich icosahedra: Effects of carbon substitution

Analysis of electronic-transport and magnetic measurements on boron carbides has indicated that charge transport occurs via bipolaron hopping between the twelve-atom icosahedral structural units of these solids. Two types of isoelectronic icosahedral units are neutral icosahedral boron clusters (B/sub 12/ icosahedra) and positively charged clusters of eleven boron atoms and a carbon atom [(B/sub 11/C)+ icosahedra]. In particular, the transport analysis assumes that it is energetically favorable for an electron pair to occupy a (B/sub 11/C)+ icosahedron rather than a B/sub 12/ icosahedron. Here, we study the energy changes and atomic displacements associated with adding two electrons to a (B/sub 11/C)+ icosahedron in neutral surroundings and compare our results with our previous study of two electrons added to an analogous B/sub 12/ icosahedron. We find that the addition of the two electrons reduces the (B/sub 11/C)+ system's energy by about 18 eV