First-principles theory of the energetics of He defects in bcc transition metals (original) (raw)
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Systematic group-specific trends for point defects in bcc transition metals: An ab initio study
Journal of Nuclear Materials, 2007
Density functional theory calculations have been performed to study the systematic trends of point defect behaviours in bcc transition metals. We found that in all non-magnetic bcc transition metals, the most stable self-interstitial atom (SIAs) defect configuration has the h1 1 1i symmetry. The calculated formation energy differences between the h1 1 0i dumbbell and the lowest-energy h1 1 1i configuration of metals in group 5B (V, Nb, Ta) are consistently larger than those of the corresponding element in group 6B (Cr, Mo, W). The predicted trends of SIA defects are fundamentally different from those in ferromagnetic a-Fe and correlate very well with the pronounced group-specific variation of thermally activated migration of SIAs under irradiation depending on the position of bcc metals in the periodic table.
Self-interstitial atom defects in bcc transition metals: Group-specific trends
Physical Review B, 2006
We present an investigation of systematic trends for the self-interstitial atom ͑SIA͒ defect behavior in body-centered cubic ͑bcc͒ transition metals using density-functional calculations. In all the nonmagnetic bcc metals the most stable SIA defect configuration has the ͗111͘ symmetry. Metals in group 5B of the periodic table ͑V, Nb, Ta͒ have significantly different energies of formation of the ͗111͘ and ͗110͘ SIA configurations, while for the group 6B metals ͑Cr, Mo, W͒ the two configurations are linked by a soft bending mode. The relative energies of SIA defects in the nonmagnetic bcc metals are fundamentally different from those in ferromagnetic bcc ␣-Fe. The systematic trend exhibited by the SIA defect structures in groups 5B and 6B transition metals correlates with the observed thermally activated mobility of SIA defects.
The non-Arrhenius migration of interstitial defects in bcc transition metals
Comptes Rendus Physique, 2008
Thermally activated migration of defects drives microstructural evolution of materials under irradiation. In the case of vacancies, the activation energy for migration is many times the absolute temperature, and the dependence of the diffusion coefficient on temperature is well approximated by the Arrhenius law. On the other hand the activation energy for the migration of self-interstitial defects, and particularly self-interstitial atom clusters, is very low. In this case a trajectory of a defect performing Brownian motion at or above room temperature does not follow the Arrhenius-like pattern of migration involving infrequent hops separated by the relatively long intervals of time during which a defect resides at a certain point in the crystal lattice. This article reviews recent atomistic simulations of migration of individual interstitial defects, as well as clusters of interstitial defects, and rationalizes the results of simulations on the basis of solutions of the multistring Frenkel-Kontorova model. The treatment developed in the paper shows that the origin of the non-Arrhenius migration of interstitial defects and interstitial defect clusters is associated with the interaction between a defect and the classical field of thermal phonons. To cite this article: S.L. Dudarev, C. R. Physique 9 (2008). Crown
Journal of Nuclear Materials, 2012
The results of DFT calculations on radiation point defects in tungsten are presented. The lowest energy configuration of the self-interstitial has exactly the h1 1 1i orientation and no tilt from this direction is observed when using appropriate cell geometry and pseudopotential. The present DFT calculations confirm that in pure tungsten the interactions between two vacancies are unexpectedly repulsive until the fifth nearest-neighbor and that the second nearest-neighbor di-vacancy is the most repulsive. The electronic entropy contribution to the free energy makes the nearest-neighbor configuration attractive at high temperature. A comparison with other bcc metals shows that the binding energies between two vacancies are strongly metal dependent and that tungsten leads to the largest deviation from empirical potential predictions. In tungsten, the effect on vacancy properties of alloying by tantalum and rhenium has been investigated using the Virtual Crystal Approximation (VCA). The effect of these alloying elements is essentially to change the filling of the d-band and the vacancy formation energy is found to be maximal and the relaxation to be minimal when the Fermi level is at the minimum of the pseudogap, as predicted by previous tight-binding calculations. Di-vacancies are shown to become attractive at first and second nearest-neighbor upon tantalum alloying and even more repulsive upon rhenium alloying.
