| Boron-rich compounds and elemental boron have amazing chemical, physical and mechanical properties, but puzzling structures. The uncertain stoichiometric value of foreign elements in boron-rich compounds greatly influences the physical and chemical properties. To study this effect, pure elemental boron which has six allotropes (alpha-r-B12, gamma-B28, alpha-t-B 51, beta-r-B106, beta-t-B190, and alpha-Ga type) reasonably becomes a suitable candidate. In this work, the electronic structures and spectroscopic properties [x-ray absorption near edge structure (XANES) and optical dielectric functions] of elemental boron were systematically studied using an ab initio orthogonalized linear combination of atomic orbitals (OLCAO) method. Based on available experimental data, the calculated electronic structure, dielectric functions and XANES spectra reproduce the experimental measurements very well. Electronic structures show that alpha-r-B 12, gamma-B28, beta-r-B106, B11C-CBC and B12O2 are indirect semiconductors; alpha-t-B 51, beta-t-B190 and alpha-Ga type boron are metallic or semi-metallic materials. Based on the electronic structures of alpha-Ga type and beta-t-B190, they both probably have poor conductivity; however the mechanisms are very different: the density of states of alpha-Ga type shows that there is a pseudogap existing, while the band structure of beta-t-B 190 at the Fermi level shows very few flat states existing. By investigating the XANES spectra for a series of perfect icosahedra built up with different bond lengths, XANES spectra of icosahedra in crystal structure of alpha-r-B 12, gamma-B28, alpha-t-B51, beta-r-B 106, and beta-t-B190 all have three dominant characteristic peak features in the energy range of 195-215eV. This result was also observed in the ico-XANES spectra of B11C-CBC and B12O2 which are typical representatives in the family of boron-rich compounds. The low energy limit increases with the atomic number of the foreign atoms rising while the upper energy limit remains constant (215eV). Ico-XANES spectra show correlation between XANES and either bond length, crystal structure, or defects. In this work, a MAX phase material becomes a good candidate to study XANES spectrum since same type of phase has same crystal structure but different elements, and different phases consist of same type of elements but different crystal structures. XANES spectra of MAX phase materials [phase (211) and (312)] show correlations between XANES and either the element type, the 1st and 2nd nearest neighbors, or the crystal structure. |
