| 1. |
- Alling, Björn, et al.
(författare)
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Metastability of fcc-related Si-N phases
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 78:13, s. 132103-132103
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Tidskriftsartikel (refereegranskat)abstract
- The phenomenon of superhardening in TiN/SiNx nanocomposites and the prediction of extreme hardness in bulk gamma-Si3N4 have attracted a large interest to this material system. Attempts to explain the experimental findings by means of first-principles calculations have so far been limited to static calculations. The dynamical stability of suggested structures of the SiNx tissue phase critical for the understanding of the nanocomposites is thus unknown. Here, we present a theoretical study of the phonon-dispersion relations of B1 and B3 SiN. We show that both phases previously considered as metastable are dynamically unstable. Instead, two pseudo-B3 Si3N4 phases derived from a L1(2)- or D0(22)-type distribution of Si vacancies are dynamically stable and might explain recent experimental findings of epitaxial SiNx in TiN/SiNx multilayers.
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| 2. |
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| 3. |
- Belov, M P, et al.
(författare)
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Ab initio lattice dynamics of CoH and NiH
- 2011
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Ingår i: Journal of Alloys and Compounds. - : Elsevier. - 0925-8388 .- 1873-4669. ; 509, s. S857-S859
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Tidskriftsartikel (refereegranskat)abstract
- Ground state parameters and lattice dynamics of CoH and NiH phases are studied by means of the state-of-the-art ab initio pseudopotentials methods. Calculated ground state parameters and phonon DOS are in rather good agreement with available experimental data. No phase transformation for CoH and NiH was found in a wide pressure and temperature range.
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| 4. |
- Belov, M P, et al.
(författare)
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Hydrogen in the approximant i-TiZrHf: The energy state, charge, and diffusion
- 2010
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Ingår i: PHYSICS OF THE SOLID STATE. - : Springer Science Business Media. - 1063-7834. ; 52:6, s. 1111-1114
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Tidskriftsartikel (refereegranskat)abstract
- The energy of hydrogen dissolution in different tetrahedral pores of a 1/1 approximant of the icosahedral TiZrHf quasicrystal has been determined in terms of the density functional theory and ab initio pseudopotentials. At low and high degrees of loading of TiZrHf with hydrogen, the charges of atoms involved in the system and the Mayer bond order have been calculated for all possible M-M and M-H pairs. The diffusion coefficient of a single hydrogen atom in the system under study has been estimated numerically.
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| 5. |
- Bleskov, I. D., et al.
(författare)
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Ab initio calculations of elastic properties of Ru1-xNixAl superalloys
- 2009
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 94:16
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Tidskriftsartikel (refereegranskat)abstract
- Ab initio total energy calculations based on the exact muffin-tin orbitals method, combined with the coherent potential approximation, have been used to study the thermodynamical and elastic properties of substitutional refractory Ru1-xNixAl alloys. We have found that the elastic constants C' and C11 exhibit pronounced peculiarities near the concentration of about 40 at. % Ni, which we ascribe to electronic topological transitions. Our suggestion is supported by the Fermi surface calculations in the whole concentration range. Results of our calculations show that one can design Ru-Ni-Al alloys substituting Ru by Ni (up to 40 at. %) with almost invariable elastic constants and reduced density.
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| 6. |
- Bleskov, I D, et al.
(författare)
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Electronic structure and ground state parameters of Ru1-x Me (x) Al refractory alloys
- 2010
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Ingår i: PHYSICS OF THE SOLID STATE. - : Springer Science Business Media. - 1063-7834. ; 52:9, s. 1803-1809
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure and ground state parameters of B2 RuAl-based refractory alloys have been investigated in the framework of the density functional theory using the exact muffin-tin orbital method in combination with the coherent potential approximation. It has been demonstrated that the number of states at the Fermi level for the Ru1 - x Me (x) Al alloys as a function of the alloying metal content has a minimum, which indicates a change in the Fermi surface topology and the presence of specific features in the behavior of elastic constants. It has been concluded that the electronic structure of the alloys can be described in terms of the rigid band model. The nonlinear variations of the lattice parameters of the alloys has been explained.
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| 7. |
- Bleskov, Ivan, et al.
