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Nonequilibrium ab initio molecular dynamics determination of Ti monovacancy migration rates in B1 TiN
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Ruhr University of Bochum, Germany.ORCID iD: 0000-0002-1379-6656
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Max Planck Institute Eisenforsch GmbH, Germany.
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. National University of Science and Technology MISIS, Russia.
2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 10, article id 104306Article in journal (Refereed) Published
Abstract [en]

We use the color diffusion (CD) algorithm in nonequilibrium (accelerated) ab initio molecular dynamics simulations to determine Ti monovacancy jump frequencies in NaCl-structure titanium nitride (TiN), at temperatures ranging from 2200 to 3000 K. Our results showthat theCDmethod extended beyond the linear-fitting rate-versus-force regime [Sangiovanni et al., Phys. Rev. B 93, 094305 (2016)] can efficiently determine metal vacancy migration rates in TiN, despite the low mobilities of lattice defects in this type of ceramic compound. We propose a computational method based on gamma-distribution statistics, which provides unambiguous definition of nonequilibrium and equilibrium (extrapolated) vacancy jump rates with corresponding statistical uncertainties. The acceleration-factor achieved in our implementation of nonequilibrium molecular dynamics increases dramatically for decreasing temperatures from 500 for T close to the melting point T-m, up to 33 000 for T approximate to 0.7 T-m

Place, publisher, year, edition, pages
AMER PHYSICAL SOC , 2017. Vol. 96, no 10, article id 104306
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:liu:diva-141712DOI: 10.1103/PhysRevB.96.104306ISI: 000411076000005OAI: oai:DiVA.org:liu-141712DiVA, id: diva2:1147321
Note

Funding Agencies|Swedish Foundation for Strategic Research (SSF) project SRL [10-0026]; Swedish Research Council (VR) [621-2011-4417, 2015-04391, 330-2014-6336]; Swedish Government Strategic Research Area Grant in Materials Science on Advanced Functional Materials [MatLiU 2009-00971]; Ministry of Education and Science of the Russian Federation [14.Y26.31.0005]; Marie Sklodowska Curie Actions [INCA 600398]; Swedish Foundation for Strategic Research; Stiftelsen Olle Engkvist Byggmastare

Available from: 2017-10-05 Created: 2017-10-05 Last updated: 2019-06-28
In thesis
1. Structural and magnetic disorder in crystalline materials: a first principles study
Open this publication in new window or tab >>Structural and magnetic disorder in crystalline materials: a first principles study
2019 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Disorder in crystalline materials can take different forms and originate from different sources. In particular, temperature introduces disorder in any kind of material. This can be observed as the appearance of vacant lattice sites in an otherwise perfect crystal, or as a random distribution of different elements on the same lattice in an alloy; at the same time, if the material is magnetic, temperature induces disorder also on the magnetic degrees of freedom. In this thesis, different levels of disorder associated to structure and magnetism are investigated by means of density functional theory and thermodynamic models.

I start with diffusion of Ti vacancies in TiN, which is studied by means of nonequilibrium ab initio molecular dynamics using the color diffusion algorithm at different temperatures. The result is an Arrhenius behavior of Ti vacancy jump rates.

A method to perform structural relaxations in magnetic materials in their hightemperature paramagnetic phase is then developed based on the disordered local moments approach in order to study vacancies, interstitial atoms, and combinations of defects in paramagnetic bcc Fe and B1 CrN, as well as the mixing enthalpy of bcc Fe1−xCrx random alloys. A correction to the energetics of every system due to the relaxation in the disordered magnetic state is observed in all cases.

Not related to temperature and disorder, but very important for an accurate description of magnetic materials, is the choice of the exchange and correlation functional to be employed in the first principles calculations. We have investigated the performance of a recently developed meta-GGA functional, the strongly constrained and appropriately normed (SCAN) functional, in comparison with the more commonly used LDA and PBE on the ferromagnetic elemental solids bcc Fe, fcc Ni, and hcp Co, and SCAN it is found to give negligible improvements, if not a worsening, in the description of these materials.

Finally, the coupling between vibrational and magnetic degrees of freedom is discussed by reviewing the literature and proposing an investigation of the influence of vibrations on longitudinal spin fluctuations. These excitations are here studied by means of thermodynamic models based on Landau expansion of the energy in even powers of the magnitude of the local magnetic moments. We find that vibrational and magnetic disorder alter the energy landscapes as a function of moment size also in bcc Fe, which is often considered a Heisenberg system, inducing a more itinerant electron behavior.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2019. p. 70
Series
Linköping Studies in Science and Technology. Licentiate Thesis, ISSN 0280-7971 ; 1837
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-156835 (URN)10.3384/lic.diva-156835 (DOI)9789176850817 (ISBN)
Presentation
2019-05-23, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
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Available from: 2019-05-14 Created: 2019-05-14 Last updated: 2019-05-14Bibliographically approved

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