| 1. |
- Kholtobina, Anastasiia S., et al.
(författare)
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Temperature dependence of (111) and (110) ceria surface energy
- 2023
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 107:3
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Tidskriftsartikel (refereegranskat)abstract
- High-temperature properties of ceria surfaces are important for many applications. Here, we report the temperature dependencies of surface energy for (111) and (110) CeO2 obtained in the framework of the extended two-stage up-sampled thermodynamic integration using Langevin dynamics. The method was used together with machine-learning potentials called moment tensor potentials (MTPs), which were fitted to the results of the ab initio molecular dynamics calculations for (111) and (110) CeO2 at different temperatures. The parameters of MTP training and fitting were tested, and the optimal algorithm for the ceria systems was proposed. We found that the temperature increases from 0 to 2100 K led to the decrease of the Helmholtz free energy of (111) CeO2 from 0.78 to 0.64 J/m2. The energy of (110) CeO2 dropped from 1.19 J/m2 at 0 K to 0.92 J/m2 at 1800 K. We show that it is important to consider anharmonicity, as simple consideration of volume expansion gives the wrong temperature dependencies of the surface energies.
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| 2. |
- Korzhavyi, Pavel A., 1966-, et al.
(författare)
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Ab Initio Study of Phase Equilibria in TiCx
- 2002
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Ingår i: Physical Review Letters. - : The American Physical Society. - 0031-9007 .- 1079-7114. ; 88:1, s. 015505-1-01505-4
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Tidskriftsartikel (refereegranskat)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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| 3. |
- Peil, Oleg E., et al.
(författare)
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Ab initio investigation of magnetic ordering and spin-glass transition in Cu-rich Cu-Mn systems
- 2007
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 310:2, s. 1561-1563
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Tidskriftsartikel (refereegranskat)abstract
- Manganese-noble metal alloys exhibit a large variety of magnetic structures as a function of Mn concentration. Ab initio studies of these structures are practically absent due to difficulties in handling simultaneously atomic and magnetic ordering effects in disordered alloys. In this work we use the generalized perturbation method (GPM) A.V.Ruban, S. Shallcross, S.I. Simak H.L. Skriver, Phys. Rev. B 70 (2004) 125115 in the framework of the KKR method and the coherent potential approximation (CPA) in order to determine both effective chemical interactions and magnetic exchange interaction parameters of the classical Heisenberg Hamiltonian in Cu-Mn alloys. The effective chemical interactions have been used in Ising Monte Carlo simulations to determine the equilibrium distribution of atoms on the FCC lattice at the experimental aging temperatures. The obtained atomic short-range order for Cu-Mn alloys is in excellent agreement with the existing experimental data. The underlying atomic distribution has subsequently been used in Heisenberg Monte Carlo simulations with the exchange interaction parameters calculated for the corresponding alloy composition by the GPM method. For alloys with low Mn content, up to 20 at%, we find a transition into a spin-glass state with a specific magnetic short order.
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| 4. |
- Peil, O. E., et al.
(författare)
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Ab initio study of atomic ordering and spin-glass transition in dilute CuMn alloys
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 79:2
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Tidskriftsartikel (refereegranskat)abstract
- An archetypical spin-glass metallic alloy, Cu0.83Mn0.17, is studied by means of an ab initio based approach. First-principles calculations are employed to obtain effective chemical, strain-induced, and magnetic exchange interactions, as well as static atomic displacements, and the interactions are subsequently used in thermodynamic simulations. It is shown that the calculated atomic and magnetic short-range order accurately reproduces the results of neutron-scattering experiments. In particular, it is confirmed that the alloy exhibits a tendency toward ordering and the corresponding ordered phase is revealed. The magnetic structure is represented by spin-spiral clusters accompanied by weaker ferromagnetic short-range correlations. The spin-glass transition temperature obtained in Monte Carlo simulations by a finite-size scaling technique, 57 K, is in reasonable agreement with experimental data, 78 K.
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| 5. |
- Peil, O. E., et al.
(författare)
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Detailed ab initio calculations of the structure and magnetic state of a metallic spin glass
- 2008
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- We present for the first time a long-sought atomic and magnetic structure of one of the most well-known and studied spin-glass systems: the Cu83Mn17 alloy, which is the prototype for a large class of metallic spin-glass systems. The structure has been determined from simulations based on atomic and magnetic interactions obtained from first-principles calculations. Being in perfect agreement with recent neutron scattering experiments, our results reveal the atomic and magnetic structure that exhibits a specific short-range order (SRO) and is believed to be responsible for a complicated magnetic cluster dynamics leading to the spin-glass behaviour in this alloy. The ordered phase underlying the atomic SRO is shown to have a Cu3Mn-type structure with 16 atoms per unit cell. The magnetic SRO is associated with incommensurate spin-spiral correlations.
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| 6. |
- Peil, O. E., et al.