(författare)
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Elastic constants of Ru100-xMexAl refractory alloys
- 2011
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Ingår i: SOLID STATE COMMUNICATIONS. - : Elsevier Science B.V., Amsterdam.. - 0038-1098. ; 151:3, s. 242-244
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Tidskriftsartikel (refereegranskat)abstract
- Elastic constants of disordered refractory Ru100-xMexAl alloys are calculated from first principles using a density-functional-theory based exact muffin-tin orbitals method combined with a coherent potential approximation. As alloying elements we considered Me = Ni, Co, Ir and Fe. We have found that peculiarities in the concentration dependencies of elastic constants on alloying metals content depend on their position in the Periodic Table with respect to Ru. These peculiarities can be connected with the changes in the electronic structure which take place upon alloying.
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| 8. |
- Isaev, Eyvas, et al.
(författare)
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Ab initio Calculations of Elastic Constants of Superalloys
- 2011
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Ingår i: JAPANESE JOURNAL OF APPLIED PHYSICS. - : Japan Society of Applied Physics / Japanese Journal of Applied Physics; 1999. - 0021-4922. ; 50:5
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Tidskriftsartikel (refereegranskat)abstract
- Using the-state-of-the-art ab initio method we have studied elastic constants of alloys potentially interesting for high temperature applications. We have shown that Cr substitutes the Al sublattice in B2 NiAl at concentration up to 40 at. %, but at higher Cr content it prefers the Ni-sublattice. Alloying of NiAl with Cr yields reduced strength but improves the ductility of the alloys. Alloying of NiAl with W which substitutes the Al sublattice, leads to a strong decrease of the shear modulus, and near 50 at.% of W the alloy becomes mechanically unstable as elastic constant C is negative. This is in agreement with our phonon calculations where we found soft modes along the [110] direction for B2 NiW. According to our calculations in (Ru,Ni)Al alloys the shear modulus is almost constant up to 40 at.% of Ni, at higher Ni concentrations it is drastically reduced. We have shown that the changes in elastic properties of (Ru-X)Al alloys are due to electronic topological transitions.
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| 9. |
- Isaev, Eyvas, et al.
(författare)
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Impact of lattice vibrations on equation of state of the hardest boron phase
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 83:13, s. 132106-
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Tidskriftsartikel (refereegranskat)abstract
- An accurate equation of state (EOS) is determined for the high-pressure orthorhombic phase of boron, B(28), experimentally as well as from ab initio calculations. The unique feature of our experiment is that it is carried out on the single crystal of B(28). In theory, we take into consideration the lattice vibrations, often neglected in first-principles simulations. We show that the phonon contribution has a profound effect on the EOS of B(28), giving rise to anomalously low values of the pressure derivative of the bulk modulus and greatly improving the agreement between theory and experiment.
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| 10. |
- Isaeva, Leyla, et al.
(författare)
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Ab initio Phonons in Magnetic Ni2MnAl
- 2011
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Ingår i: Japanese Journal of Applied Physics. - : Japan Society of Applied Physics / Japanese Journal of Applied Physics; 1999. - 0021-4922 .- 1347-4065. ; 50:5, s. 05FE07-
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Tidskriftsartikel (refereegranskat)abstract
- In the present work we report the results of ab initio studies of electronic and dynamic properties of nickel based Heusler alloy Ni2MnAl. The total magnetic moment and elastic constants were also evaluated and compared to experimental results where possible. We found that the phonon dispersion relations calculated in this work within the linear response method did not reveal any softening of the transversal acoustic mode TA(2) along [xi, xi, 0] direction in accordance with the experiment and in controversy to previous theoretical studies.
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| 11. |
- Isaeva, L E, et al.
(författare)
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Dynamic stability of palladium hydride: An ab initio study
- 2011
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Ingår i: INTERNATIONAL JOURNAL OF HYDROGEN ENERGY. - : Elsevier Science B.V., Amsterdam.. - 0360-3199. ; 36:1, s. 1254-1258
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Tidskriftsartikel (refereegranskat)abstract
- We present results of our ab initio studies of electronic and dynamic properties of ideal palladium hydride PdH and its vacancy ordered defect phase Pd(3)VacH(4) ("Vac" - vacancy on palladium site) with L1(2) crystal structure found experimentally and studied theoretically. Quantum and thermodynamic properties of these hydrides, such as phonon dispersion relations and the vacancy formation enthalpies have been studied. Dynamic stability of the defect phase Pd(3)VacH(4) with respect to different site occupation of hydrogen atoms at the equilibrium state and under pressure was analyzed. It was shown that positions of hydrogen atoms in the defect phase strongly affect its stability and may be a reason for further phase transitions in the defect phase.
