| 1. |
- Korzhavyi, Pavel A., 1966-, et al.
(author)
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Ab Initio Study of Phase Equilibria in TiCx
- 2002
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In: Physical Review Letters. - : The American Physical Society. - 0031-9007 .- 1079-7114. ; 88:1, s. 015505-1-01505-4
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Journal article (peer-reviewed)abstract
- The phase diagram for the vacancy-ordered structures in the substoichiometric TiCx (x=0.5-1.0) has been established from Monte Carlo simulations with the long-range pair and multisite effective interactions obtained from ab initio calculations. Three ordered superstructures of vacancies (Ti2C, Ti3C2, and Ti6C5) are found to be ground state configurations. Their stability has been verified by full-potential total energy calculations of the fully relaxed structures.
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| 2. |
- Pourovskii, L. V., et al.
(author)
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Antisite-defect-induced surface segregation in ordered NiPt alloy
- 2003
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 90:2
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Journal article (peer-reviewed)abstract
- By means of first principles simulations we demonstrate that tiny deviations from stoichiometry in the bulk composition of the NiPt-L1(0) ordered alloy have a great impact on the atomic configuration of the (111) surface. We predict that at T=600 K the (111) surface of the Ni51Pt49 and Ni50Pt50 alloys corresponds to the (111) truncation of the bulk L1(0) ordered structure. However, the (111) surface of the nickel deficient Ni49Pt51 alloy is strongly enriched by Pt and should exhibit the pattern of the 2x2 structure. Such a drastic change in the segregation behavior is due to the presence of different antisite defects in the Ni- and Pt-rich alloys and is a manifestation of the so-called off-stoichiometric effect.
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| 3. |
- Pourovskii, L. V., et al.
(author)
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Bulk ordering and surface segregation in Ni50Pt50
- 2001
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In: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 6403:3
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Journal article (peer-reviewed)abstract
- Interatomic interactions obtained from the effective screened generalized-perturbation method have been applied in Monte Carlo simulations to derive the bulk and surface-alloy configurations for Ni50Pt50 The calculated order-disorder transition temperature and short-range order parameters in the bulk compare well with experimental data. The surface-alloy compositions for the (111) and (110) facets above the ordering transition temperature are also found to be in a good agreement with experiments. It is demonstrated that the segregation profile at the (110) surface of NiPt is mainly caused by the unusually strong segregation of Pt into the second layer and the interlayer ordering due to large chemical nearest-neighbor interactions.
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| 4. |
- Poyurovskii, L. V., et al.
(author)
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Application of the Monte Carlo method to the problem of surface segregation simulation
- 2001
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In: JETP Letters. - : Pleiades Publishing Ltd. - 0021-3640 .- 1090-6487. ; 73:8, s. 415-419
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Journal article (peer-reviewed)abstract
- A generalization of the Monte Carlo method to the case of grand canonical ensemble allowing the elimination of the problem of determination of the chemical potential of alloy components was proposed. The method is particularly convenient for the calculations of surface segregations because it excludes time-consuming calculation of the temperature-dependent bulk chemical potential mu (T). The new method was used for calculating segregations at the (100), (110), and (111) surfaces of the Ni50Pd50 alloy using the Ising model with ab initio effective interatomic interaction potentials.
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| 5. |
- Bligaard, T., et al.
(author)
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Pareto-optimal alloys
- 2003
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 83:22, s. 4527-4529
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Journal article (peer-reviewed)abstract
- Large databases that can be used in the search for new materials with specific properties remain an elusive goal in materials science. The problem is complicated by the fact that the optimal material for a given application is usually a compromise between a number of materials properties and the cost. In this letter we present a database consisting of the lattice parameters, bulk moduli, and heats of formation for over 64 000 ordered metallic alloys, which has been established by direct first-principles density-functional-theory calculations. Furthermore, we use a concept from economic theory, the Pareto-optimal set, to determine optimal alloy solutions for the compromise between low compressibility, high stability, and cost.
