| 21. |
- Osorio Guillén, Jorge Mario, 1970-
(författare)
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Density Functional Theory in Computational Materials Science
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The present thesis is concerned to the application of first-principles self-consistent total-energy calculations within the density functional theory on different topics in materials science. Crystallographic phase-transitions under high-pressure has been study for TiO2, FeI2, Fe3O4, Ti, the heavy alkali metals Cs and Rb, and C3N4. A new high-pressure polymorph of TiO2 has been discovered, this new polymorph has an orthorhombic OI (Pbca) crystal structure, which is predicted theoretically for the pressure range 50 to 100 GPa. Also, the crystal structures of Cs and Rb metals have been studied under high compressions. Our results confirm the recent high-pressure experimental observations of new complex crystal structures for the Cs-III and Rb-III phases. Thus, it is now certain that the famous isostructural phase transition in Cs is rather a new crystallographic phase transition. The elastic properties of the new superconductor MgB2 and Al-doped MgB2 have been investigated. Values of all independent elastic constants (c11, c12, c13, c33, and c55) as well as bulk moduli in the a and c directions (Ba and Bc respectively) are predicted. Our analysis suggests that the high anisotropy of the calculated elastic moduli is a strong indication that MgB2 should be rather brittle. Al doping decreases the elastic anisotropy of MgB2 in the a and c directions, but, it will not change the brittle behaviour of the material considerably. The three most relevant battery properties, namely average voltage, energy density and specific energy, as well as the electronic structure of the Li/LixMPO4 systems, where M is either Fe, Mn, or Co have been calculated. The mixing between Fe and Mn in these materials is also examined. Our calculated values for these properties are in good agreement with recent experimental values. Further insight is gained from the electronic density of states of these materials, through which conclusions about the physical properties of the various phases are made. The electronic and magnetic properties of the dilute magnetic semiconductor Mn-doped ZnO has been calculated. We have found that for an Mn concentration of 5.6%, the ferromagnetic configuration is energetically stable in comparison to the antiferromgnetic one. A half-metallic electronic structure is calculated by the GGA approximation, where Mn ions are in a divalent state leading to a total magnetic moment of 5 μB per Mn atom.
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| 22. |
- Osorio-Guillen, J. M., et al.
(författare)
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Structural phase transitions in heavy alkali metals under pressure
- 2004
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Ingår i: ChemPhysChem. - : Wiley. - 1439-4235 .- 1439-7641. ; 5:9, s. 1411-1415
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Tidskriftsartikel (refereegranskat)abstract
- We performed a theoretical study of the crystal structures of cesium and rubidium under high compressions. Our results confirm the recent high-pressure experimental observations of new complex crystal structures for the Cs lll and Rb lll phases. The calculated transition pressures agree extremely well with the measured data. Thus, it is now certain that the famous isostructural phase transition in cesium is actually a new crystallographic phase transition. A d-orbital occupation number of about 0.52 is crucial for the occurrence of these complex structures.
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| 23. |
- Schmitt, Thorsten, et al.
(författare)
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Electronic structure studies of V6O13 by soft X-ray emission spectroscopy : band-like and excitonic vanadium states
- 2004
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 69:12, s. 125103-
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Tidskriftsartikel (refereegranskat)abstract
- Resonant soft x-ray emission (SXE) spectra of the mixed valence vanadium oxide V6O13 have been recorded for a series of excitation energies across the V L-absorption band. Resonant excitation allows one to distinguish between charge neutral low-energy excitations and continuum-excited, more band-like V 3d valence band states in the spectra. We find that the V L-emission spectra of V6O13 consist of two distinct components that can be assigned to nearly pure V 3d states, and to V 3dstates that are strongly hybridized with O 2p states, respectively. Band structure calculations of the density functional theory support the assignment of these features. At threshold excitation the V L-emission spectra of V6O13 show strong signatures from excitonic states, the energy dependence of which shows Raman-like behavior. We compare these spectral features in the resonant SXE spectra with cluster model calculations and assign them to dd excitations and charge-transfer excitations, respectively. Finally, we discuss changes in the V L-absorption and emission spectra that take place when changing the sample temperature from 295 K to 120 K. We relate the changes to redistributions in the V 3d partial density of states, which occur at the transition temperature for the metal-semiconductor-transition TMST=150K and find support in our temperature dependent band structure calculations.
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| 24. |
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| 25. |
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| 26. |
- Skorodumova, Natalia V., et al.
(författare)
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Electronic, bonding, and optical properties of CeO2 and Ce2O3 from first principles - art. no. 115108
- 2001
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Ingår i: PHYSICAL REVIEW B. - : AMERICAN PHYSICAL SOC. - 0163-1829. ; 6411:11, s. 5108-+ Language: English
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Tidskriftsartikel (refereegranskat)abstract
- First-principles electronic structure calculations of cerium oxide in two forms, CeO2 and Ce2O3, are Presented. The 4f state of Ce is treated as a part of the inner core in Ce2O3 and as a valence-band-like state in CeO2,. The calculated ground-state and m
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| 27. |
- Souvatzis, Petros, et al.
(författare)
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Elastic properties of Mg(1-x)AlxB2 from first principles theory
- 2004
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 16:29, s. 5241-5250
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Tidskriftsartikel (refereegranskat)abstract
- Elastic properties of Mg(1−x)AlxB2 have been studied from first principles. The elastic constants (c11, c12, c13, c33 and c55) have been calculated, in the regime of x = 0 to 0.25. From these calculations the ratio between the bulk modulus and shear modulus (B/G) as well as the ratio between the two directional bulk moduli (Ba/Bc) have been evaluated. Our calculations show that the ratio Ba/Bc decreases monotonically as the aluminium content is increased, whereas the ratio B/G is well below the empirical ductility limit, 1.75, for all concentrations. In addition, we analyse the electronic structure and the nature of the chemical bonding, using the balanced crystal orbital overlap population (BCOOP) (Grechnev et al 2003 J. Phys.: Condens. Matter 15 7751) and the charge densities. Our analysis suggests that, while aluminium doping decreases the elastic anisotropy of MgB2 in the a and c directions, it will not change the brittle behaviour of the material considerably.
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| 28. |
- Souvatzis, Petros, et al.
(författare)
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First-principles prediction of superplastic transition-metal alloys
- 2004
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 70:1, s. 012201-
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Tidskriftsartikel (refereegranskat)abstract
- Superplastic transition metal alloys and compounds are predicted from first principles calculations. Provided a suitable tuning of the alloying is done, materials with vanishingly low shear modulus C[prime] have recently been identified among the 3d, 4d, and 5d elements if the valence electron average number is close to 4.24 (i.e., Ti-Ta-Nb-V-Zr-O and Ti-Nb-Ta-Zr-O alloys). The vanishingly low C[prime] elastic constant of these bcc alloys is, according to the joint experimental and theoretical studies [T. Saito et al., Science 300, 464 (2003)], the crucial material parameter that is responsible for the superplasticity. We predict here, using first principles calculations, that superplastic alloys should also be found for alloys with drastically different valence electron concentrations, i.e., for W-Re-, W-Tc-, Mo-Re-, Mo-Tc-, and Fe-Co-based alloys.
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| 29. |
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