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Electronic Structure and Lattice Dynamics of Elements and Compounds
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The elastic constants of Mg(1-x)AlxB2 have been calculated in the regime 0<x<0.25. The calculations show that the ratio, B/G, between the bulk- and the shear-modulus stays well below the empirical ductility limit, 1.75, for all concentrations, indicating that the introduction of Al will not change the brittle behaviour of the material considerably. Furthermore, the tetragonal elastic constant C’ has been calculated for the transition metal alloys Fe-Co, Mo-Tc and W-Re, showing that if a suitable tuning of the alloying is made, these materials have a vanishingly low C'. Thermal expansion calculations of the 4d transition metals have also been performed, showing good agreement with experiment with the exception of Nb and Mo. The calculated phonon dispersions of the 4d metals all give reasonable agreement with experiment. First principles calculations of the thermal expansion of hcp Ti have been performed, showing that this element has a negative thermal expansion along the c-axis which is linked to the closeness of the Fermi level to an electronic topological transition. Calculations of the EOS of fcc Au give support to the suggestion that the ruby pressure scale might underestimate pressures with ~10 GPa at pressures ~150 GPa. The high temperature bcc phase of the group IV metals has been calculated with the novel self-consistent ab-initio dynamical (SCAILD) method. The results show good agreement with experiment, and the free energy resolution of < 1 meV suggests that this method might be suitable for calculating free energy differences between different crystallographic phases as a function of temperature.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2007. , p. 127
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 338
Keywords [en]
Atomic and molecular physics, electronic structure, lattice dynamics, first-principles theory, self-consistent lattice dynamical calculation, elasticity, super plasticity, electronic topological transition, equation of state
Keywords [sv]
Atom- och molekylfysik
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-8198ISBN: 978-91-554-6960-3 (print)OAI: oai:DiVA.org:uu-8198DiVA, id: diva2:170665
Public defence
2007-10-05, Siegbahnsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Box 530, Uppsala University, SE-75121, Uppsala, 10:15
Opponent
Supervisors
Available from: 2007-09-13 Created: 2007-09-13 Last updated: 2012-04-01Bibliographically approved
List of papers
1. Elastic properties of Mg(1-x)AlxB2 from first principles theory
Open this publication in new window or tab >>Elastic properties of Mg(1-x)AlxB2 from first principles theory
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2004 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 16, no 29, p. 5241-5250Article in journal (Refereed) Published
Abstract [en]

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. 

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-92691 (URN)10.1088/0953-8984/16/29/015 (DOI)
Available from: 2005-03-10 Created: 2005-03-10 Last updated: 2017-12-14Bibliographically approved
2. First-principles prediction of superplastic transition-metal alloys
Open this publication in new window or tab >>First-principles prediction of superplastic transition-metal alloys
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2004 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 70, no 1, p. 012201-Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-96174 (URN)10.1103/PhysRevB.70.012201 (DOI)000222996300003 ()
Available from: 2007-09-13 Created: 2007-09-13 Last updated: 2017-12-14Bibliographically approved
3. Calculation of the equation of state of fcc Au from first principles
Open this publication in new window or tab >>Calculation of the equation of state of fcc Au from first principles
2006 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 73, no 5, p. 054110-Article in journal (Refereed) Published
Abstract [en]

The electron and phonon contributions to the free energy of gold are calculated from a first-principles method. From the free energy the equation of state (EOS) along with the shock Hugoniot with initial conditions at ambient pressure and room temperature are calculated. The results show good agreement with the corresponding experimental EOS and Hugoniot up to compressions of V/V-0=0.65-i.e., up to the solid-liquid phase transition in the Hugoniot. Optimal agreement between experiment and theory is obtained after a small adjustment of the calculated equilibrium lattice constant to the experimental lattice constant. (the correction is of the order of a few percent). The current calculations are consistent with recent suggestions of an underestimation of the pressures obtained with the ruby pressure scale R-1 line.

Keywords
ELASTIC CONSTANTS, PRESSURE GAUGE, GOLD, CALIBRATION, TEMPERATURE, SYSTEMS, AG, CU
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-96175 (URN)10.1103/PhysRevB.73.054110 (DOI)
Available from: 2007-09-13 Created: 2007-09-13 Last updated: 2017-12-14Bibliographically approved
4. Anomalous Thermal Expansion in α-Titanium
Open this publication in new window or tab >>Anomalous Thermal Expansion in α-Titanium
2007 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 99, no 1, p. 015901-Article in journal (Refereed) Published
Abstract [en]

We provide a complete quantitative explanation for the anisotropic thermal expansion of hcp Ti at low temperature. The observed negative thermal expansion along the c axis is reproduced theoretically by means of a parameter free theory which involves both the electron and phonon contributions to the free energy. The thermal expansion of titanium is calculated and found to be negative along the c axis for temperatures below ∼170 K, in good agreement with observations. We have identified a saddle point van Hove singularity near the Fermi level as the main reason for the anisotropic thermal expansion in α-titanium.

Keywords
Thermal expansion; thermomechanical effects, Phonon states and bands, normal modes, and phonon dispersion, Transition metals and alloys
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-96176 (URN)10.1103/PhysRevLett.99.015901 (DOI)000247819900024 ()
Available from: 2007-09-13 Created: 2007-09-13 Last updated: 2017-12-14Bibliographically approved
5. Entropy driven stabilization of energetically unstable crystal structures explained from first principles theory
Open this publication in new window or tab >>Entropy driven stabilization of energetically unstable crystal structures explained from first principles theory
2008 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 100, no 9, p. 095901-Article in journal (Refereed) Published
Abstract [en]

Conventional methods to calculate the thermodynamics of crystals evaluate the harmonic phonon spectra and therefore do not work in frequent and important situations where the crystal structure is unstable in the harmonic approximation, such as the body-centered cubic (bcc) crystal structure when it appears as a high-temperature phase of many metals. A method for calculating temperature dependent phonon spectra self-consistently from first principles has been developed to address this issue. The method combines concepts from Born's interatomic self-consistent phonon approach with first principles calculations of accurate interatomic forces in a supercell. The method has been tested on the high-temperature bcc phase of Ti, Zr, and Hf, as representative examples, and is found to reproduce the observed high-temperature phonon frequencies with good accuracy.

Keywords
Thermal expansion; thermomechanical effects, Phonon states and bands, normal modes, and phonon dispersion, Transition metals and alloys
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-96177 (URN)10.1103/PhysRevLett.100.095901 (DOI)000253764500047 ()
Available from: 2007-09-13 Created: 2007-09-13 Last updated: 2017-12-14Bibliographically approved
6. Ab-initio calculations of the phonon spectrum and the thermal expansion for the 4d metals
Open this publication in new window or tab >>Ab-initio calculations of the phonon spectrum and the thermal expansion for the 4d metals
(English)In: Phys. Rev. B, p. 1-7Article in journal (Refereed) Submitted
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-96178 (URN)
Available from: 2007-09-13 Created: 2007-09-13 Last updated: 2012-04-01Bibliographically approved

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