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| 12. |
- Korzhavyi, Pavel A., 1966-, et al.
(författare)
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Exploring monovalent copper compounds with oxygen and hydrogen
- 2012
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 109:3, s. 686-689
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Tidskriftsartikel (refereegranskat)abstract
- New important applications of copper metal, e.g., in the areas of hydrogen production, fuel cell operation, and spent nuclear fuel disposal, require accurate knowledge of the physical and chemical properties of stable and metastable copper compounds. Among the copper(I) compounds with oxygen and hydrogen, cuprous oxide Cu(2)O is the only one stable and the best studied. Other such compounds are less known (CuH) or totally unknown (CuOH) due to their instability relative to the oxide. Here we combine quantum-mechanical calculations with experimental studies to search for possible compounds of monovalent copper. Cuprous hydride (CuH) and cuprous hydroxide (CuOH) are proved to exist in solid form. We establish the chemical and physical properties of these compounds, thereby filling the existing gaps in our understanding of hydrogen- and oxygen-related phenomena in Cu metal.
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| 13. |
- Krasilnikov, O M, et al.
(författare)
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Elastic phase transitions in metals at high pressures
- 2012
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Ingår i: Journal of Physics. - : Institute of Physics. - 0953-8984 .- 1361-648X. ; 24:19, s. 195402-
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Tidskriftsartikel (refereegranskat)abstract
- The elastic phase transitions of cubic metals at high pressures are investigated within the framework of Landau theory. It is shown that at pressures comparable with the magnitude of the bulk modulus the phase transition is connected with the loss of stability relative to uniform deformation of the crystalline lattice. Discontinuity of the order parameter at the transition point and its equilibrium value are expressed through the second-to fourth-order elastic constants. The second-, third- and fourth-order elastic constants and phonon dispersion curves of vanadium under hydrostatic pressure are obtained by first-principles calculations. Structural transformation in vanadium under pressure is studied using the obtained results. It is shown that the experimentally observed at P approximate to 69 GPa phase transition in vanadium is the first-order phase transition close to a second-order phase transition.
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| 14. |
- Loa, I, et al.
(författare)
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Lattice Dynamics and Superconductivity in Cerium at High Pressure
- 2012
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 108:4, s. 045502-
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Tidskriftsartikel (refereegranskat)abstract
- We have measured phonon dispersion relations of the high-pressure phase cerium-oC(4) (alpha phase with the alpha-uranium crystal structure) at 6.5 GPa by using inelastic x-ray scattering. Pronounced phonon anomalies are observed, which are remarkably similar to those of alpha-U. First-principles electronic structure calculations reproduce the anomalies and allow us to identify strong electron-phonon coupling as their origin. At the low-pressure end of its stability range, Ce-oC4 is on the verge of a lattice-dynamical instability and possibly a charge density wave. The superconducting transition temperatures of the fcc, oC4, and mC4 phases of Ce have been calculated, and the superconductivity observed experimentally by Wittig and Probst is attributed to the oC4 phase.
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| 15. |
- Marten, Tobias, et al.
(författare)
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First-principles study of the SiNx/TiN(001) interface
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 85:10, s. 104106-
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Tidskriftsartikel (refereegranskat)abstract
- The structure of the SiNx tissue phase in superhard TiN/SiNx nanocomposites has been debated in the literature. We present a theoretical investigation of the possibility of crystalline and coherent ( 001) interfaces that satisfies the two necessary criteria, stability with respect to lattice vibrations as well as to variations in stoichiometry. It is found that one monolayer of Si tetrahedrally coordinated by N in a B3-like geometry embedded between B1-TiN( 001) surfaces is both dynamically stable and thermodynamically stable with respect to vacancy formation. However, with increasing layer thickness the B3-type structure becomes unstable with respect to Si vacancy formation. Instead we suggest that a tetragonal D0(22)-like order of Si vacancies can stabilize the interface. These structures are in line with the experimental findings of the crystalline tissue phase which has coherent interfaces with TiN.
