Papers by Keyword: Ab Initio

Abstract: Metal oxide semiconducting compounds have potential application as photocatalyst materials to decompose many types of dyes and pollutants in the water. Zn₂SnO₄ and SnO₂ are semiconducting materials that have photocatalytic properties and the properties of those two semiconducting materials in the composite form have been studied. Metal oxide compounds of Zn₂SnO₄ and SnO₂ have been prepared through sonochemical methods using ZnCl₂ and SnCl₄.5H₂O as precursors. After sonication and heat treatment at 1000 °C, we could obtain Zn₂SnO₄ and SnO₂ compound in the sample as confirmed by x-ray diffraction measurement. The volume fraction of Zn₂SnO₄ and SnO₂ phases in the sample were found to be at 60 % and 40 %, respectively. The absorption spectra revealed that the band gap of the composite materials is 3.7 eV. This material could degrade all of the methylene blue with concentration of 6.0 x 10^-6 M in 120 minutes. The band structure calculation revealed that the comparable band gap values are found for Zn₂SnO₄ and SnO₂ compounds. However, the absorption edges for those compounds are slightly different, with absorption edge at 3.2 eV for SnO₂ and 3.6 eV for Zn₂SnO₄, respectively.

Abstract: In FCC metals a single parameter – stacking fault energy (SFE) – can help to predict the expectable way of deformation such as martensitic deformation, deformation twinning or pure dislocation glide. At low SFE one can expect the perfect dislocations to dissociate into partial dislocations, but at high SFE this separation is more restricted. The role of the magnitude of the stacking fault energy on the deformation microstructures and tensile behaviour of different austenitic steels have been investigated using uniaxial tensile testing and electron backscatter diffraction (EBSD). The SFE was determined by using quantum mechanical first-principles approach. By using plasticity models we make an attempt to explain and interpret the different strain hardening behaviour of stainless steels with different stacking fault energies.

Abstract: We investigated the crystal structure, vibrational and elastic properties of crystals with a rare-earth sublattice related to different structural types at ab initio level of modeling: elpasolite Cs₂NaRF₆ −> pyrochlore R₂Ti₂O₇ −> ferroborate RFe₃(BO₃)₄, where R is a rare-earth ion or yttrium. The calculations were performed in the framework of a density functional theory using the hybrid functionals containing local and non-local contribution (i.e. Hartree-Fock exchange term) to the exchange energy. We used CRYSTAL program for simulating periodic structures in the MO LCAO approximation. To describe the internal shell of a rare-earth ion up to 4f, we used the nonrelativistic pseudopotential («4f-in-core») that describes the effect of internal electrons on the outer valence shells. The results of the calculations are in good agreement with the available experimental data of IR and Raman experiments, X-ray diffraction analysis for the rows of elpasolites, pyrochlores and ferroborates.

Abstract: The electronic structure of the perovskite manganites LaMnO₃ and La_2/3 Al_1/3 MnO₃ was presented. The calculations were made within density functional theory (DFT) and PBE exchange correlations energy approximation. It was found that inclusion of Al dopants add additional states near the Fermi level and decreasing the resistivity values for all temperature range.

Abstract: We have performed ab-initio calculations to investigate the structural and electronic behavior of TiN in the stable B1 and high pressure B2 phases using pseudo-potential plane wave approach within the framework of density functional theory. The calculated results show agreement with the experimental data. The present electronic behavior, determined by total energy calculations with generalized gradient approximation for exchange and correlation interactions, is observed to be similar in both B1 and B2 phases showing metallic, covalent as well as ionic bonding of TiN. The investigations in B2 phase need validation experimentally as well as theoretically.

Abstract: We use quantum-mechanical calculations to study single-crystalline elastic properties of (Ca,Mg)CO₃ crystals with concentrations ranging from calcite CaCO₃ to magnesite MgCO₃. By analyzing results for a dense set of distributions of Ca and Mg atoms within 30-atom supercells, our theoretical study shows that those atomic configurations, that minimize the total energy for a given concentration, are characterized by elastic constants that either increase with the Mg content or remain nearly constants. Employing these ab initio calculated single-crystalline elastic parameters, the polycrystalline elastic properties of (Ca,Mg)CO₃ aggregates are determined using a mean-field self-consistent homogenization method. The computed integral elastic moduli (bulk and shear) show a significant stiffening impact of Mg atoms on calcite crystals. Our analysis also demonstrates that it is not advantageous to use a granular two-phase composite of stoichiometric calcite and magnesite instead of substituting individual Ca and Mg atoms. Such two-phase aggregates are not significantly thermodynamically favorable and do not offer any strong additional stiffening effect.

Abstract: In the framework of a unified approach the diffusion coefficient (the prefactor and activation barrier) of an interstitial oxygen O_i, the hydrogen molecule H₂, vacancy, oxygen dimer in silicon crystals and O_i in Si_1-xGe_x solid solutions, silicon nanotubes and nanowires has been calculated. For all the above cases, the calculated values of the diffusion coefficient are in good agreement with the experimental data. The calculated equilibrium structures, electrical activity, the vibrational spectrum, the mechanism of diffusion of oxygen dimer fully describe the experimental results. Our study has revealed that the diffusivity of impurities (defects) in alloys can decrease considerably and this variation results from the fact that the prefactor depends on the concentration of component elements of the alloy.

Abstract: The structural and elastic properties of the cubic spinel Ge₃N₄ semiconductor have been investigated using the ab initio scheme within the generalized gradient approximation in the framework of density functional theory. Quantities such as lattice constants and elastic constants of interest are calculated. A good agreement is found between our results and the other data. Through the lattice dynamics, in which the finite displacement method is used, we have obtained successfully the thermal properties such as the phonon curve, free energy, heat capacity and Debye temperature in the whole temperature range from 0 to 1000K. It is the authors ambition that these results will inspire further experimental study on the Ge-based semiconductors. * Corresponding author: CHEN Dong

Abstract: Ab initio electronic structures for β-Ge₃N₄ are investigated using ultrasoft pseudo-potential method within the generalized gradient approximation functional. The lattice parameters are predicted theoretically, and are compared with available experimental data and the other theoretical results. The elastic constants calculations reveal that the phenacite structure is a stable phase in the pressure range of 020GPa. The high bulk modulus (B=180.2GPa) indicates that β-Ge₃N₄ is a relatively hard material. β-Ge₃N₄ has a direct band gap of 2.94eV, indicating its promising applications as a semiconductor in electronics and optical fibers. On the other hand, the density of states analysis is also included in this paper.* Corresponding author: CHEN Dong

Abstract: We report ab initio calculations of the structural, elastic and optical properties of the compound LaB6 as a function of pressure. The computation is based on the density functional theory in combination with the generalized gradient approximation functional. The calculated lattice constants and elastic moduli are compared with the theoretical results and a good agreement is found. LaB6 can retain its mechanical stability in the pressure range of 0-20GPa. Besides, the frequency-dependent dielectric function, absorption coefficient and loss function of LaB6 are also obtained. The calculated static dielectric function is 8.8 at 0GPa and 5GPa. The computed results should be testified by experiments.