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Ab initio study of phase stability in metals
Káňa, Tomáš ; Hüger, E. ; Šob, Mojmír
We employ ab initio calculations to study the behavior of total energy along the transformation paths between the hexagonal close-packed (hcp) structure and face-centered cubic (fcc) structure in Al, Co, Ni, Cu and Pd. We found one peculiar transformation path wit h extremely low energy barrier of only 0.010 eV/atom for Al, 0.020 eV/atom for Pd, 0.030 eV/atom for Cu, 0.033 eV/atom for Ni and 0.050 eV/atom for Co.
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First principles study of polymorfy of Ni and Co thin layers
Zelený, Martin ; Šob, Mojmír
The total energy profiles of trigonal and hexagonal bulk Ni and Co are employed to advance our understanding of epitaxial growth of Ni and Co thin films on various fcc(111) substrates. For nickel films, we predict the fcc stacking in the neighbourhood of the ground state and for the distance of nearest neighbors larger than 5.18 a.u. the film should exhibit hcp layer stacking. Cobalt films behave differently. The hcp phases are energetically more favorable in whole region studied.
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Analýza elektronové struktury Lavesových fází obsahujících chrom
Houserová, Jana ; Šob, Mojmír
Binary systems containing chromium can be found in many kinds of steels where the Laves-phase can be considered as a potential candidate for strengthening provided size and distribution of the particles are within certain limits. We performed electronic structure calculations to study the thermodynamic and structural properties of the C14 Laves phase with various compositions. In the frame of the thermodynamic study, we have evaluated the total energies of formation of Laves phase structure with different occupations of sublattices (Cr2Cr, Cr2X, X2Cr, X2X, where X stands for Fe, Mo, Si, Ta and W) with respect to the Standard Element Reference (SER) states. These energies of formation are of great importance for prediction of stability of studied configurations.
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Energetika tvorby sigma fází v Mo-X (X=Co, Cr, Fe) systémech
Houserová, Jana ; Šob, Mojmír
In systems Mo-Co, Mo-Cr and Mo-Fe, we performed calculations of the total energies of formation of all 32 possible occupations of sublattices in sigma phase structure. The optimisation of the lattice parameters of all structures was included. Sigma phases in studied systems exhibit positive values of energies of formation and it is supposed that the stability of the really existing sigma phases (i.e. Mo-Fe and Mo-Co) is due to configurational and vibrational entropies.
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Studium struktury a magnetismu FeCo při vysokých deformacích z prvních principů
Čák, Miroslav ; Legut, Dominik ; Šob, Mojmír
Total energies and magnetic moments of FeCo along the tetragonal deformation path are calculated using first-principles electronic structure methods. Total energies are displayed in a contour plot as a function of tetragonal distortion and volume; borderline between the ferromagnetic and nonmagnetic states is shown. The calculated energies may be used to predict the lattice parameters of FeCo thin films on various (001) substrates; here we study the FeCo film on the MgO(001) substrate. The calculated results are compared with available experimental data. The loss of magnetism of FeCo at low atomic volumes and large shape deformations is observed.
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Možné transformační dráhy spojující struktury C11b, C40 and C54 v MoSi2
Káňa, Tomáš ; Legut, Dominik ; Šob, Mojmír
We suggest three transformation paths between the ideal C11b , C40 and C54 structures of transition-metal disilicides. These three structures can be regarded as different stackings of identical planes of atoms. Each suggested transformation path consists in shifting these atomic planes in three directions. The shift of planes is described by one parameter p <0;1>. Along each path, we calculate the total energy of MoSi2 as a function of p. Our results confirm the structural order C11b → C40 →C54 in MoSi2. The C11b–C54 path exhibits the largest energy barrier, 2.5 eV/f.u (f.u. means the formula unit). The energy barrier of the C11b–C40 path is lower, 1.7 eV/f.u. and finally the energy barrier of the C40–C54 path is the lowest one, 1.4 eV/f.u.
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