| 1. |
- Mattesini, M., et al.
(författare)
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Electronic structure and optical properties of solid C-60
- 2009
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Ingår i: Physica. B, Condensed matter. - : Elsevier BV. - 0921-4526 .- 1873-2135. ; 404:12-13, s. 1776-1780
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure and the optical properties of face-centered-cubic C-60 have been investigated by using an all-electron full-potential method. Our ab initio results show that the imaginary dielectric function for high-energy values looks very similar to that of graphite, revealing close electronic structure similarities between the two systems. We have also identified the origin of different peaks in the dielectric function of fullerene by means of the calculated electronic density of states. The computed optical spectrum compares fairly well with the available experimental data for the Vis-UV absorption spectrum of solid C-60.
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| 2. |
- Hugosson, Håkan Wilhelm, et al.
(författare)
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Restricting dislocation movement in transition metal carbides by phase stability tuning
- 2001
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 293:5539, s. 2434-2437
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Tidskriftsartikel (refereegranskat)abstract
- A mechanism to enhance hardness in multilayer coatings is proposed. Using the technologically important hard transition metal carbides as prototypes, although the principle is transferable also to other systems, we demonstrate, from first-principles calculations, that by suitable alloying the energy difference between several competing structures in the transition meta[ carbides is small or tunable. This creates multiphase/polytypic compounds with a random or controllable layer stacking sequence, systems in which the propagation of dislocations can be strongly suppressed by a large number of interfaces between structures with different glide systems, accordingly allowing the possibility of a greatly enhanced hardness. With modern thin-film technologies, it should therefore be possible to deposit such materials that will express multilayer characteristics with only minor changes in the chemical constitution of the material, which is in contrast to conventional superlattices.
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| 3. |
- Hugosson, Håkan Wilhelm, 1972-, et al.
(författare)
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Theoretical studies of substitutional impurities in molybdenum carbide
- 1999
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 60:22, s. 15123-15130
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Tidskriftsartikel (refereegranskat)abstract
- First-principles full-potential linear muffin-tin orbital calculations have been used to study the effect on the cohesion and electronic structure of cubic δ-MoC when 25% of the carbon is substituted for boron, nitrogen, or oxygen and when 25% of the molybdenum is substituted for niobium, tungsten, or ruthenium. A thorough study of the changes in the electronic structure and the effect of these on the properties of the compounds is made. Special attention is paid to the character (ionic, covalent, or metallic) of the states becoming occupied (or unoccupied) due to the substitution. A study is also made on the properties of the quaternary alloy Mo0.75W0.25C0.75N0.25. This substitution is shown to harden δ-MoC.
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| 4. |
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| 5. |
- 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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| 6. |
- Hugosson, Håkan Wilhelm, 1972-, et al.
(författare)
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Phase stabilities and homogeneity ranges in 4d-transition-metal carbides: A theoretical study
- 2001
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Ingår i: Physical Review B Condensed Matter. - : American Physical Society. - 0163-1829 .- 1095-3795. ; 63:13
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Tidskriftsartikel (refereegranskat)abstract
- First-principles full-potential linear muffin-tin orbital calculations have been used to study the 4d-transition-metal carbides ZrC, NbC, and MoC. The experimental phase diagrams at T=0 of the refractory compounds ZrC, NbC, and MoC have been reproduced with great accuracy from first principles theory. The energy of formation for these compounds has been calculated for several phases and stoichiometries in order to understand the differences in phase stabilities and the changes in homogeneity ranges found between these systems is explained. The results can be regarded as theoretical zero-temperature phase stability diagrams for the three compounds containing not only the experimentally verified but also hypothetical phases and many of the experimental properties and trends are reproduced and explained. A study of the changes and differences in electronic structure and bonding of the studied compounds, phases and stoichiometries is also presented. As a part of this study the hexagonal Me2C (Me being Zr, Nb, or Mo) phases were studied and the theoretical structures, with relaxed interlayer distances and lattice parameters, were obtained. The phase stabilities and electronic structure of the experimentally reported orthorhombic Nb2C and Mo2C phases were also studied.
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| 7. |
- Hugosson, Håkan Wilhelm, 1972-, et al.
(författare)
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Phase stabilities and structural relaxations in substoichiometric TiC1−x
- 2001
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Ingår i: Physical Review B Condensed Matter. - : American Physical Society. - 0163-1829 .- 1095-3795. ; 63:16
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Tidskriftsartikel (refereegranskat)abstract
- First-principles calculations have been used to study the effect of vacancies and relaxation around the vacancy sites in substoichiometric TiC1−x. The effect of relaxation on phase stabilities, equilibrium volumes, and electronic structure of the substoichiometric phases was studied using a combined approach of pseudopotential plane wave and full-potential linear muffin-tin orbital methods. A relaxation away from the vacancies was found for the titanium atoms, the magnitude of which increased with vacancy concentration and the inclusion of nearest-neighbor carbon atom relaxation. The inclusion of local relaxations was found to correctly predict the off-stoichiometric equilibrium composition of titanium carbide. The anomalous volume behavior of TiC at small vacancy concentration is explained as an effect of the local relaxation of the atoms surrounding the vacancy sites, but we do not find that the lattice parameter of any of the studied stoichiometries is larger than that of ideal stoichiometric TiC.
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| 8. |
- Hugosson, Håkan Wilhelm, 1972-, et al.
(författare)
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Theory of phase stabilities and bonding mechanisms in stoichiometric and substoichiometric molybdenum carbide
- 1999
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 86:7, s. 3758-3767
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Tidskriftsartikel (refereegranskat)abstract
- ABSTRACTFirst principles, total energy methods have been applied to predict the relative stabilities of the four experimentally verified MoC phases: the cubic δ(NaCl) phase and the three hexagonal γ(WC), η and γ′(TiAs) phases. The effect of vacancies on the relative stability of these four phases was investigated using a model structure with ordered vacancies within the carbon sublattice. For stoichiometric MoC, the γ phase was found to be the most stable followed by γ′, δ, and η, but for substoichiometric MoC0.75,MoC0.75, the order of relative stability was changed and the substoichiometric δ phase was found to have the lowest energy followed by γ′ and γ. A study of the electronic structure revealed vacancy induced peaks in the density of state and the electron density attached to these peaks was analyzed and found to emanate from unscreened Mo–Mo bonds through the carbon vacancy site. Finally, the oxygen stabilization of the γ′ MoC phase was studied.
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