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Qantum-mechanical study of structural stability of Ni4N allotropes
Hemzalová, P. ; Friák, Martin ; Šob, Mojmír ; Neugebauer, J.
Parameter-free density functional theory (DFT) calculations of Ni4N in eight crystallographic phases were performed using the pseudopotential approach implemented in the VASP code; the exchange-correlation energy was evaluated within the generalized gradient approximation (GGA). In agreement with experiments, the cubic structure with Pearson symbol cP5, space group Pm-3m (221), has been found to be the most stable. It is also the only thermodynamically stable structure at T=0 K with respect to decomposition into elemental Ni crystal and N2 gas phase. We determine structural, thermodynamic, electronic, magnetic and elastic properties of all eight Ni4N allotropes studied. The thermodynamic stability and bulk modulus is found to be anti-correlated. For the cubic allotropes, we predict a complete set of single-crystalline elastic constants, directional dependence of the single-crystalline Young modulus and homogenized polycrystalline elastic moduli.
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Investigating ground state of nickel nitrides NiN and Ni2N with the help of quantum-mechanical calculations
Elstnerová, P. ; Friák, Martin ; Šob, Mojmír ; Neugebauer, J.
We have employed quantum mechanical calculations to identify ground-state structures of nickel nitrides NiN and Ni2N for which experimental data are lacking. In total 19 crystalline phases have been calculated for which not only thermodynamic but also structural and selected elastic properties have been determined. Employing density functional theory (DFT) methods, the total energies were calculated by means of a pseudopotential approach implemented in the VASP code and selected states were benchmarked by the full-potential linearized augmented plane wave (FP-LAPW) method implemented in the WIEN2k code. For the exchange-correlation energy the generalized gradient approximation (GGA) has been used.
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Nanoindentation and theoretical strength in metals and intermetallics
Šob, Mojmír ; Legut, Dominik ; Friák, Martin ; Fiala, J. ; Vitek, V. ; Hafner, J.
The present contribution gives an account of applications of quantum-mechanical (first-principles) electronic structure calculations to the problem of theoretical strength in metals and intermetallics. First, we briefly describe the way of simulating the tensile test and the electronic structure calculational method. Then we discuss the theoretical strength values in a number of elemental metals and intermetallics and compare them with available experimental data, both from measurements on whiskers and from nanoindentation experiments.
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Structure and magnetism of iron and iron overlayers from the first principles
Friák, Martin ; Šob, Mojmír ; Vitek, V.
A detailed theoretical study of magnetic behavior of iron along the bcc fcc (Bain's) transformation paths at various atomic volumes is presented. The total energies are calculated by spin polarized full potential LAPW method and are displayed in contour plots as functions of tetragonal distortion c/a and volume; borderlines between various magnetic phases are shown. Stability of tetragonal magnetic phases of fl Fe is discussed. The calculated phase boundaries are used to predict the lattice parameters and magnetic states of iron overlayers on various (001) substrates.
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Ab initio simulation of three-axial deformation of perfect iron crystal
Černý, M. ; Šandera, P. ; Pokluda, J. ; Friák, Martin ; Šob, Mojmír
Ab initio electronic structure calculations of ideal strength, bulk modulus and equilibrium lattice parameter of iron in the body-centered-cubic lattice under three-axial tension are performed using the linear muĆn-tin orbitals method in atomic sphere ap proximation (LMTO-ASA) and the full-potential linearized augmented plane waves method (FLAPW). Magnetic ordering was taken into account by means of spin-polarized calculation. Two exchange-correlation energy approximations were employed, namely the local (spin) den-sity approximation (LDA) and the generalized gradient approximation (GGA). Computed values are compared with experimental data.
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Ab initio simulation of a tensile test in iron
Friák, Martin ; Šob, Mojmír ; Vitek, V.
A tensile test in ferromagnetic and nonmagnetic iron is simulated by ab initio electronic structure calculations using all-electron full potential linearized augmented plane wave method (FLAPW) within generalized gradient approximation (GGA). The theoretical tensile strength of ferromagnetic iron for [001] loading is determined and compared with that of other materials. The magnetic behavior of iron under tensile loading is studied in detail and compared with results for triaxial loading.
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