The Vacancy Energy in Metals: Cu, Ag, Ni, Pt, Au, Pd, Ir and Rh
Physical Science International Journal
The predictive calculations of vacancy formation energies in metals: Cu, Ag, Ni, Pt, Au, Pd, Ir and Rh are presented. The energy is given as a function of electron density. Density functional theory underestimates the vacancy formation energy when structural relaxation is included. The unrelaxed mono-vacancy formation, unrelaxed di-vacancy formation, unrelaxed di-vacancy binding and low index surface energies of the fcc transition metals Cu, Ag, Ni, Pt, Au, Pd, Ir and Rh has been calculated using embedded atom method. The values for the vacancy formation energies agree with the experimental value. We also calculate the elastic constants of the metals and the heat of solution for the binary alloys of the selected metals. The average surface energies calculated by including the crystal angle between planes (hkl) and (111) correspond to the experiment for Cu, Ag, Ni, Pt and Pd. The calculated mono-vacancy formation energies are in reasonable agreement with available experimental values...
Physical Review B, 1999
The unrelaxed vacancy formation energies have been calculated for group-IV elements ͑Ti, Zr, Hf͒ in the hexagonal close packed ͑hcp͒ and body centered cubic ͑bcc͒ structures within the local density approximation to the density functional theory using the full-potential linear muffin-tin orbital method. In hcp-Hf the calculated value of 2.37 eV is in excellent agreement with the experimental value of 2.45Ϯ0.2 eV. The results found in hcp-Ti and hcp-Zr, i.e., 2.14 eV and 2.07 eV, respectively, can therefore be considered as reliable predictions. In the more open bcc structure, after very conclusive validations of the present procedure in Mo and W by comparison with experiments and other ab initio calculations, vacancy formation energies of 2.2-2.4 eV are obtained in Ti, Zr, and Hf. These energies, which are very similar to those in the hcp structure, are significantly larger than the experimental activation energies for self-diffusion in the bcc structure. Assuming that the monovacancy mechanism is dominant in -Ti,-Zr, and -Hf, this demonstrates that structural relaxations with particularly large amplitudes are expected around the vacancy. ͓S0163-1829͑99͒00313-6͔
Vacancies at the surfaces of FCC metals
Russian Physics …, 1997
The vacancy formation energy is calculated using the inserted atom method for faces with high and low indices and in the near-surface atomic layers of aluminum, nickel, copper, palladium, silver, platinum, and gold. The transition of the calculated quantities to the bulk quantities is traced for successive insertion of a defect deeper into the near-surface layer of the material. The relaxation contribution to the energetics is revealed for each type of surface. A correlation is seen between the energies of formation at surfaces with high and low indices.
Stability in bcc Transition Metals: Madelung and Band-Energy Effects due to Alloying
Physical Review Letters, 2009
The phase stability of group VB (V, Nb, and Ta) transition metals is explored by first-principles electronic-structure calculations. Alloying with a small amount of a neighboring metal can either stabilize or destabilize the body-centered-cubic phase relative to low-symmetry rhombohedral phases. We show that band-structure effects determine phase stability when a particular group VB metal is alloyed with its nearest neighbors within the same d-transition series. In this case, the neighbor with less (to the left) and more (to the right) d electrons destabilize and stabilize bcc, respectively. When alloying with neighbors of higher d-transition series, electrostatic Madelung energy dominates and stabilizes the body-centeredcubic phase. This surprising prediction invalidates current understanding of simple d-electron bonding that dictates high-symmetry cubic and hexagonal phases.
Physical Review B, 2014
A density functional theory (DFT) study of the 1/2 111 screw dislocation was performed in the following body-centered cubic transition metals: V, Nb, Ta, Cr, Mo, W, and Fe. The energies of the easy, hard, and split core configurations, as well as the pathways between them, were investigated and used to generate the two-dimensional (2D) Peierls potential, i.e. the energy landscape seen by the dislocation as a function of its position in the (111) plane. In all investigated elements, the nondegenerate easy core is the minimum energy configuration, while the split core configuration, centered in the immediate vicinity of a 111 atomic column, has a high energy near or above that of the hard core. This unexpected result yields 2D Peierls potentials very different from the usually assumed landscapes. The 2D Peierls potential in Fe differs from the other transition metals, with a monkey saddle instead of a local maximum located at the hard core. An estimation of the Peierls stress from the shape of the Peierls barrier is presented in all investigated metals. A strong group dependence of the core energy is also evidenced, related to the position of the Fermi level with respect to the minimum of the pseudogap of the electronic density of states.