Ab initio computational applications of electronic structure and spectroscopic Properties of bulk Crystalline materials

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 (α-r-B12, γ-B28, α-t-B51, β-r-B106, β-t-B190, and α-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 α-r-B12, γ-B28, β-r-B106, B11C-CBC and B12O2 are indirect semiconductors; α-t-B51, β-t-B190 and α-Ga type boron are metallic or semi-metallic materials. Based on the electronic structures of α-Ga type and β-t-B190, they both probably have poor conductivity; however the mechanisms are very different: the density of states of α-Ga type shows that there is a pseudogap existing, while the band structure of β-t-B190 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 α-r-B12, γ-B28, α-t-B51, β-r-B106, and β-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

Name thesis:degree_name

Ph. D.

Level thesis:degree_level

Doctoral

Discipline thesis:degree_discipline

Physics (UMKC)

Grantor

University of Missouri--Kansas City

Year dc:date.issued

2013

Author dc:creator

• Wang, Liaoyuan

Advisor dc:contributor.advisor

• Ching, Wai-Yim

Handle dc:identifier.uri

http://hdl.handle.net/10355/43473

OAI identifier oai:identifier

oai:mospace.umsystem.edu:10355/43473