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Elastic properties of Mg(1-x)AlxB2 from first principles theory
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
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2004 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 16, no 29, 5241-5250 p.Article 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. 

Place, publisher, year, edition, pages
2004. Vol. 16, no 29, 5241-5250 p.
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-92691DOI: 10.1088/0953-8984/16/29/015OAI: oai:DiVA.org:uu-92691DiVA: diva2:165862
Available from: 2005-03-10 Created: 2005-03-10 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Theoretical Studies of Two-Dimensional Magnetism and Chemical Bonding
Open this publication in new window or tab >>Theoretical Studies of Two-Dimensional Magnetism and Chemical Bonding
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is divided into two parts. In the first part we study thermodynamics of the two-dimensional Heisenberg ferromagnet with dipolar interaction. This interaction breaks the conditions of the Mermin-Wagner theorem, resulting in a finite transition temperature. Our calculations are done within the framework of the self-consistent spin-wave theory (SSWT), which is modified in order to include the dipolar interaction. Both quantum and classical versions of the Heisenberg model are considered.

The second part of the thesis investigates the chemical bonding in solids from the first principles calculations. A new chemical bonding indicator called balanced crystal orbital overlap population (BCOOP) is developed. BCOOP is less basis set dependent than the earlier indicators and it can be used with full-potential density-functional theory (DFT) codes. We apply BCOOP formalism to the chemical bonding in the high-T_c superconductor MgB2 and the theoretically predicted MAX phase Nb3SiC2. We also study how the chemical bonding results in a repulsive hydrogen–hydrogen interaction in metal hydrides. The role of this interaction in the structural phase transition in Ti3SnHx is investigated.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2005. vii + 87 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 21
Keyword
Physics, spin Hamiltonians, quantized spin models, Heisenberg model, spin waves, self-consistent spin-wave theory, dipolar interaction, density functional theory, chemical bonding, overlap population, MAX phases, metal hydrides, Fysik
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-4815 (URN)91-554-6164-6 (ISBN)
Public defence
2005-04-01, Siegbahnsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 10:15
Opponent
Supervisors
Available from: 2005-03-10 Created: 2005-03-10Bibliographically approved
2. Density Functional Theory in Computational Materials Science
Open this publication in new window or tab >>Density Functional Theory in Computational Materials Science
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2004. 49 p.
Series
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 1001
Keyword
Physics, Density Functional Theory, High Pressure, Phase Transitions, Elastic Properties, Lithium Batteries, Dilute Magnetic Semiconductors, Fysik
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-4496 (URN)91-554-6016-X (ISBN)
Public defence
2004-09-24, Polhemsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 10:15
Opponent
Supervisors
Available from: 2004-09-01 Created: 2004-09-01 Last updated: 2012-03-28Bibliographically approved
3. Electronic Structure and Lattice Dynamics of Elements and Compounds
Open this publication in new window or tab >>Electronic Structure and Lattice Dynamics of Elements and Compounds
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. 127 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 338
Keyword
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, Atom- och molekylfysik
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-8198 (URN)978-91-554-6960-3 (ISBN)
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

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Souvatzis, PetrosAhuja, RajeevEriksson, Olle

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