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| 6. |
- Christoffersen, E., et al.
(author)
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Anode materials for low-temperature fuel cells : A density functional theory study
- 2001
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In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 199:1, s. 123-131
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Journal article (peer-reviewed)abstract
- Based on density functional calculations, we discuss the effect of alloying Pt with other metals for use as anode catalyst materials in low-temperature fuel cells. We discuss why a few parts per million of CO in the H-2 fuel can poison Pt surfaces and how this problem can be alleviated by alloying, and an extensive data base of the effect of alloying on the reactivity that includes all binary combinations of the transition metals to the right in the periodic table is given. We also discuss the effect of surface segregation and give a calculated data base of segregation energies of binary transition metal alloys. Based on extensive Monte Carlo simulations we show that while the adsorbate-free surface of a Ru0.5Pt0.5 alloy has no Ru in the first layer, the presence of CO can move some Ru to the surface, but all these Ru atoms are covered by CO.
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| 7. |
- Hugosson, Håkan Wilhelm, et al.
(author)
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Surface energies and work functions of the transition metal carbides
- 2004
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In: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 557:03-jan, s. 243-254
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Journal article (peer-reviewed)abstract
- We have performed an ab initio study of the surface energies, surface electronic structures and work functions for the (10 0) surface of the, existent and hypothetical, cubic 3d (Sc-Cu), 4d (Zr-Ag) and 5d (La-Au) transition metal carbides. The calculated surface energies have been compared to predictions using a so-called bond-cutting model and a model based on the so-called bonding energies. The absolute values and rough trends of the surface energies are fairly well predicted within the simple bond-cutting model, as compared to fully self-consistent calculations, while both trends and absolute values are well reproduced within the bonding energy model. The electronic structure (densities of states) of the transition metal carbides at the surface and in the bulk have been calculated. The trends are discussed in relation to the behavior of the surface energy and the work function across the series.
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| 8. |
- Johannesson, G. H., et al.
(author)
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Combined electronic structure and evolutionary search approach to materials design
- 2002
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 88:25
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Journal article (peer-reviewed)abstract
- We show that density functional theory calculations have reached an accuracy and speed making it possible to use them in conjunction with an evolutionary algorithm to search for materials with specific properties. The approach is illustrated by finding the most stable four component alloys out of the 192 016 possible fcc and bcc alloys that can be constructed out of 32 different metals. A number of well known and new super alloys are identified in this way.
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| 9. |
- Korzhavyi, Pavel A., 1966-, et al.
(author)
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Constitutional and thermal point defects in B2 NiAl
- 2000
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 61:9, s. 6003-6018
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Journal article (peer-reviewed)abstract
- The formation energies of point defects and the interaction energies of various defect pairs in NiAl are calculated from first principles within an order N, locally self-consistent Green's-function method in conjunction with multipole electrostatic corrections to the atomic sphere approximation. The theory correctly reproduces the ground state for the off-stoichiometric NiAl alloys. The constitutional defects (antisite Ni atoms and Ni vacancies in Ni-rich and Al-rich NiAl, respectively) are shown to form ordered structures in the ground state, in which they tend to avoid each other at the shortest distance on their sublattice. The dominant thermal defects in Ni-rich and stoichiometric NiAl are calculated to be triple defects. In Al-rich alloys another type of thermal defect dominates, where two Ni vacancies are replaced by one antisite Al atom. As a result, the vacancy concentration decreases with temperature in this region. The effective defect formation enthalpies for different concentration regions of NiAl are also obtained.
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| 10. |
- Landa, A., et al.