(författare)
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Self-consistent supercell approach to alloys with local environment effects
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 85:16, s. 165140-
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Tidskriftsartikel (refereegranskat)abstract
- We present an efficient and accurate method for calculating electronic structure and related properties of random alloys with a proper treatment of local environment effects. The method is a generalization of the locally self-consistent Green's-function technique for the exact muffin-tin orbital method. An alloy system in the calculations is represented by a supercell with a certain set of atomic-distribution correlation functions. The Green's function for each atom in the supercell is obtained by embedding the cluster of neighboring atoms lying within a local interaction zone (LIZ) into an effective medium and solving the cluster Dyson equation exactly. The key ingredients of the method are locality, which makes it linearly scaling with the number of atoms in the supercell, and coherent-potential self-consistency of the effective medium, which results in a fast convergence of the electronic structure with respect to the LIZ size. To test the performance and accuracy of the method, we apply it to two systems: Fe-rich bcc-FeCr random alloy with and without a short-range order, and a Cr impurity on the Fe surface. Both cases demonstrate the importance of taking into account the local environment effects for correct description of magnetic and bulk properties.
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| 7. |
- Pourovskii, L. V., et al.
(författare)
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Fully relativistic spin-polarized exact muffin-tin-orbital method
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 71:9
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Tidskriftsartikel (refereegranskat)abstract
- The exact muffin-tin-orbital (EMTO) method is generalized for fully relativistic (FR) spin-polarized calculations. In the present implementation we solve self-consistently the four-component Dirac equation by using the Green's function formalism. Substitutional disorder is treated within the coherent potential approximation. To obtain accurate total energies we use the full-charge density technique. We apply the FR-EMTO Green's function method to calculate the ground-state properties of delta-Pu. We also calculate spin and orbital magnetic moments in random bcc, fcc, and hcp Fe-Co alloys, as well as in the B2 ordered and partially ordered phase.
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| 8. |
- Ruban, Andrei V., et al.
(författare)
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Theoretical investigation of bulk ordering and surface segregation in Ag-Pd and other isoelectornic alloys
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75, s. 054113-1-05411316
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Tidskriftsartikel (refereegranskat)abstract
- Bulk ordering in Ag-Pd and other isoelectronic alloys is investigated theoretically by a number of first-principles techniques. The electronic structure and total energy have been calculated by the Green's function Korringa-Kohn-Rostocker and full-potential plane wave methods. The effective cluster interactions of the Ising-type Hamiltonian have been obtained by the screened generalized perturbation method. They reveal a complex concentration-dependent ordering behavior in these alloys due to band filling and Fermi surface effects. In particular we show that long-period superstructures are gradually stabilized by a great number of relatively weak long-range effective pair- and three-site interactions, which can be seen as collective effect. A similar complex concentration dependence is also found for surfaces of Ag-Pd alloys. The surface composition of the (111) and (100) surface of Ag75Pd25, Ag50Pd50, and Ag33Pd67 alloys have been then investigated by the surface Green's function technique and the screened generalized perturbation method for the effective interactions of the Ising-type Hamiltonian and the grand canonical Monte Carlo method for statistical thermodynamic simulations at finite temperatures. We compare our results with experimental data and other theoretical calculations.
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| 9. |
- Dmitriev, V. V., et al.
(författare)
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Ab initio based investigation of interstitial interactions and Snoek relaxation in Nb-O
- 2012
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Ingår i: Journal of Physics and Chemistry of Solids. - : Elsevier BV. - 0022-3697 .- 1879-2553. ; 73:2, s. 182-187
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Tidskriftsartikel (refereegranskat)abstract
- Chemical and strain-induced effective pair interactions of interstitial oxygen atoms in bcc Nb have been determined in supercell first-principles calculations using Vienna ab initio simulation package (VASP). The strain-induced interactions are in reasonable agreement with those obtained earlier within a phenomenological microscopic Krivoglaz-Kanzaki-Khachaturyan model (KKKM). At the same time, the chemical interactions, which have been considered to be small in earlier theoretical considerations, turned out to be dominating at the first several coordination shells. The obtained interactions have been used in calculations of the concentration- and temperature-dependence of the internal friction Snoek peak. The theoretical results are found to be in good agreement with the existing experimental data.
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| 10. |
- Gorbatov, O. I., et al.
(författare)
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Vacancy-solute interactions in ferromagnetic and paramagnetic bcc iron : Ab initio calculations
- 2011
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Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 419:1-3, s. 248-255
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Tidskriftsartikel (refereegranskat)abstract
- Vacancy-solute interactions play a crucial role in diffusion-controlled processes, such as ordering or decomposition, which occur in alloys under heat treatment or under irradiation. Detailed knowledge of these interactions is important for predicting long-term behavior of nuclear materials (such as reactor steels and nuclear-waste containers) as well as for advancing our general understanding of kinetic processes in alloys. Using first-principles calculations based on the density functional theory and employing the locally self-consistent Green's function technique, we develop a database of vacancy-solute interactions in dilute alloys of bcc Fe with 3p (Al, Si, P, S), 3d (Sc-Cu), and 4d (Y-Ag) elements. Unrelaxed interactions within the first three coordination shells have been computed in the ferromagnetic state as well as in the paramagnetic (disordered local moment) state of the iron matrix. Magnetism is found to have a strong effect on the vacancy-solute interactions. Implications of the obtained results for interpreting the effects of vacancy trapping and enhanced impurity diffusion are discussed.
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