Papers by Keyword: Atomic Structure

Abstract: This paper presents an efficient and accurate method for solving the time-dependent Hartree-Fock equations for the helium atom in the ground and single excited states. The radial coordinate is discretized by the discrete variable representation (DVR), which is constructed from Coulomb wave functions. To solve the equation, the spectral method with Coloumb wave functions is used as a basis. We illustrate that the calculated electronic energies for the helium atom are in good agreement with the best available experimental values. The CWDVR method proves to be more economical and efficient as it uses the optimal few numbers of grid points compared to other numerical calculations.

Abstract: In the paper, the atomic structure of amorphous and nanocrystalline alloys of the electrolytically obtained CoP, NiP, CoNiP, CoW, and CoNiW systems has been studied. The structure was investigated by electron microscopy and diffraction using a Libra 200 HR FE transmission electron microscope at an accelerating voltage of 200 kV within a temperature range of 50-35 °C. The obtained radial atom distribution function and the coordination sphere radii are in good agreement with the data for the cobalt structure in the cubic and hexagonal modifications. The high coordination numbers of the third and fourth coordination spheres allow suggesting a predominantly cubic structure of the local atom environment in CoP samples but somewhat lower, which is explained by the presence of free volume and phosphorus atoms distorting the local structure. When heating, the near atomic order also corresponds to the cubic phase of cobalt, and the ordering occurs in the second, third, and fourth coordination spheres. The data obtained for CoNiP alloys indicate that by configuration, the local atomic environment is closer to the hexagonal structure of nickel. In general, the structure of the CoP-CoNiP system alloy films obtained by electrolytic deposition is already in one of the local minima of the total system energy, which is confirmed by the near atomic order similar to the cubic phase of cobalt or hexagonal phase of nickel. This determines the good stability of the structure and properties during thermal exposure.

Abstract: The atomic structure and morphology of cerium oxide nanoparticles obtained by laser ablation are experimentally and theoretically investigated. Using transmission electron microscopy, X-ray diffractometry, and electron energy loss spectroscopy, it has been shown that particles are enriched in oxygen vacancies that stabilize their internal structure. The density functional method was used to study the dependence of the unit cell parameter of CeO₂ nanoparticles on their size. An analysis of the charge density distribution shows a different structural distribution of Ce³⁺ and Ce⁴⁺ atoms in nanoparticles.

Abstract: We have performed density-functional calculations in order to clarify atomic structures and energetics of surface steps on SiC. The obtained energetics of distinct step types on vicinal 3C-SiC(111) surfaces which correspond to 4H- and 6H-SiC(0001) surfaces reveals the atom-scale reason for the experimental observation in the past that the step morphology is straight for the SiC(0001) surfaces inclined toward the 〈1-100〉 direction while it is meandering for the 〈11-20〉 inclined surfaces. The calculations clarify the rebonding between upper- and lower-terrace edge atoms, which is decisive for the energetics of the atomic steps.

Abstract: The structure of electrolytically deposited nanocrystalline alloys of the CoP-CoNiP systems under low-temperature heating was investigated by means of high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM), and analytical methods such as energy dispersive x-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS). Structural relaxation and crystallization were investigated at temperatures from 150°C to 300°C. Structural and compositional inhomogeneities were found in the CoP-CoNiP alloys, while the local changes in composition were found to reach 15 at.%. Nanocrystals in the alloys grew most intensely in the presence of a free surface. It was determined that the local diffusion coefficient ranged from 1.2 to 2.4 10⁻¹⁸ m²/s, which could be explained by the surface diffusion prevalence. The data gathered in these investigations can be further used to predict the thermal stability of CoP-CoNiP alloys.

Abstract: Cu₆Sn₅ is an important intermetallic compound in soldering and electronic packaging. It is formed at the interface between molten solder and substrate during the soldering process, and the evolution of microstructure and properties also occurs in service. Previous studies revealed that Au and Ni are stabilization alloying elements for hexagonal η-Cu₆Sn₅ intermetallic. For better understanding of stabilization mechanisms at atomic resolution level, in this work, we made an attempt atomic structure analysis on a stoichiometric (Cu, Au, Ni)₆Sn₅ intermetallic prepared by direct alloying. High-angle annular dark-field (HAADF) imaging and atomic-resolution chemical mapping were taken by the aberration-corrected (Cs-corrected) scanning transmission electron microscopy (STEM). It is found that Au and Ni doped Cu₆Sn₅ has hexagonal structure. The atom sites of Cu1 and Sn can be distinguished in atomic-resolution images after being observed from orientation [2110], which is also confirmed by atomic-resolution chemical mapping analysis. Importantly, atomic-resolution about distribution of alloying Au atom was directly observed, and Au atoms occupy the Cu1 sites in η-Cu₆Sn₅.

