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Influence of stresses and impurities on thermodynamic and elastic properties of metals and alloys from ab initio theory
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Stresses and impurities may influence elastic properties, phase stability and magnetic behavior of metals and their alloys. A physical understanding of this influence is of great importance to both fundamental science and technological applications. The diverse methods used in this work allowed us to shed light on the various aspects of the problem. In particular, in this work the thermodynamic, magnetic and elastic properties of Fe and Fe-Ni alloys at Earth’s inner core conditions were investigated by means of the ab initio theory. The main features of these calculations are on one side the extreme pressure-temperature conditions; on the other side the strong-correlation effects, which at these conditions may play an unexpected role. That is why I used different approaches, ranging from molecular dynamics to the dynamical mean field theory.

Interesting possibility for the effect of non-hydrostatic stresses on the stability of the body-centered cubic (bcc) phase of iron was observed. If detected, it could allow for an explanation of striking contradictions in high-pressure experiments. The influence of the alloying with Ni on the stability of Fe was studied. It was shown that the observed reverse of the stability trend under pressure is associated with the suppression of ferromagnetism at conditions of Earth’s inner core.

The strong correlation effects were observed in Fe3Ni by means of the dynamical mean field theory, revealing that the local environment of iron atoms is crucial for the strength of the on-site electronic correlations.

There is also an exciting experimental finding of our colleagues indicating that magnetism in pure nickel survives at very high pressures up to 260 GPa, i.e. up to the highest pressure at which magnetism in any material has ever been observed. Our calculations of the pressure dependence of the effective exchange interaction parameter and the hyperfine field support the picture of the ordered ferromagnetic state in Ni at multimegabar pressures.

Further, hydrogen is believed to be an important light impurity in Earth’s core. Thereupon the hydrogen containing FeOOH was also investigated. The prediction of the effect of symmetrization of the hydrogen bond under pressure was made.

The universality of applied methods allowed us to study the elastic constants of TiN, which is of high relevance to the industry of cutting tools. The importance of taking into account the finite temperature effects in the calculations of the elastic properties was demonstrated. Another case of practical interest is the Fe-Cr system, a prototype of many industrial steels. For instance, it is used in cooling pipes of pressure vessel reactors. We studied the effect of hydrostatic pressure on the phase stability of Fe-Cr alloys and revealed intriguing differences in the ordering tendencies depending on the Cr concentration and magnetic state of the alloy. We showed how variation of the ordering tendency between the Fe and Cr atoms emerges due to suppression of the local magnetic moment on the Cr atoms.

Noteworthy, hydrogen is not only the basic material playing fundamental role on and in the Earth, it is also a very promising source of fuel, which does not pollute the environment. In this sense the problem of hydrogen storage in Pd is of separate but related interest and it was theoretically investigated in the present work. The effect of vacancies on the energetically preferable position of hydrogen in the Pd cell was addressed. My theoretical results supported the experimental suggestion of multiple occupation of Pd vacancies by hydrogen.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2013. , 85 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1531
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:liu:diva-97383ISBN: 9789175195575 (print)OAI: oai:DiVA.org:liu-97383DiVA: diva2:647381
Public defence
2013-09-19, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2013-09-11 Created: 2013-09-11 Last updated: 2017-12-14Bibliographically approved
List of papers
1. Influence of Ni on the lattice stability of Fe-Ni alloys at multimegabar pressures
Open this publication in new window or tab >>Influence of Ni on the lattice stability of Fe-Ni alloys at multimegabar pressures
2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 22, 224107- p.Article in journal (Refereed) Published
Abstract [en]

The lattice stability trends of the primary candidate for Earths core material, the Fe-Ni alloy, were examined from first principles. We employed the exact muffin-tin orbital method (EMTO) combined with the coherent potential approximation (CPA) for the treatment of alloying effects. It was revealed that high pressure reverses the trend in the relative stabilities of the body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close-packed (hcp) phases observed at ambient conditions. In the low pressure region the increase of Ni concentration in the Fe-Ni alloy enhances the bcc phase destabilization relative to the more close-packed fcc and hcp phases. However, at 300 GPa (Earths core pressure), the effect of Ni addition is opposite. The reverse of the trend is associated with the suppression of the ferromagnetism of Fe when going from ambient pressures to pressure conditions corresponding to those of Earths core. The first-principles results are explained in the framework of the canonical band model.