(author)
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First-principles phase diagram of the Ce-Th system
- 2004
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 70:22
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Journal article (peer-reviewed)abstract
- Ab initio total energy calculations based on the exact muffin-tin orbitals (EMTO) theory are used to determine the high pressure and low-temperature phase diagram of Ce and Th metals as well as the Ce43Th57 disordered alloy. The compositional disorder for the alloy is treated in the framework of the coherent potential approximation. The equation of state for Ce, Th, and Ce43Th57 has been calculated up to 1 Mbar in good comparison with experimental data: upon compression the Ce-Th system undergoes crystallographic phase transformation from a fcc to a body-centered-tetragonal structure and the transition pressure increases with Th content in the alloy.
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| 11. |
- Landa, A., et al.
(author)
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Monte Carlo simulations of the stability of delta-Pu
- 2003
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In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 15:24, s. L371-L376
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Journal article (peer-reviewed)abstract
- The transition temperature (T-c) for delta-Pu has been calculated for the first time. A Monte Carlo method is employed for this purpose and the effective cluster interactions are obtained from first-principles calculations incorporated with the Connolly-Williams and generalized perturbation methods. It is found that at T-c similar to 548 K, delta-Pu undergoes transformation from a disordered magnetic state to a structure with an anti ferromagnetic spin alignment that is mechanically unstable with respect to tetragonal distortion. The calculated transition temperature is in good agreement with the temperature measured at the gamma --> delta transition (593 K).
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| 12. |
- Ruban, Andrei V., et al.
(author)
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Atomic and magnetic configurational energetics by the generalized perturbation method
- 2004
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 70:12
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Research review (peer-reviewed)abstract
- It is shown that, using the generalized perturbation method (GPM) with screened Coulomb interactions that ensures its consistency with the force theorem, one is able to obtain effective interactions that yield an accurate and physically transparent description of configurational energetics in the framework of the Korringa-Kohn-Rostoker method within the atomic sphere and coherent potential approximations. This is demonstrated with calculations of ordering energies, short-range order parameters, and transition temperatures in the CuZn, CuAu, CuPd, and PtCo systems. Furthermore, we show that the GPM can be used to obtain Heisenberg exchange interaction parameters, which, for instance, capture very well the magnetic configurational energy in bcc Fe.
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| 13. |
- Ruban, Andrei V., et al.
(author)
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Local lattice relaxations in random metallic alloys : Effective tetrahedron model and supercell approach
- 2003
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 67:21
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Journal article (peer-reviewed)abstract
- We present a simple effective tetrahedron model for local lattice relaxation effects in random metallic alloys on simple primitive lattices. A comparison with direct ab initio calculations for supercells representing random Ni0.50Pt0.50 and Cu0.25Au0.75 alloys as well as the dilute limit of Au-rich CuAu alloys shows that the model yields a quantitatively accurate description of the relaxtion energies in these systems. Finally, we discuss the bond length distribution in random alloys.
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| 14. |
- Ruban, Andrei V., et al.
(author)
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Screened Coulomb interactions in metallic alloys. I. Universal screening in the atomic-sphere approximation
- 2002
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 66:2
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Journal article (peer-reviewed)abstract
- We have used the locally self-consistent Green's-function (LSGF) method in supercell calculations to establish the distribution of the net charges assigned to the atomic spheres of the alloy components in metallic alloys with different compositions and degrees of order. This allows us to determine the Madelung potential energy of a random alloy in the single-site, mean-field approximation. The Madelung potential makes density-functional calculations by the conventional single-site, coherent potential approximation practically identical to the more rigorous LSGF supercell results obtained with a single-site local interaction zone. We demonstrate that the basic mechanism that governs the charge distribution is the screening of the net charges of the alloy components that makes the direct Coulomb interactions short ranged. In the atomic-sphere approximation, this screening appears to be almost independent of the alloy composition, lattice spacing, and crystal structure. A formalism which allows a consistent treatment of the screened Coulomb interactions within the single-site mean-field approximation is outlined. We also derive the contribution of the screened Coulomb interactions to the S-(2) formalism and the generalized perturbation method.
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| 15. |
- Ruban, Andrei V., et al.
