Papers by Keyword: Ab Initio Calculation

Abstract: This research work focuses on the atomic study of hexagonal titanium (Ti) in order to estimate the relative accuracy of DFT (Density Functional Theory) and Molecular Statics (MS) approaches to better understand the interactions between solute atoms and twins. Four twins (2 tensile twins and 2 compressive twins) were modeled and then doped with the following elements: hydrogen, oxygen, nitrogen, aluminum and vanadium (H, O, N, Al, V). The formation energies of the twins as well as the segregation energies of the solute atoms were calculated to better predict the concentration heterogeneities of these elements in the material and their possible influence on local mechanical properties.

Abstract: Isolated clusters, fragments of the bcc structure of the compounds of the TiVZrNbMo, TiVZrNbHf and TiVZrNbTa systems, have been constructed and studied from the first principles. An approach has been developed and applied to predicting the lattice parameter and its distortion in solid-state structures based on the results of an Ab initio study of fragments of their structure. It has been established that the predicted values of the lattice constant of the equiatomic compounds TiVZrNbMo, TiVZrNbTa, TiVZrNbHf agree with the available experimental values with high reliability. The developed approach, together with the constructed models of isolated clusters, can be recommended for predicting the lattice parameter and its distortion in new predictive multicomponent metalic compounds.

Abstract: Ab initio calculations based on the Density Function Theory (DFT) have been performed to study the interaction between helium and helium, helium and vacancy, migration of helium, and the stability of small helium-vacancy clusters in tantalum. The following results are found: (I) The tetrahedral interstitial helium atoms have weak interactions in tantalum, suggesting that no stable covalent bond is formed between this two helium atoms; (II) The stability of small helium-vacancy clusters is investigated. The interstitial helium atom and vacancy to the clusters are found to be positive in almost all case, i.e., all interactions are attractive; (III) The activation energies for a substitutional helium atom migration by the dissociation or vacancy mechanisms are estimated under the irradiation condition.

Abstract: In the present work, we report an ab initio investigation of the structural, electronic and linear optical properties of (Na_0.5Bi_0.5)TiO₃ (NBT) in its rhombohedral phase, using a Full Potential Augmented Plane Waves (FP-LAPW) method in the frame work of Density Functional Theory (DFT) with the TB-mBJ potential for a better description of the electronic properties. Firstly a Full structure optimization was performed with a relaxation of atomics positions to minimize the Hellmann-Feynman forces exerted over the atoms. The calculated lattice parameters of the rhombohedral phase of NBT are in very good agreement with experimental values with a deviation of 0.9%. The electronic density of states are presented and commented. The calculated band structure shows that our compound has an indirect band gap of 3.30eV. Furthermore, the optical properties were presented, compared with experimental ones present in the literature and commented.

Abstract: A model has been developed to predict the interdiffusion behaviour of elements between a substrate and a coating. This model, however, relies on knowing accurate diffusion coefficients. However, only limited diffusion data are available in the literature. Recently, it has been demonstrated that Density Functional Theory (DFT) can be used to calculate relevant diffusion coefficients with reasonable accuracy. According to the vacancy diffusion mechanism , diffusion coefficient has an Arrhenius form. The diffusion activation energy can be written as a sum of the diffusion energy barrier and the vacancy formation energy adjacent to a solute.

Abstract: The effects of interstitial nitrogen atoms on the adsorption of atomic oxygen on fcc Fe (001) surface have been studied using ab initio density functional theory calculations to understand the initial stage of oxidation on nitrogenous austenitic stainless steel. It has been found out that the N atoms can improve the adsorption ability of the O atom at the hollow site on the surface, and thus promote the rapid passivation of nitrogenous austenitic stainless steel. This improvement is possibly because the Coulombic interactions between the O atom and the neighboring Fe and Cr atoms are enhanced due to the electron transfer from the Fe and Cr atoms to the N atoms.

Abstract: We have performed First-principles density functional calculation by using full potential linearized augmented plane wave (FP-LAPW) method within generalized gradient approximation (GGA) of B₂- AlGd (Aluminum compound). The ground state properties along with electronic and elastic properties are studied. The energy ranges are given for bands which are crossing the Fermi level and explained whether the Fermi surface is formed by hole pocket or electron pocket. Bonding properties are analyzed by charge density plot. By B/G_H ratio the brittleness of the material is determined.

Abstract: The ground state behavior of rare earth intermetallic compound TmPb_3, which crystallize in AuCu₃ type structure, has been examined using first principles density functional theory based on full potential linearized augmented plane wave (FP-LAPW) method. Very few study on structural and electronic properties of TmPb₃ compound has been available in the literature, which motivated us to perform the present study. The spin polarized calculations are carried out within the PBE-GGA and LSDA for the exchange correlation (XC) potential. Our calculated ground state properties such as lattice constant (a₀), bulk modulus (B) and its pressure derivative (B’) are in good agreement with the experimental results. The value of bulk modulus of TmPb₃ is found to be 44.32 GPa and 55.01GPa by PBE-GGA and LSDA respectively. The electronic band structure (BS) and density of states (DOS) verify the metallic nature of this compound. The calculated density of states at the fermi level is found to be 0.16 states/eV and 19.50 states/eV for spin-up and spin-down modes respectively. The magnetic moment of TmPb₃ is found to be 0.95.

Abstract: A density functional theory based method was used to model dynamics of high frequency delocalized normal mode and discrete breathers in graphene at T = 0 K. For the normal mode the comparison of results was made with modeling by means of classical molecular dynamics. Discrete breathers have been found only in presence of uniaxial strain applied in “zigzag” direction. The oscillations of breather core atoms appeared to be polarized along “arm-chair” direction. In the case of “arm-chair” uniaxial strain there were no breathers found. The frequency on the amplitude dependency of DBs in graphene corresponds to the soft nonlinearity type that is due to the soft nonlinearity type of the high frequency normal mode on which breather is constructed.

Abstract: The influence of Co and Cu doping on Ni-Mn-Ga alloy is investigated using the different ab initio methods for description of chemical disorder. The exact muffin-tin orbital method in combination with the coherent-potential approximation provides almost identical profiles of total energies along the tetragonal deformation path compared to the supercell approach used within the projector-augmented wave method. On the other hand, the simple virtual crystal approximation exhibits different results and thus it is not able to describe doping in Ni-Mn-Ga alloy properly.