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| 16. |
- Marten, Tobias, et al.
(författare)
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Single-monolayer SiNx embedded in TiN : A first-principles study
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 81:21, s. 212102-
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Tidskriftsartikel (refereegranskat)abstract
- The dynamical and thermodynamic stability of a single monolayer of SiNx sandwiched isostructurally between B1-TiN(001) and (111) oriented slabs are investigated by means of density functional theory. Possible dynamical stabilization of the (001) interface, by distortion of the Si-N bonds, is considered and found to almost, but not completely, remove the phonon instabilities. The (111) interface on the other hand is found to be dynamically stable. We furthermore relax the stoichiometry degree of freedom by allowing for Si vacancies in the lattice and show that the ideal 1:1 SiN stoichiometry in both interfaces are thermodynamically unstable with respect to Si vacancy formation regardless if the system is grown under nitrogen-rich or nitrogen-poor conditions, and therefore ruling out its relevance for performance of real materials.
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| 17. |
- Ponomareva, A V, et al.
(författare)
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Site preference and effect of alloying on elastic properties of ternary B2 NiAl-based alloys
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 85:14, s. 144117-
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Tidskriftsartikel (refereegranskat)abstract
- Using the exact muffin-tin orbitals method in conjunction with the coherent potential approximation, we study the site preference of transition metal impurities X (X = Sc, Ti, V, Cr, W, Re, Co) in B2 NiAl and their effect on its elastic properties. Analyzing interatomic bonding of NiAl-X alloys and elastic characteristics evaluated from the elastic constants C-11, C-12, and C-44, we predict that the addition of W, V, Ti, and Re atoms could yield improved ductility for B2 NiAl-X alloys without significant changes in the macroscopic elastic moduli.
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| 18. |
- Razumovskiy, Vsevolod I., et al.
(författare)
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Ab initio calculations of elastic properties of Pt-Sc alloys
- 2008
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Ingår i: Intermetallics (Barking). - : Elsevier. - 0966-9795 .- 1879-0216. ; 16:8, s. 982-986
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Tidskriftsartikel (refereegranskat)abstract
- Elastic properties of substitutionally disordered Pt-rich Pt-Sc alloys and L1(2)-ordered Pt3Sc compound are derived from the first-principles calculations based on the exact muffin-tin orbitals (EMTO) method. We demonstrate that these alloys should exhibit a ductile behavior, which combined with relatively high melting temperature and strong cohesive properties make them a very promising candidate for high-temperature applications.
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| 19. |
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| 20. |
- Stishov, Sergei M., et al.
(författare)
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Lost Heat Capacity and Entropy in the Helical Magnet MnSi
- 2010
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 105:23, s. 236403-
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Tidskriftsartikel (refereegranskat)abstract
- The heat capacity of MnSi at B = 0 and B = 4 T was measured in the temperature range 2.5-100 K. To analyze the data, calculations of the phonon spectrum and phonon density of states in MnSi were performed. The calculated phonon frequencies were confirmed by means of inelastic neutron scattering. The analysis of the data suggests the existence of negative contributions to the heat capacity and entropy of MnSi at T > T-c that may imply a specific ordering in the spin subsystem in the paramagnetic phase of MnSi.
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| 21. |
- Zarechnaya, E. Y., et al.
(författare)
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Theoretical study of linear monoatomic nanowires, dimer and bulk of Cu, Ag, Au, Ni, Pd and Pt
- 2008
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Ingår i: Computational materials science. - : Elsevier BV. - 0927-0256 .- 1879-0801. ; 43:3, s. 522-530
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Tidskriftsartikel (refereegranskat)abstract
- The binding and electronic properties of monoatomic nanowires, dimers and bulk structures of Cu, Ag, Au and Ni, Pd, Pt have been studied by the projector augmented-wave method (PAW) within the density functional theory (DFT) using the local density approximation (LDA) as well as generalized gradient approximation (GGA) in both Perdew-Wang (PW91) and Perdew-Burke-Ernzerhof (PBE) parameterizations. Our results show that the formation of atomic chains is not equally plausible for all the studied elements. In agreement with experimental observations Pt and An stand out as most likely elements to form monoatomic wires. Changes in the electronic structure and magnetic properties of metal chains at stretching are analyzed.
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