Place, publisher, year, edition, pages
American Physical Society, 2012
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-86890 (URN)10.1103/PhysRevB.86.224107 (DOI)000312064300001 ()
Note

Funding Agencies|Swedish Research Council (VR)|621-2008-55352011-42-59|Goran Gustafsson Foundation for Research in Natural Sciences and Medicine||Swedish Government Strategic Research Area Grant in Materials Science||Functional Materials and VR Linnaeus Grant LiLi-NFM||Swedish Foundation for Strategic Research through the SRL program||Ministry of Education and Science of the Russian Federation|14.B37.21.089014.A18.21.0893|Russian Foundation for Basic Researches|10-02-00-194a|Swedish National Infrastructure for Computing SNIC||

Available from: 2013-01-07 Created: 2013-01-07 Last updated: 2017-12-06
2. Effect of non-uniform stresses on bcc Fe and Fe-Ni alloys at Earth's core conditions
Open this publication in new window or tab >>Effect of non-uniform stresses on bcc Fe and Fe-Ni alloys at Earth's core conditions
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Elucidation of Earth's core composition and structure is needed for the understanding of various phenomena, from geodynamics to geomagnetism. Geochemical data suggest that solid Fe alloyed with Ni is Earth's core main constituent, however, the exact content and crystal structure remain unclear. We report the results of ab initio molecular dynamics study of the mechanical stability of the body-centered cubic (bcc) phase of Fe and Fe0.89Ni0.ll. A striking saw-toothed effect in the dependence of stresses on finite distortions is revealed. The calculated region of mechanical stability of the bcc iron under non- hydrostatic stress at Earth's core conditions is narrow, and about 5 GPa stresses should destabilize it. While it is unlikely that stresses in Earth's core are sufficient to mechanically destabilize bcc iron, experimental controversies and puzzling stabilization of diverse phases may be well explained by the observed saw-toothed stress effect. It might be common for different materials close to dynamical instability and has crucial importance for determination of equilibrium phase boundaries.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-97378 (URN)
Available from: 2013-09-11 Created: 2013-09-11 Last updated: 2013-09-11Bibliographically approved
3. Electronic correlations in Fe at Earth's inner core conditions: effect of alloying with Ni
Open this publication in new window or tab >>Electronic correlations in Fe at Earth's inner core conditions: effect of alloying with Ni
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We have studied the body-centered cubic (bcc), face-centered cubic (fcc) and hexagonal close-packed (hcp) phases of Fe alloyed with 25 at. % of Ni at Earth's core conditions using an ab initio local density approximation + dynamical mean-field theory (LDA+DMFT) approach. The alloys have been modeled by ordered crystal structures based on the bcc, fcc, and hcp unit cells with minimum possible cell size allowing for the proper composition. 0ur calculations demonstrate that the strength of electronic correlations on the Fe 3d shell is highly sensitive to the phase and local environment. In the bcc phase the 3d electrons at the Fe site with Fe only nearest neighbors remain rather strongly correlated even at extreme pressure-temperature conditions, with the local and uniform magnetic  susceptibility exhibiting a Curie-Weiss-like temperature evolution and the quasi-particle lifetime r featuring a non-Fermi-liquid temperature dependence. In contrast, for the corresponding Fe site in the hcp phase we predict a weakly-correlated Fermi-liquid state with a temperature- independent local susceptibility and a quadratic temperature dependence of r. The iron sites with nickel atoms in the local environment exhibit behavior in the range between those two extreme cases, with the strength of correlations gradually increasing along the hcp-fcc- bcc sequence. Further, the inter-site magnetic interactions in the bcc and hcp phases are also strongly affected by the presence of Ni nearest neighbors. The sensitivity to the local environment is related to modifications of the Fe partial density of states due to mixing with Ni 3d-states.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-97380 (URN)
Available from: 2013-09-11 Created: 2013-09-11 Last updated: 2013-09-11Bibliographically approved
4. Hyperfine Splitting and Room-Temperature Ferromagnetism of Ni at Multimegabar Pressure
Open this publication in new window or tab >>Hyperfine Splitting and Room-Temperature Ferromagnetism of Ni at Multimegabar Pressure
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2013 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 111, no 15Article in journal (Refereed) Published
Abstract [en]