(author)
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Screened Coulomb interactions in metallic alloys. II. Screening beyond the single-site and atomic-sphere approximations
- 2002
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 66:2
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Journal article (peer-reviewed)abstract
- A quantitative description of the configurational part of the total energy of metallic alloys with substantial atomic size difference cannot be achieved in the atomic-sphere approximation: It needs to be corrected at least for the multipole-moment interactions in the Madelung part of the one-electron potential and energy. In the case of a random alloy such interactions can be accounted for only by lifting the atomic-sphere and single-site approximations, in order to include the polarization due to local environment effects. Nevertheless, a simple parametrization of the screened Coulomb interactions for the ordinary single-site methods, including the generalized perturbation method, is still possible. We obtained such a parametrization for bulk and surface NiPt alloys, which allows one to obtain quantitatively accurate effective interactions in this system.
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| 16. |
- Ruban, Andrei V.
(author)
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Surface composition of ordered alloys : An off-stoichiometric effect
- 2002
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 65:17
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Journal article (peer-reviewed)abstract
- I predict the existence of an off-stoichiometric effect in ordered alloys in the form of a distinct transition in the surface segregation behavior of alloy components near the bulk stoichiometric composition. It is caused by the discontinuity in the effective chemical potential at the stoichiometric composition. The effect is predicted to occur at the (111) surface of ordered Ni3Al and Pt3Fe alloys.
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| 17. |
- Skriver, H. L., et al.
(author)
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Steps, kinks, and segregation at metallic surfaces
- 2000
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In: Progress in Surface Science. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0079-6816 .- 1878-4240. ; 64:08-mar, s. 193-198
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Journal article (peer-reviewed)abstract
- We have used density functional theory to establish databases of surface energies for low index surfaces of monoatomic metals and of surface segregation energies of single transition metal impurities in transition metal hosts. The data form one consistent starting point for models of surface science phenomena.
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| 18. |
- Soderlind, P., et al.
(author)
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First-principles elastic constants and phonons of delta-Pu
- 2004
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 70:14
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Journal article (peer-reviewed)abstract
- Elastic constants and zone-boundary phonons of delta-plutonium have been calculated within the density-functional theory. The paramagnetic state of delta-Pu is modeled by disordered magnetism utilizing either the disordered local moment or the special quasirandom structure techniques. The anomalously soft C-' as well as a large anisotropy ratio (C-44/C-') of delta-Pu is reproduced by this theoretical model. Also the recently measured phonons for delta-Pu compare relatively well with their theoretical counterpart at the zone boundaries.
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| 19. |
- Xu, Y., et al.
(author)
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Adsorption and dissociation of O-2 on Pt-Co and Pt-Fe alloys
- 2004
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 126:14, s. 4717-4725
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Journal article (peer-reviewed)abstract
- Self-consistent periodic density functional theory calculations (GGA-PW91) have been performed to study the adsorption of O and O-2 and the dissociation of O-2 on the (111) facets of ordered Pt3Co and Pt3Fe alloys and on monolayer Pt skins covering these two alloys. Results are compared with those obtained on two Pt(111) surfaces, one at the equilibrium lattice constant and the other laterally compressed by 2% to match the strain in the Pt alloys. The absolute magnitudes of the binding energies of O and O-2 follow the same order in the two alloy systems: Pt skin < compressed Pt(111) < Pt(111) < Pt3CO(111) or Pt-3-Fe(111). The reduced activity of the compressed Pt(111) and Pt skins for oxygen can be rationalized as being due to the shifting of the d-band center increasingly away from the Fermi level. We propose that an alleviation of poisoning by O and enhanced rates for reactions involving O may be some of the reasons why Pt skins are more active for the oxygen reduction reaction in low-temperature fuel cells. Finally, a linear correlation between the transition-state and final-state energies of O-2 dissociation on monometallic and bimetallic surfaces is revealed, pointing to a simple way to screen for improved cathode catalysts.
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