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Electronic correlations in Fe at Earth's inner core conditions: effect of alloying with Ni
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
Centre de Physique Theorique, CNRS, Ecole Polytechnique, Palaiseau, France.
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
(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
URN: urn:nbn:se:liu:diva-97380OAI: diva2:647328
Available from: 2013-09-11 Created: 2013-09-11 Last updated: 2013-09-11Bibliographically approved
In thesis
1. Influence of stresses and impurities on thermodynamic and elastic properties of metals and alloys from ab initio theory
Open this publication in new window or tab >>Influence of stresses and impurities on thermodynamic and elastic properties of metals and alloys from ab initio theory
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.
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1531
National Category
Condensed Matter Physics
urn:nbn:se:liu:diva-97383 (URN)978–91–7519–557–5 (print) (ISBN)
Public defence
2013-09-19, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Available from: 2013-09-11 Created: 2013-09-11 Last updated: 2013-09-13Bibliographically approved

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