Metal impurities in crystallographic voids of beta-rhombohedral boron lattice: Binding energies and electron levels

Applying quasi-classical approach, the binding energies and electron levels of metal impurities (Li, Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Hf, Ta, and Re) introduced into crystallographic voids of types A, D and E in the beta-rhombohedral boron lattice are calculated. Binding energies are estimated as similar to 1 eV -60 eV per bond. The most of the obtained metal-boron bond lengths are very close to the mean radii of voids in the undoped crystal. Relatively light impurities (from Li to Cu) are found to form donor electron states directly inside the conduction band, i.e., they cause metallization of the material being introduced at sufficiently high concentrations. Heavy impurities (from Zr to Re) form shallow or deep donor levels inside the band gap or even valence band depending on dopants and voids of accommodation. (C) 2012 Elsevier Masson SAS. All rights reserved.