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Entropy driven stabilization of energetically unstable crystal structures explained 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.
2008 (English)In: Physical Review Letters, ISSN 0031-9007, Vol. 100, no 9, 095901- p.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.

Place, publisher, year, edition, pages
2008. Vol. 100, no 9, 095901- p.
Keyword [en]
Thermal expansion; thermomechanical effects, Phonon states and bands, normal modes, and phonon dispersion, Transition metals and alloys
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-96177DOI: 10.1103/PhysRevLett.100.095901ISI: 000253764500047OAI: oai:DiVA.org:uu-96177DiVA: diva2:170663
Available from: 2007-09-13 Created: 2007-09-13 Last updated: 2012-04-01Bibliographically approved
In thesis
1. 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, PetrosEriksson, Olle

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