Magnetic and elastic properties of Ni metal have been studied up to 260 GPa by nuclear forward scattering of synchrotron radiation with the 67.4 keV Mossbauer transition of Ni-61. The observed magnetic hyperfine splitting confirms the ferromagnetic state of Ni up to 260 GPa, the highest pressure where magnetism in any material has been observed so far. Ab initio calculations reveal that the pressure evolution of the hyperfine field, which features a maximum in the range of 100 to 225 GPa, is a relativistic effect. The Debye energy obtained from the Lamb-Mossbauer factor increases from 33 me V at ambient pressure to 60 me V at 100 GPa. The change of this energy over volume compression is well described by a Gruneisen parameter of 2.09.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2013
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-97381 (URN)10.1103/PhysRevLett.111.157601 (DOI)000326051100011 ()
Available from: 2013-09-11 Created: 2013-09-11 Last updated: 2017-12-06Bibliographically approved
5. Pressure-induced hydrogen bond symmetrization in iron oxyhydroxide
Open this publication in new window or tab >>Pressure-induced hydrogen bond symmetrization in iron oxyhydroxide
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2013 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 111, no 17Article in journal (Refereed) Published
Abstract [en]

Under high pressures the hydrogen bonds were predicted to transform from a highly asymmetric soft O-H center dot center dot center dot O to a symmetric rigid configuration in which the proton lies midway between the two oxygen atoms. Despite four decades of research on hydroxyl containing compounds, pressure induced hydrogen bond symmetrization remains elusive. Following single crystal x-ray diffraction, Mossbauer and Raman spectroscopy measurements supported by ab initio calculations, we report the H-bonds symmetrization in iron oxyhydroxide, FeOOH, resulting from the Fe3+ high-to-low spin crossover at above 45 GPa.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-97382 (URN)10.1103/PhysRevLett.111.175501 (DOI)000326148500007 ()
Available from: 2013-09-11 Created: 2013-09-11 Last updated: 2017-12-06Bibliographically approved
6. First-principles study of vacancy-hydrogen interaction in Pd
Open this publication in new window or tab >>First-principles study of vacancy-hydrogen interaction in Pd
2009 (English)In: PHYSICAL REVIEW B, ISSN 1098-0121, Vol. 80, no 2, 024101- p.Article in journal (Refereed) Published
Abstract [en]

Hydrogen absorption in face-centered-cubic palladium is studied from first principles, with particular focus on interaction between hydrogen atoms and vacancies, formation of hydrogen-vacancy complexes, and multiple hydrogen occupancy of a Pd vacancy. Vacancy formation energy in the presence of hydrogen, hydrogen trapping energy, and vacancy formation volume have been calculated and compared to existing experimental data. We show that a vacancy and hydrogen atoms form stable complexes. Further we have studied the process of hydrogen diffusion into the Pd vacancy. We find the energetically preferable position for hydrogen to reside in the palladium unit cell in the presence of a vacancy. The possibility of the multiple hydrogen occupancy (up to six hydrogen atoms) of a monovacancy is elucidated. This theoretical finding supports experimental indication of the appearance of superabundant vacancy complexes in palladium in the presence of hydrogen.

