Lizhi Ouyang

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.

Ab initio tensile experiment on a model of an intergranular glassy film in β-Si3N4 with prismatic surfaces

We report the results of a large-scale ab initio simulation of an intergranular glassy film (IGF) model in β-Si3N4. It is shown that the stress-strain behavior under uniaxial load in the model with prismatic surfaces and few defective bonds is very different from an earlier IGF model with basal planes. The results are explained by the fundamental electronic structure of the model.

Properties of non-equivalent sites and bandgap of spinel-phase silicon nitride

γ-Si3N4 is a nitrogen-based ultra-hard ceramic with Si atoms occupying both tetrahedral and octahedral sites in a spinel structure. Soft-x-ray emission and absorption spectra of γ-Si3N4 are presented, which together provide an experimentally determined bandgap of 4.30 ± 0.25 eV. Information about localized partial density of states (LPDOS) of the non-equivalent Si sites is presented.

Electronic structure and bonding in vitamin B12, cyanocobalamin

The electronic structure of the vitamin B12 molecule, cyanocobalamin, is calculated by a first-principles method, which provides an interpretation of the optical absorption data and is consistent with soft X-ray fluorescence measurements. A fixed accurately determined geometry was used for the calculation. The large difference in the HOMO–LUMO gap compared to existing calculations, using only the analogous corrin derivative, is evidence that the side chains (particularly the nucleotide loop) strongly influence the electronic structure.

Electronic structure and dielectric properties of dielectric gate material (ZrO2)x(SiO2)1-x

We have investigated the electronic structure and dielectric properties of (ZrO2)x(SiO2)1−x with x less than 0.5 using first-principles methods. Initial models of (ZrO2)x(SiO2)1−x were obtained by selecting random distributions of Zr and Si atoms on the cation sites of tetragonal ZrSiO4 according to x. These models were relaxed using the Vienna ab initio simulation package with high accuracy. Subsequent electronic structure and dielectric properties analysis was performed using the ab initio orthogonalized linear combination of atomic orbitals method. Our results indicate that for x less than 1/8, the SiO2 matrix is not significantly changed and that there are no signs of defect states being introduced into the band gap. Meanwhile, the optical dielectric constant was significantly increased compared to pure SiO2. For x greater than 1/8, the optical dielectric constant enters a plateau region. Our results confirm the experimental findings that low-x Zr silicate can be a viable candidate for high-k dielectr...

Ab initio calculations of strain fields and failure patterns in silicon nitride intergranular glassy films

Theoretical experiments were performed on silicon nitride intergranular glassy film (IGF) models subjected to tensile loading using an accurate ab initio method. The results were used to investigate the strain fields within IGF models. The Green–Lagrange strain fields were calculated from the displacement gradients of the IGF models under various loads. Significant deviations from the first-order Cauchy–Born rule were observed for IGF models even under small load. The strain fields were also analysed to understand atomic-scale mechanisms that lead to intergranular and intragranular failures.

Comparative Study of the Electronic Structure of Ternary Superconductors MoRuP and ZrRuP in the Orthorhombic and Hexagonal Phases

The electronic structures of the ternary superconductors MoRuP and ZrRuP in the orthorhombic (o-) and hexagonal (h-) phases were calculated by a first-principles method and then compared. For h-MoRuP, the theoretically predicted structure was used. Based on the comparative studies of the electronic structures of the known superconductors of o-MoRuP (Tc=15.5 K), o-ZrRuP (Tc=4 K), and h-ZrRuP (Tc=13 K), it is argued that h-MoRuP may have a transition temperature Tc exceeding 20 K.

Computational Studies of Physical Properties of Boron Carbide

The overall goal is to provide valuable insight in to the mechanisms and processes that could lead to better engineering the widely used boron carbide which could play an important role in current plight towards greener energy. Carbon distribution in boron carbide, which has been difficult to retrieve from experimental methods, is critical to our understanding of its structure-properties relation. For modeling disorders in boron carbide, we implemented a first principles method based on supercell approach within our G(P,T) package. The supercell approach was applied to boron carbide to determine its carbon distribution. Our results reveal that carbon prefers to occupy the end sites of the 3-atom chain in boron carbide and further carbon atoms will distribute mainly on the equatorial sites with a small percentage on the 3-atom chains and the apex sites. Supercell approach was also applied to study mechanical properties of boron carbide under uniaxial load. We found that uniaxial load can lead to amorphization. Other physical properties of boron carbide were calculated using the G(P,T) package.