Abstract: The morphological and structural transitions in CdSe hollow nanoparticles (hNPs) with zinc blende structure have studied by molecular dynamics (MD) simulation method under heating. The seven samples of CdSe-hNPs are constructed with different thicknesses from the solid NPs at 10nm and 15nm sizes. Morphological changes in CdSe-hNPs have presented by describing the first stage melting in hollow semiconductor NPs. The thermal effect on the atomic arrangement has also examined by the cubic zinc blende-to-wurtzite transformation occurred during the melting in hNPs. MD results show that the inner shells of those with thin walls have begun to melt at lower temperatures due to the thickness of the NPs. The first stage melting, which resulted in the filling of the void within the particle, takes place almost at the same temperature for hNPs with the thick wall thickness. Then, the melting of the particles is completed at higher temperatures. The cubic diamond structure disappears with the collapse of the inner cavity, and the hcp structure begins to appear at later temperatures.

Abstract: The research presents the atomic structure investigation of amorphous rapidly quenched Co₅₈Ni₁₀Fe₅Si₁₁B₁₆ at.% alloys. The alloys were quenched with linear velocity of cooper wheel surface from 22 to 38 m/s. We found a nonlinear dependence of local atomic ordering from linear velocity of cooling wheel. The average lateral density of ordered atomic clusters of 5 nm size changes from 4% to 8%. The amorphous alloy with metastable disordered structure with lower level of free energy is more stable against the external conditions. This approach can be used to determine the best technological parameters for preparing amorphous metallic alloy with metastable structure.

Abstract: The article is about regularities of structure formation of carbon nanotubes (7,7) in their ground state in the process of the gradually increasing of the concentration of sulfur atoms. The fragment of nanotube is investigated within the framework of the density functional theory with the using of periodic boundary conditions. It contains 112 carbon atoms and n = 1 ÷ 9 sulfur atoms. It is shown that the most energetically favorable configurations of n = 1 ÷ 6 sulfur atoms can be considered as a configuration uncombined sulfur clusters S_n with the slight deformation by tube field. From n = 6, there is a tendency toward the formation of a ribbon structures with 2 sulfur atoms wide, it has a tendency in the moment of increasing n to be guided by transversely to the tube axis. In all considered cases the sulfur atoms are located at a distance from the surface of the tube more than 0.28 nm, it is excluded the possibility of the formation of the strong (covalent) chemical bonds with the carbon nanotube.

Abstract: A first principles calculation method was used to investigate the site preference of Ruthenium (Ru) at the γ/γ′ interface in Ni-based single-crystal superalloys. The calculation results show that the addition of Ru can decrease the total energy and the binding energy of γ/γ′ interface, which may result in an improved microstructure stability of Ni-based single-crystal superalloys. Moreover, by calculation, it is also found that Ru can stabilize both γ and γ′ phases and have a preference for Ni site at the coherent γ/γ′ interface. When Ru substitutes the central Ni at the γ/γ′ interface, a reverse partitioning of W, Re and Cr occurs; while the partitioning behavior of Mo is not affected. The influence of Ru on the partitioning behavior of W, Re and Cr in γ′-Ni3Al was studied by Dmol3 calculation as well. The calculation results show that W, Re and Cr have a preference for Ni site in γ′- Ni3Al with Ru alloying. When Ru substitutes the central Ni atom, the site preference of W, Re and Cr varies accordingly. Furthermore, electronic structure analysis of γ/γ′ interface and γ′-Ni3Al in terms of Mulliken population and partial density of states (PDOS) was performed to understand the alloying mechanism of Ru in Ni-based single-crystal superalloys. The results show that the strengthening effect of Ru alloying is mainly due to the reduction in binding energy of Ru as well as a p-orbital hybridization between Ru and the host atoms.