Papers by Keyword: Ab Initio

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Authors: Martin Friák, Li Fang Zhu, Liverios Lymperakis, Hajjir Titrian, Ugur Aydin, Anna Maria Janus, Helge Otto Fabritius, Andreas Ziegler, Svetoslav Nikolov, Pavlina Hemzalová, Dierk Raabe, Joerg Neugebauer
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.
Authors: M.M. De Souza, E.M. Sankara Narayanan
Authors: Tokuteru Uesugi, Kenji Higashi
Abstract: We investigate the energy of segregation of solute Ca at symmetric tilt grain boundary in aluminum from the first-principles calculations. As energy of segregation of Ca is negative, Ca atoms tend to segregate at the grain boundary. Furthermore, on basis of the Rice-Wang model, we study the effect of the segregation of Ca on the grain boundary embrittlement of aluminum. Our first-principles calculations of energies of segregation at grain boundary and free surface show that Ca behaves as embrittler.
Authors: S. Fujita, Tokuteru Uesugi, Yorinobu Takigawa, Kenji Higashi
Abstract: The segregation energy of solute Ga in the staking fault in Cu-Ga alloy was calculated from the first principles. Then, we presented numerical results of the stacking fault energy for Cu-Ga alloy using the value of the segregation energy as a input parameter to a expression in the equilibrium state. The numerical results of the stacking fault energy were in good agreement with the experimental values.
Authors: A.B. van Oosten, A.M. Frens, J. Schmidt
Authors: Vladislav P. Petrov, Vladimir A. Chernyshev, Anatoly E. Nikiforov
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.
Authors: Mamta Chauhan, Dinesh Chandra Gupta, Idris Hamid Bhat
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.
Authors: Xue Jie Liu, Ling Ling Zhao, Yuan Ren, Engelbert Westkaemper, Xin Tan, Hui Ling Jia
Abstract: In order to study the evolution of Ti-Si-N film growth, the total energies and absorption energies of the Ti-Si-N islands on TiN(001) surface and the activation energy of the configuration evolution have been calculated with the first principle method. Four configurations of Si-2Ti-2N island have been studied, which are a silicon atom in a 2Ti2N island (Si-in-2Ti2N), a silicon atom by a 2Ti2N island (Si-by-2Ti2N), a titanium atom by a 2N1Ti1Si island (Ti-by-2N1Ti1Si), and a nitrogen atom by a 2Ti1N1Si island (N-by-2Ti1N1Si), respectively. The investigation presents some results. In the growth process of Ti-Si-N film, (1) the Ti and N atoms bonding together to form islands and Si atoms staying outside of TiN islands will lead to the most stable configuration; (2) the Si atom tends to separate from TiN phase, but the configuration evolution is not very easy, the activation energy of the transition from Ti-by-2N1Ti1Si to Si-2Ti2N is about 1.94eV; (3) it shows a tendency for Si atoms to bond with N atoms, rather than with Ti atoms.
Authors: Mariya G. Ganchenkova, V.A. Borodin, S. Nicolaysen, Risto M. Nieminen
Abstract: In this paper we study the effect of chemical environment and elastic strains, which can arise in layered heterostructures due to the lattice parameter mismatch, on the vacancy formation energy in random Si-Ge compounds. Ab initio calculations demonstrate a number of simple trends characterizing the vacancy formation energy dependence on vacancy charge, the number of Ge atoms in its neighbourhood and on the magnitude of elastic strains. The obtained parameters of vacancy-germanium interaction indicate, in particular, a tendency for preferential vacancy accumulation in SiGe region of Si/SiGe/Si layered structures, which is confirmed here by Monte- Carlo simulation of high-temperature vacancy annealing and agrees well with recent experimental observations.
Authors: Rodolfo A. Pérez, D.N. Torres, Fanny Dyment, M. Weissman
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