Shang-Di Mo

X-ray absorption near-edge structure in alpha-quartz and stishovite: Ab initio calculation with core–hole interaction

Ab initio calculation of the XANSE/ELNES spectra for α quartz and stishovite were carried out using a large-supercell approach that includes the electron–core–hole interaction. Excellent agreements with experimental spectra were obtained for Si–K, Si–L2,3, and O–K edges. The usual interpretation using orbital-resolved local density of states in the conduction band is unsatisfactory.

Electron energy loss near-edge structures of cubic Si3N4

Electron energy loss near-edge structures of the newly discovered cubic-Si3N4 at the Si L2,3, edge and N K edge have been measured. The same edges were calculated using a first-principles supercell approach, including the core–hole interaction. The experimental spectra at the two edges were satisfactorily reproduced by the calculations, confirming that the present calculation has sufficient predictive power. The difference in spectral shapes between c-Si3N4 and β-Si3N4 is more clear for the Si L2,3, edge. However, the difference cannot be simply explained by the difference in coordination numbers of Si.

Electronic and optical properties of the cubic spinel phase of c − Si 3 N 4 , c − Ge 3 N 4 , c − SiGe 2 N 4 , and c − GeSi 2 N 4

The electronic and optical properties of the new cubic spinel nitrides c-Si{sub 3}N{sub 4}, c-Ge{sub 3}N{sub 4}, and that of the predicted double nitrides c-SiGe{sub 2}N{sub 4} and c-GeSi{sub 2}N{sub 4} are studied by a first-principles method. They are all semiconductors with band gaps between 1.85 and 3.45 eV and a bulk modulus between 258 and 280 GPa. From the total-energy calculations, it is shown that c-SiGe{sub 2}N{sub 4} should be a stable compound while c-GeSi{sub 2}N{sub 4} could be metastable. The compound c-SiGe{sub 2}N{sub 4} is of particular interest because of a favorable direct band gap of 1.85 eV and a conduction-band effective mass of 0.49. The crystal has a very strong covalent bonding character as revealed by the calculated Mulliken effective charge and bond order. The strong covalent bonding in c-SiGe{sub 2}N{sub 4} is attributed to the optimal arrangement of the cations. The smaller Si ion occupies the tetrahedrally coordinated (8a) site and the larger Ge ion occupies the octahedrally coordinated (16d) site.

Optical properties of a near- axis tilt grain boundary in

The electronic structure and the optical properties of a near-611 grain boundary in -Al2O3 are studied by means of first-principles calculations based on a structural model constructed by Kenway. Results on the orbital-resolved partial density of states are presented for the grain boundary model and also for a perfect bulk supercell model containing the same number of atoms. An effective-charge calculation indicates an increased ionic character for atoms in the grain boundary region which is attributed to the distorted bonding pattern and a decrease in coordination number. The calculated optical properties show very similar results for the grain boundary model and the bulk supercell model. The electron energy-loss spectra are consistent with the recent experimental data on the near-611 grain boundary obtained by spatially resolved valence electron energy-loss spectroscopy.