Keyword
density functional theory, diffusion, hydrogen, palladium, vacancies (crystal)
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-19890 (URN)10.1103/PhysRevB.80.024101 (DOI)
Note
Original Publication: Olga Vekilova, D I Bazhanov, Sergey Simak and Igor Abrikosov, First-principles study of vacancy-hydrogen interaction in Pd, 2009, PHYSICAL REVIEW B, (80), 2, 024101. http://dx.doi.org/10.1103/PhysRevB.80.024101 Copyright: American Physical Society http://www.aps.org/ Available from: 2009-08-28 Created: 2009-08-14 Last updated: 2013-09-11Bibliographically approved
7. Temperature dependence of TiN elastic constants from ab initio molecular dynamics simulations
Open this publication in new window or tab >>Temperature dependence of TiN elastic constants from ab initio molecular dynamics simulations
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2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 9Article in journal (Refereed) Published
Abstract [en]

Elastic properties of cubic TiN are studied theoretically in a wide temperature interval. First-principles simulations are based on ab initio molecular dynamics (AIMD). Computational efficiency of the method is greatly enhanced by a careful preparation of the initial state of the simulation cell that minimizes or completely removes a need for equilibration and therefore allows for parallel AIMD calculations. Elastic constants C11, C12, and C44 are calculated. A strong dependence on the temperature is predicted, with C11 decreasing by more than 29% at 1800 K as compared to its value obtained at T=0 K. Furthermore, we analyze the effect of temperature on the elastic properties of polycrystalline TiN in terms of the bulk and shear moduli, the Young's modulus and Poisson ratio. We construct sound velocity anisotropy maps, investigate the temperature dependence of elastic anisotropy of TiN, and observe that the material becomes substantially more isotropic at high temperatures. Our results unambiguously demonstrate the importance of taking into account finite temperature effects in theoretical calculations of elastic properties of materials intended for high-temperature applications.

Place, publisher, year, edition, pages
American Physical Society, 2013
National Category
Natural Sciences Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-78754 (URN)10.1103/PhysRevB.87.094114 (DOI)000316791600001 ()
Note

Funding Agencies|Swedish Research Council|621-2008-5535621-2011-4426|Swedish Foundation for Strategic Research (SSF) programs|10-0026|project Designed Multicomponent Coatings (MultiFilms)||Erasmus Mundus doctoral program DocMase||Ministry of Education and Science of the Russian Federation within the framework of Program Research and Pedagogical Personnel for Innovative Russia|14.B37.21.089010.09.2012|

Available from: 2012-06-20 Created: 2012-06-20 Last updated: 2017-12-07Bibliographically approved
8. Effect of pressure on phase stability in Fe-Cr alloys
Open this publication in new window or tab >>Effect of pressure on phase stability in Fe-Cr alloys
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2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 9, 094422- p.Article in journal (Refereed) Published
Abstract [en]

The effect of hydrostatic pressure on the phase stability of Fe-Cr alloys has been studied using ab initio methods. We show that while pressure decreases the tendency toward the phase separation in the paramagnetic state of bcc alloys, in the ferromagnetic state it reduces the alloy stability at low Cr concentration and vice versa, makes the solid solution more stable at higher concentrations. This behavior of the phase stability can be predicted from the deviation of the lattice parameter from Vegards law in bcc Fe-Cr alloys. On the atomic level, the pressure effect can be explained by the suppression of the local magnetic moments on Cr atoms, which gives rise to a decrease of the Fe-Cr magnetic exchange interaction at the first coordination shell and, as a result, to the observed variation of the ordering tendency between the Fe and Cr atoms.

Place, publisher, year, edition, pages
American Physical Society, 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-71218 (URN)10.1103/PhysRevB.84.094422 (DOI)000295006300011 ()
Available from: 2011-10-07 Created: 2011-10-07 Last updated: 2017-12-08

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